40202092 the Chemistry of Aroma Therapeutic Oils

The Chemistry ofAromatherapeutic Oils

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The Chemistry ofAromatherapeutic

Oils3rd Edition

E. Joy Bowles

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This book is intended for educational and reference purposes, and is not providedin order to diagnose, prescribe or treat any illness or injury. The information

contained in the book is technical and is in no way to be considered as a substitutefor consultation with a recognised health-care professional. As such the author andothers associated with this book accept no responsibility for any claims arising from

the use of any remedy or treatment mentioned here.

This edition first published 2003Copyright E. Joy Bowles, 2003

All rights reserved. No part of this book may be reproduced ortransmitted in any form or by any means, electronic or mechanical,including photocopying, recording or by any information storageand retrieval system, without prior permission in writing from the

publisher. The Australian Copyright Act 1968 (the Act) allows amaximum of one chapter or 10 per cent of this book, whichever is the

greater, to be photocopied by any educational institution for itseducational purposes provided that the educational institution (or

body that administers it) has given a remuneration notice toCopyright Agency Limited (CAL) under the Act.

Allen & Unwin83 Alexander Street

Crows Nest NSW 2065Australia

Phone: (61 2) 8425 0100Fax: (61 2) 9906 2218

Email: [email protected]: www.allenandunwin.com

National Library of AustraliaCataloguing-in-Publication entry:

Bowles, E. Joy.The chemistry of aromatherapeutic oils.

Includes index.ISBN 1 74114 051X

1. Essences and essential oilsTherapeutic use. 2. Aromatherapy.I. Title

Set in 11/13pt Sabon by Midland Typesetters, VictoriaPrinted by Griffin Press, South Australia

10 9 8 7 6 5 4 3 2 1

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CONTENTS

List of Illustrations xiiiList of Tables xviiAcknowledgments xixForeword xxIntroduction 1

1 What is Chemistry? 3Atomic theory of elements 4

Rutherfords atomic model 5Types of atoms found in essential oils 6

Structure of a hydrogen atom 6Structure of a carbon atom 6Structure of an oxygen atom 8

Bonding 9Atoms that dont bond 9Types of bonding 9

Symbols, formulae and chemical drawings 11Chemical formulae 12Drawing chemical structures 13Chemical names 15Functional group names 16

Further reading 16

2 Plants and Essential Oils 21Plant classification 21

How to write botanic names 21Plant anatomy 23

Essential oil storage structures 27Plant physiology 27

Photosynthesis 28Secondary products 28

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How plants make essential oil molecules 29Terpenoid molecules 29Monoterpenes 30Sesquiterpenes 31Phenols and phenyl propanoids 32Non-terpenoid aliphatic molecules 33Heterocyclic compounds 33

Variation in essential oil composition 34Chemotype and effects of geo-climatic location 34Harvesting time 36

A brief overview of the essential oil industry 38Further reading 38

3 Terpenes 41Physical characteristics of terpenoid molecules 41

Polarity 42Solubility 45

Molecular structure and information 50Monoterpenes 53

Structure 53Distribution of monoterpenes in common essential oils 53Naming 53Solubility 55Volatility 55Reactivity 56Toxic effects on the human body 56Therapeutic effects of monoterpenes 57Essential oils with high percentages of monoterpenes 59

Sesquiterpenes 59Structure 59Naming 60Solubility 61Volatility 61Reactivity 61Toxic effects on the body 61Therapeutic effects of sesquiterpenes 61Essential oils with high percentages of sesquiterpenes 63

Further reading 63

4 Functional groups 65Alcohols 65

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Monoterpenols 65Structure 65Naming 66Solubility 66Volatility 66Reactivity 67Toxic effects on the body 68Therapeutic effects of monoterpenols 68Essential oils with high percentages of monoterpenols 70

Sesquiterpenols 70Structure 70Naming 70Solubility 70Volatility 73Reactivity 73Toxic effects on the body 73Therapeutic effects of sesquiterpenols 73Essential oils with high percentages of sesquiterpenols 75

Phenols 76Structure 76Naming 76Solubility 76Volatility 78Reactivity 78Toxic effects on the body 78Therapeutic effects of phenols 78Essential oils with high percentages of phenols 79

Aldehydes 80Structure 80Naming 80Solubility 82Volatility 82Reactivity 82Toxic effects on the body 82Therapeutic effects of aldehydes 83Essential oils with high percentages of aldehydes 84

Ketones 84Structure 84Naming 85Solubility 86Volatility 86

CONTENTS VII

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Reactivity 86Toxic effects on the body 86Therapeutic effects of ketones 87Essential oils with high percentages of ketones 89

Acids and esters 89Naming 90Solubility 90Volatility 91Reactivity 91Toxic effects on the body 92Therapeutic effects of acids and esters 92Essential oils with high percentages of esters 93

Phenyl methyl ethers 93Structure 93Naming 94Solubility 95Volatility 95Reactivity 95Toxic effects on the body 95Therapeutic effects of phenyl methyl ethers 96Essential oils with high percentages of phenyl methyl ethers 97

Cyclic ethers or oxides 98Structure 98Naming 98Solubility 99Volatility 99Reactivity 100Toxic effects on the body 100Therapeutic effects of oxides 101Essential oils with high percentages of cyclic ethers

1,8-cineole 101Lactones 101

Structure 101Naming 102Solubility 103Volatility 104Reactivity 104Toxic effects on the body 104Therapeutic effects of lactones 105Essential oils with high percentages of lactones 106

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Coumarins 106Structure 106Naming 106Solubility 107Volatility 107Reactivity 108Toxic effects on the body 108Therapeutic effects of coumarins 109Essential oils with high percentages of coumarins 110

Summary of hazardous and therapeutic properties of functionalgroups 110

Further reading 110

5 The pharmacology of essential oils 113Pharmacodynamics 113

Binding affinity and specificity 114Selectivity 114Drug potency and efficacy 115Plasma concentration 116

Pharmacokinetics 118Absorption 118Distribution 122Metabolism 124Excretion 125

Pharmacokinetic pathway of essential oils 127Determining the therapeutic dosage range of essential oils 127

Interactions of essential oils with pharmaceutical drugs 133Pharmacological targets of essential oil constituents 134

Interactions with lipophilic cell membranes 135Solvent effects on cholesterol 136Anti-oxidant effects 136Anticoagulant effects 137Effects on excretion 137Interaction with glutathione 137

Enzyme effects 138Inhibition of nuclear transcription factors 138Inhibition of cyclo-oxygenase and 5-lipoxygenase enzymes 139Induction of adenylate cyclase 141Induction or inhibition of cytochrome P450 enzymes 141Interactions with UDP-glucuronyl transferase 141Inhibition of mevalonate pathway 142

CONTENTS IX

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Inhibition of isoprenylation enzymes 143Acetylcholinesterase inhibition 143

Interactions with DNA and developmental processes 143Phototoxicity 144Hyperplasia 144Developmental defects 144

Cell membrane ion channel effects 145Calcium ion flow inhibition 145Antispasmodic effects 145

Receptor interactions 146Histamine receptors 146Antagonism of H1 receptors 147Agonism of H1 receptors 147Oestrogen receptors 148Neurotransmitter effects 150

Psychopharmacology 151Indirect effects of essential oils on mood and cognition via the

olfactory pathway 151The limbic system, mood, odour conditioning and memory 152

Summary of health conditions and applicable pharmacodynamiceffects of essential oil molecules 155

Further reading 155

6 Quality control 159Quality control of essential oils 159

Adulteration 159How can we be sure we are buying good quality oils? 160Physical measures of essential oil quality 160Effects of extraction method on essential oil quality 164Degradation of essential oils 170

7 Isomers and naming 171Geometric isomers 171Enantiomers 172Chirality 172

Optical activity of enantiomers 174Systematic chemical nomenclature 175Further reading 176

X THE CHEMISTRY OF AROMATHERAPEUTIC OILS

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8 Molecular structures 179Common monoterpenes 179Common sesquiterpenes 180Common monoterpenols 181Common sesquiterpenols 182Common phenols 183Common aldehydes 183Common ketones 184Common esters 185Common ethers 186Common cyclic ethers and oxides 186Common lactones 187Common coumarins 187Common furocoumarins 187

Appendix: Ready reference list of essential oils 189Notes 201Index 225

CONTENTS XI

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LIST OF ILLUSTRATIONS

Figure 1.1 Two ways to represent atomic structure 5Figure 1.2 A hydrogen atom 6Figure 1.3 A carbon atom 7Figure 1.4 An oxygen atom 8Figure 1.5 Two ways of representing a hydrogen molecule 10Figure 1.6 Two ways of representing a water molecule 10Figure 1.7 Two ways of representing an ethene

molecule, C2H4 12Figure 1.8 A water molecule 12Figure 1.9 An ethanol molecule 13Figure 1.10 Two ways of representing a methane molecule 13Figure 1.11 An ethene molecule 14Figure 1.12 An ethyne molecule 14Figure 1.13 Variability in structure of carbon-based molecules 14Figure 1.14 Two ways of representing a linalool

(C10H17OH) molecule, showing ease of thesimplified stick method 15

Figure 1.15 A benzene ring (C6H6), showing the way electronsare shared between carbon atoms in the ring 15

Figure 2.1 Taxonomic levels used in botany, with exampleof names for Scotch pine (Pinus sylvestris) 22

Figure 2.2 Anatomical structures of Salvia officinalis(family Lamiaceae) 24

Figure 2.3 Structure of an isoprene molecule and anisoprene unit 30

Figure 2.4 The monoterpene alpha-myrcene. The dotted linerepresents the bond formed between two isopreneunits 31

Figure 2.5 The sesquiterpene (E)-beta-farnesene. The dottedlines represent the bonds formed between threeisoprene units 31

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Figure 2.6 Structure of phenols and phenyl propanoids.Thymol is a phenol, anethole is a phenyl propanoid 32

Figure 2.7 Structure of the aliphatic aldehyde octanol(C8H16O) 33

Figure 2.8 Structure of the heterocyclic molecule indole,showing the position of the nitrogen atom 33

Figure 3.1 Polarisation of charge in a water molecule 43Figure 3.2 Three water molecules with electrostatic liaisons

represented by broken lines 44Figure 3.3 Structures of menthol and glucose, showing

water-soluble OH groups 47Figure 3.4 Structure of ethanol 47Figure 3.5 Schematic representation of an emulsifier

molecule 48Figure 3.6 Cross-section view of a micelle of emulsifier

molecules: non-polar ends in centre of sphere,polar ends on surface of micelle able to liaisewith polar water molecules 49

Figure 3.7 Essential oil chemistry road map 52Figure 4.1 Structure of the carboxylic acid, acetic acid (also

known as ethanoic acid) 89Figure 4.2 The esterification reaction of linalool and acetic

acid leading to formation of an ester 90Figure 4.3 Structure of epoxy ether and furan rings, showing

placement of oxygen atom within carbon chain 98Figure 4.4 Structure of warfarin, showing similarity

to coumarin 108Figure 5.1 Typical plasma concentrationtime curve of a

therapeutic dose of a hypothetical terpenoidcompound 117

Figure 5.2 Schematic representation of the layers of theskin and underlying structures 120

Figure 5.3 Pharmacokinetic pathway of essential oilsthrough different parts of the body fromabsorption to excretion 123

Figure 5.4 Distribution of drug molecules between bodyfluid compartments 123

Figure 5.5 Pharmacokinetic route of essential oilsthrough the body 128

Figure 5.6 A hypothetical drug concentrationresponsecurve 130

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Figure 5.7 Plasma concentrationtime curve after severalrepeated doses showing the steady-state pattern 130

Figure 5.8 Structure of some anti-inflammatory molecules 139Figure 5.9 Eicosanoid molecules 140Figure 5.10 The structure of histamine 147Figure 5.11 Structures of two female steroidal hormones 149Figure 5.12 Structure of the diterpenol, sclareol 150Figure 5.13 The flow of information to different brain

structures during olfaction 153Figure 6.1 Gas chromatograph of Tea-tree (Melaleuca

alternifolia) oil 162Figure 6.2 One of the isomers of gingerol 168Figure 7.1 Cis- and trans-isomers 172Figure 7.2 Enantiomers of carvone, showing chiral carbon

atoms as * 173Figure 7.3 Enantiomers of menthol, showing different

configurations for chiral carbon atoms 174Figure 7.4 Alpha-terpinene, showing numbering of carbon

atoms and bonds 175Figure 7.5 A molecule of para-cymene or 1-methyl-4-

isopropyl benzene, showing positions ofgroups attached to the ring 176

Figure 8.1 Common monoterpenes 179Figure 8.2 Common sesquiterpenes 180Figure 8.3 Common monoterpenols 181Figure 8.4 Common sesquiterpenols 182Figure 8.5 Common phenols 183Figure 8.6 Common aldehydes 183Figure 8.7 Common ketones 184Figure 8.8 Common esters 185Figure 8.9 Common ethers 186Figure 8.10 Common cyclic ethers and oxides 186Figure 8.11 Common lactones 187Figure 8.12 Common coumarins 187Figure 8.13 Common furocoumarins 187

LIST OF ILLUSTRATIONS XV

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LIST OF TABLES

Table 1.1 Examples of elements and compounds 4Table 1.2 Numbers of electrons involved in bonding 11Table 1.3 Simple carbon compounds 17Table 1.4 Functional groups found in essential oil molecules 18Table 2.1 Common plant families and examples of essential

oil producing plants from each family 25Table 2.2 Variation in the composition of Rosemary

essential oil, chemotypes 1 and 2 35Table 2.3 Comparison of four Thymus vulgaris chemotypes 36Table 2.4 Comparison of Geranium oils from China and

Reunion 37Table 3.1 Comparison of solubilities of hydrocarbons and

alcohols in water 46Table 3.2 Examples of monoterpenes 54Table 3.3 Essential oils containing high percentages of

monoterpenes 58Table 3.4 Examples of sesquiterpenes 60Table 3.5 Essential oils containing high percentages of

sesquiterpenes 62Table 4.1 Examples of monoterpenols 67Table 4.2 Essential oils containing high percentages of

monoterpenols 71Table 4.3 Examples of sesquiterpenols 72Table 4.4 Essential oils containing high percentages of

sesquiterpenols 76Table 4.5 Examples of phenols 77Table 4.6 Essential oils containing high percentages of

phenols 80Table 4.7 Examples of aldehydes 81Table 4.8 Essential oils containing high percentages of

aldehydes 84

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Table 4.9 Examples of ketones 85Table 4.10 Essential oils containing high percentages of

ketones 88Table 4.11 Examples of esters 91Table 4.12 Essential oils containing high percentages of

esters 93Table 4.13 Examples of phenyl methyl ethers 94Table 4.14 Essential oils containing high percentages of

phenyl methyl ethers 97Table 4.15 Examples of cyclic ethers and oxides 99Table 4.16 Essential oils containing high percentages of

1,8-cineole 102Table 4.17 Examples of lactones 103Table 4.18 An essential oil containing high percentages of

lactones 106Table 4.19 Examples of coumarins 107Table 4.20 An essential oil containing high percentages of

coumarins 110Table 4.21 Summary of therapeutic properties of essential

oil constituents 111Table 5.1 Comparison of potency and efficacy of some

essential oils as local anaesthetics 115Table 5.2 Summary of research on percutaneous

absorption enhancement 122Table 5.3 Application methods and dosages used in

non-medical aromatherapy 132Table 5.4 List of possible essential oil/drug interactions 134Table 5.5 Health conditions and pharmacodynamic effects 156Table 6.1 Ratio of linalool to linalyl acetate obtained with

different extraction techniques 165Major and minor constituents of essential oils 190

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ACKNOWLEDGMENTS

This book is the product of eleven years of research, teaching andlearning. As with all creative labours, its writing has required muchenergy, grace and generosity from my teachers, mentors, colleagues,students, friends and family. For everyones gifts of inspiration overtime I am deeply thankful.

I hesitate to name people individually, as I may forget some, butI do wish to credit Dr D. Pnol and Pierre Franchomme, whoseseminal work Laromathrapie exactement started me off on myjourney. I also acknowledge my publisher Emma Cant and the teamat Allen & Unwin who have made my transition from self-publishingto commercially published author a pleasant and easy one. I amindebted to my reviewers including Hans Wohlmuth, David de Vries,Corinne Border and Bob Harris who made valuable comments andcorrections in the editing process.

It is my hope that this book will aid communication betweenaromatherapists and other health professionals, and contribute to anunderstanding of the therapeutic principles of the essential oils.

E. Joy Bowles, Lismore, AustraliaJanuary 2003

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FOREWORD

A recent poll of scientists opinions about alternative medicinerevealed a perhaps surprising general level of acceptance, but this wasbiased in favour of serious therapies such as acupuncture andosteopathy. Aromatherapy was not given much credence, although itwas awarded some credit for relaxation.

The popular perception of aromatherapy is pretty far removedfrom the kind of aromatherapy being taught, practised and writtenabout in books such as this. It is not really surprising that the lack ofknowledge about, and understanding of, aromatherapy is wide-spread, and shared even by scientists. The volume of clinical researchconcerning essential oils is small, almost all of it has been publishedsince 1990, and very few books even refer to it.

We need more books that discuss the scientific literature which, ifyou step beyond the clinical realm, is bordering on the substantial,especially in relation to certain essential oils and aromachemicals.Any study of aromatherapy and essential oils is hollow, unless thebasic chemistry is understood, and this text does an excellent job ofimparting that knowledge.

This is not a new text, but is a substantial and very welcome clabo-ration of the previous edition. The new edition has a depth and breadththrough which the writer dovetails chemistry with pharmacology andtoxicology. It is no secret that I am not a proponent of the idea that allthe chemical constituents in a particular category share certain proper-ties, and I am pleased to find that Joy has taken a more balancedapproach in this new edition. At the same time it is always fascinatingto discover that a certain property is explainable because of somemolecular foible, and Joy does an excellent job of joining up the knots.

It is monumentally difficult to write about chemistry when most ofyour target audience finds it an unfamiliar and obtuse subject, butJoy manages to convey her enthusiasm and insight in a way that botheducates and inspires.

Robert TisserandCalifornia, September 2003

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INTRODUCTION

People in todays highly pressurised society are increasingly beingaffected by new forms of disease that are less amenable to treatmentwith mainstream medicine. Doctors and specialists seem to have lessand less time to spend on consultations, and are often perceived as beingtoo readily influenced by the pharmaceutical industrys incentives.

Many people disappointed with mainstream treatments turn tocomplementary therapies, finding there a different approach to healthand disease. Most complementary therapies, however, rely onhistorical and empirical evidence rather than scientific evidence. Thescientific method uses randomised controlled trials to evaluateefficacy of a drug or therapy. Randomisation aims to reduce any biasin selection of participants, and using a control group of people withthe same characteristics as your treatment group allows you tocompare the effect of a treatment with a placebo, or with anothertreatment. A placebo or non-active treatment is used to control forthe effect that peoples beliefs about the treatment being tested mayhave on the outcome of the treatment.

Scientific evaluation of aromatherapy faces some difficulties: findinga fragrant placebo that has no effect; assessing the effect of the consul-tation and practitionerclient relationship; and separating olfactoryeffects from pharmacological effects and the effects of massage.However until there have been enough randomised clinical trials, thereare many studies of essential oils and their components that can beused to hypothesise the likely effects of essential oils in humans.

Why study essential oil chemistry?

The purpose of this book is to offer you a way of understandingessential oil chemistry so that you can make sense of the research

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literature. A knowledge of essential oil chemistry also enables you to:

make educated guesses about the properties of unfamiliar oilsonce you can identify their chemical composition. Lets take theAustralian oil Rosalina, Melaleuca ericifolia, as an example.A quick search on the Internet yields several sites that list one ofits major components as linalool. Given this as a starting point,you can research the known effects of linalool by a search of theMedline database, and begin to get a feel for how the oil may beused. If you knew more of the constituents, you could begin tobuild up a composite profile of possible effects of the whole oil.1

make judgements about the safety of different oils. If an adverseevent occurs while using an essential oil, you can look at thehazards or toxicity associated with its individual constituents,and estimate whether any were likely to have caused the event.

What does this book cover?

The book starts with a brief overview of chemistry and a simplifiedexplanation of how atoms bond to form the molecules found inessential oils. Chapter 2 describes the aspects of plant anatomy andphysiology that are relevant to essential oils, and the four main typesof molecule found in essential oils: terpenoids, phenols and phenylpropanoids, non-terpenoid aliphatic molecules and heterocycliccompounds. Chapter 3 discusses terpenes and the polarity and solu-bility of essential oil molecules. Chapter 4 illustrates the differentfunctional groups found in essential oil molecules with examples ofeach from the research literature.

Chapter 5 offers a brief overview of pharmacology and thenexplores possible actions of essential oils on the human body giventhe information in Chapters 3 and 4. Chapter 6 is concerned withquality control, Chapter 7 with some more advanced details aboutthe chemical naming system for essential oil molecules. Chapter 8has a Ready Reference list of the top 3 constituents found in94 different essential oils, and chemical structures for several of thecommon constituents from each functional group.

2 THE CHEMISTRY OF AROMATHERAPEUTIC OILS


1WHAT IS CHEMISTRY?

Chemistry is the study of the composition, properties and trans-formations of substances. One has to distinguish between physicalproperties (shape, colour, texture, etc.) and chemical properties suchas the fundamental transformations a substance, for example, wood,undergoes when tested.

One of the simplest transformational experiments is to see whathappens when they are heated. If you heat a piece of wood suff-iciently, it will catch alight and burn with a yellow flame, producingsmoke and some grey or black ash. If you heat a piece of glasssufficiently, it may melt and change shape, but it wont burn with ayellow flame or produce smoke and ash.

Chemists have systematically studied thousands of different typesof materials using many different types of transformational exper-iments. The result of their research is the discovery that there areabout 105 naturally occurring fundamental substances, known aselements. Some of these elements occur as everyday materials, butmost elements more usually combine with other elements to formcompounds. Examples of elemental and compound substances areshown in Table 1.1. The formulae are a chemistry short-handshowing the types of atoms present in the substances.


Atomic theory of elements

To explain how elements combined to form compounds with verydifferent properties than those of the component elements, a theoryof atoms was gradually developed during the 1800s.1 The theory goeslike this:

Elements are made up of indivisible particles called atoms.Atoms of a given element all share the same properties.

Chemical changes occur when atoms are combined, separated orrearranged.

Atoms of different elements have different properties. Atoms combine together in fixed whole number ratios to form

larger particles known as molecules.

In 1909, Ernest Rutherford proposed that atoms were mainlyempty space, with a tiny, positively charged central nucleus orbitedby a series of negatively charged electrons. This particle theory ofatoms has been superseded by quantum mechanics, but it is still


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Elements Compounds

Substance Formula Type of Substance Formula Typesname atom name of atom

Hydrogen gas H2 Hydrogen Water H2O Hydrogen,oxygen

Oxygen gas O2 Oxygen Carbon dioxide CO2 Carbon,oxygen

Diamond C Carbon Butane C4H10 Carbon,hydrogen

Sulphur S Sulphur Hydrogen H2S Hydrogen,sulphide sulphur(rotten egg gas)

Silver metal Ag Silver Silver oxide AgO Silver,(black tarnish oxygenon silver)

Nitrogen gas N2 Nitrogen Ammonia NH3 Nitrogen,hydrogen


useful for explaining how different elements combine to formmolecules.

Rutherfords atomic model

In Rutherfords model of the atom, atoms are made up of three types ofsubatomic particles of matter called protons, neutrons and electrons.Each subatomic particle can be thought of as a building block, but singleprotons, neutrons and electrons do not manifest the characteristics ofany element. It is the specific number and ratio of protons, neutrons andelectrons that together give rise to atoms of different elements.

Protons and electrons are electrically charged. Positively chargedprotons attract negatively charged electrons, but repel other protons.Electrons, being negatively charged, repel other electrons. The rule isopposite charges attract, similar charges repel. Neutrons, as the namesuggests, are neutral: they have no charge and are not affected by thecharge of protons or electrons. In Rutherfords model, protons, neutronsand electrons are arranged within atoms in the following ways:

The number of protons equals the number of electrons, so thetotal charge on the atom, when they are added together, is zero.

The protons are in the centre or nucleus of the atom. The neutrons are in the nucleus and seem to prevent the positive

charges from repelling each other. The electrons orbit the nucleus at defined energy levels or

orbitals.

Imagine an electron in an orbital as a yoyo being whirled on itsstring. Most of the time, in any one spot on the circumference of theyoyos path, there is empty space, but if you put your hand in the wayof the whirling yoyo, it does not feel like empty space. Figure 1.1shows two ways of representing atomic structure. The dots are thesubatomic particles.

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WHAT IS CHEMISTRY? 5


Types of atoms found in essential oils

Like most molecules made by living organisms, essential oilmolecules are mainly made up of hydrogen and carbon atoms.Several types of essential oil molecule also contain oxygen atoms,though these are usually in the functional groups attached to thecarbon skeleton of the molecule (see Table 1.4 Functional groups onpage 18).

Some essential oil molecules also contain sulphur atoms (forexample, diallyl sulphide, from garlic and the onion family), andsome contain nitrogen atoms (for example, indole, from Jasmine, andmethyl N-methyl anthranilate, from Mandarin and Sweet orange peeloils). We will now look at the structures of hydrogen, carbon andoxygen atoms in more detail.

Structure of a hydrogen atom

Hydrogen is the smallest atom. It has only one proton orbited by oneelectron. It is unique among atoms in that it doesnt have anyneutrons in its nucleus. The electron is located in the first orbital at aset distance from the proton. This first orbital has the potential tocontain two electrons, so there is a vacancy in the first orbital ofhydrogen atoms. The vacancy determines the capacity of hydrogenatoms to form molecules. Figure 1.2 is a two-dimensional diagram ofa hydrogen atom.

Structure of a carbon atom

The next atom we concern ourselves with is carbon. It has sixprotons and six electrons. There are also six neutrons in the nucleusof a carbon atom which act as a kind of bubble wrap or insulating


+

orbital

nucleus

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layer to prevent the six positively charged protons from repellingeach other. Two electrons fill the first orbital. Four electrons arelocated in the second orbital. The electrons in the second orbital havemore energy than those in the first. They are depicted as being furtheraway from the nucleus. Electrons can jump between orbitals if theyare sufficiently energised, which is where the term quantum leapcomes from, as there is no continuum between orbitals. The secondorbital can contain as many as eight electrons, so there are fourvacancies in the second orbital of a carbon atom.

Figure 1.3 below shows the structure of a carbon atom, with itstwo electron shells.


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Element Carbon

Symbol CNumber of protons (+) 6Number of neutrons 6Number of electrons (-) 2 in first orbital

4 in second orbitalNumber of electron vacancies 4 in second orbitalNumber of bonds possible 4

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Element Hydrogen

Symbol HNumber of protons (+) 1Number of neutrons 0Number of electrons (-) 1 in first orbitalNumber of electron vacancies 1 in first orbitalNumber of bonds possible 1

6+

Secondorbital

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Structure of an oxygen atom

An oxygen atom has eight protons and eight neutrons in the nucleus,and also eight electrons. As with the carbon atom, the first electronorbital is full, containing two electrons. The second orbital containssix electrons, leaving two vacancies. Figure 1.4 shows the structure ofan oxygen atom.


8+

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Element Oxygen

Symbol ONumber of protons (+) 8Number of electrons (-) 2 in first orbital

6 in second orbitalNumber of neutrons 8Number of electron vacancies 2 in second orbitalNumber of bonds possible 2

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Ions are atoms that have either gained or lost one or more electrons. Forexample, free radical cations (positively charged) are formed when an atom isbombarded by sufficient energy to allow an electron to escape from the atomaltogether. Anions are negatively charged because they receive an extra electron.

Ionic bonding occurs when electrons are transferred between ions ofopposite charges so that their outermost orbitals are full. Metallic elements andnon-metals bind ionically to form substances known as salts. Salts usually occurin a crystalline form, where many positive and negative ions form a lattice heldtogether by electrical charge. Ionic compounds do not exist as single moleculesin the solid state.


Bonding

As atoms are neutrally charged, you would not expect them to attractor repel each other. However, most atoms do not exist as singleatoms. They bond together with other atoms to form molecules.Hydrogen, carbon and oxygen atoms all have electron vacanciesin their outermost orbital. When atoms have electron vacancies, theybind with other atoms to get electrons to fill the vacancies. Thisis because a full orbital is more stable energetically than a partiallyfull one.

Atoms that dont bond

Helium and neon are examples of substances which do exist as singleatoms. A helium atom has two protons, two neutrons and twoelectrons. This means that its only electron orbital is full, so it has noneed to bond with any other atom. A neon atom has ten protons, tenneutrons and ten electrons, which means that both its first andsecond orbitals are full. You can imagine helium and neon as self-actualised atoms that dont need other atoms to feel fulfilled.Neither helium or neon atoms have yet been discovered in molecules,so they are considered inert or un-reactive elements.

Types of bonding

Bonding is an interaction between two atoms. There are two maintypes of bonding: ionic and covalent. Information Box 1.1 describesions and ionic bonding, but as essential oils are covalently bonded,we will not consider ionic bonding further here.

Covalent bonding occurs between atoms by sharing electrons.In order for this to happen, atoms have to get close enough to


An example of an ionically bonded substance is common table salt, sodiumchloride (NaCl). When dissolved in water, the positively charged sodium ions(Na+) and negatively charged chlorine ions (Cl) dissociate from each other,reforming into the crystal lattice only when the water molecules evaporate.Bodily fluids have many different types of ions dissolved in them, and the rightbalance of positive and negative ions in body fluids is crucial for cell function.


overlap their outer electron orbitals. Hydrogen gas consists ofmolecules of hydrogen, each molecule containing two hydrogenatoms bonded together. A hydrogen molecule is the simplestcovalently bonded molecule. Figure 1.5 shows two ways of repre-senting a hydrogen atom. The first demonstrates the overlap oforbitals, and how each electron still belongs to its own atom whilebeing accessible to the other atom. The second drawing uses astick to show the bond between the two atoms, with an electronat either end of the stick.

Figure 1.6 shows two ways of drawing a molecule of water. Noticehow the oxygen atom requires two electrons to fill its outer orbital(eight electrons), and in this case has bonded with two differenthydrogen atoms to get them. Notice also that each hydrogen atomnow has acquired an extra electron and has a full orbital.


HH

O

oxygen electrons

hydrogen electrons

HH

O

oxygen electrons

hydrogen electrons

FFiigguurree 11..66 TTwwoo wwaayyss ooff rreepprreesseennttiinngg aa wwaatteerr mmoolleeccuullee

+ +

+ +

FFiigguurree 11..55 TTwwoo wwaayyss ooff rreepprreesseennttiinngg aa hhyyddrrooggeenn mmoolleeccuullee


Table 1.2 lists the number of bonds that hydrogen, carbon andoxygen atoms need to make with other atoms in order to gain fullouter orbitals.

Double and triple covalent bonds can be formed between twoatoms if there are enough electrons to share. A double bond is wheretwo pairs of electrons are shared between the two atoms. Carbonatoms quite often form double bonds with other carbon atoms, as willbe seen in the examples of essential oil molecules in Chapters 3 and 4.Figure 1.7 shows two ways of drawing a molecule of ethene, whichhas two carbon atoms and four hydrogen atoms. Ethene has a doublebond between the two carbon atoms. Black dots represent electronsfrom carbon atoms, diamonds electrons from hydrogen atoms. Theinner orbital electrons on the carbon atoms are not shown.

The types of bonds between atoms in a molecule affect its three-dimensional structure. This in turn affects its chemical propertiesand, as we will see in Chapters 3 and 4, it also affects the odour andtherapeutic properties of essential oil molecules.

Symbols, formulae and chemical drawings

When written down, chemistry is a curious mixture of letters andsymbols, which allows for a two-dimensional representation of three-dimensional molecular structures, and also provides a short-hand forease of communication. Atoms are usually represented by the initialof their Latin or Greek name, or first two letters if more thantwo elements share the same initial. For example, Au is the symbolfor gold, from the Latin aurum, gold.

Figures 1.1 to 1.7 used dots for subatomic particles and curvedlines for electron orbitals, but for large molecules this drawing


H

O

H

TTaabbllee 11..22 NNuummbbeerrss ooff eelleeccttrroonnss iinnvvoollvveedd iinn bboonnddiinngg

Atom No. of electrons Maximum possible Bonds neededin outermost no. of electrons for molecularorbital in outer orbital stability

Hydrogen 1 2 1

Carbon 4 8 4

Oxygen 6 8 2


method would get very tedious (and take up far too much space). Thesimplest way to represent a molecule is by drawing sticks or barsbetween the atoms, each stick representing one bond.

Water molecules (one atom of oxygen, two atoms of hydrogen)can therefore be represented as in Figure 1.8.

Chemical formulae

When chemists want to refer to the atomic components of amolecule, they use its formula. This is an even shorter way ofexpressing types and numbers of atoms present. For example, wateris written H2O, and ethene C2H4. The subscript numbers indicate thenumber of atoms of each type present (for example, H4 means thereare four atoms of hydrogen present).

For more complex molecules, there are different ways of writingformulae. Let us take ethanol, the substance found in wine referred toas alcohol by the general public. Figure 1.9 shows the arrangementof the two carbon atoms, six hydrogen atoms and one oxygen atomthat make up an ethanol molecule.


H H

H H

C C

H

H

H

H

C C

FFiigguurree 11..77 TTwwoo wwaayyss ooff rreepprreesseennttiinngg aann eetthheennee mmoolleeccuullee,, CC22HH44

H

O

H

FFiigguurree 11..88 AA wwaatteerr mmoolleeccuullee


According to convention, in the simplest written formulae, carbonis mentioned first, hydrogen second, and oxygen last. Thus ethanol iswritten as C2H6O. The formula can also be written to give moreinformation about the molecules structure, for example,CH3CH2OH. If you look at the stick structure in Figure 1.9, youcan see how the carbon atoms form the skeleton of the molecule. Yetanother way of writing the formula highlights the placement ofoxygen atoms in the molecule, for example, C2H5OH.

Drawing chemical structures

Chemists are also interested in representing the physical structure ofmolecules more closely. In three dimensions, the molecules are not instraight lines as implied in Figure 1.8. The atoms are joined to eachother at angles to ensure their maximum separation. This is because theouter electrons round each atom repel electrons round the other atoms.

Thus, if a carbon atom has four other atoms (say, hydrogen atoms)joined to it, the three-dimensional shape of the molecule will be tetra-hedral, as in the molecule methane (CH4) shown in Figure 1.10.Imagine the solid black arrowhead coming out of the page surfaceand the dashed arrowhead going back behind the page. The diagramnext to it in the shape of a cross is how the molecule is representedwith just straight lines.

At the site of a double bond, the molecule will have a planar trian-gular shape like ethene (C2H4), as shown in Figure 1.11. Rememberthat hydrogen cannot make double bonds as it requires only one


C C

H

H

H H

H

O

H

FFiigguurree 11..99 AAnn eetthhaannooll mmoolleeccuullee

C

H

H H

H

H

C H

H

H

FFiigguurree 11..1100 TTwwoo wwaayyss ooff rreepprreesseennttiinngg aa mmeetthhaannee mmoolleeccuullee


additional electron to achieve a full outer orbital. In essential oils,therefore, only carbon and oxygen atoms will make double bonds.

Two carbon atoms can even share three bonds with each other, inwhat is called a triple bond. The net result is a linearly shapedmolecule known as ethyne (C2H2), as shown in Figure 1.12. Triplebonds are not very stable physically, which makes them reactive.Carbon-carbon triple bonds are very uncommon in essential oils.

Carbon atoms are capable of forming many complex structures,due to their ability to form four bonds. Figure 1.13 shows isobutane(C4H10) as a branched structure and cyclohexane (C6H12) as a closedring.

Most essential oil molecules have long carbon chains and requirean even more abbreviated form of representation. In the first illus-tration in Figure 1.14, each stick represents a bond between twocarbon atoms. The attachment of hydrogen atoms is presumed andnot shown, whereas the position of oxygen atoms is shown. Themolecule is linalool, a constituent of Lavender oil and many otheressential oils.

Phenolic and coumarin essential oil molecules feature structuresknown as benzene rings. The molecule C6H6 is the compound


C C

H

H

H

H

FFiigguurree 11..1111 AAnn eetthheennee mmoolleeccuullee

C C HH

FFiigguurree 11..1122 AAnn eetthhyynnee mmoolleeccuullee

CC

C

C

HH

H

HH

H

HH

H

H

isobutane

C

CC

C

CC

HH

H

H

H

HH

H

H

H

H

H

cyclohexane

FFiigguurree 11..1133 VVaarriiaabbiilliittyy iinn ssttrruuccttuurree ooff ccaarrbboonn--bbaasseedd mmoolleeccuulleess


benzene. Its structure is similar to the closed 6-carbon ring of cyclo-hexane (see Figure 1.13), but the bonds are neither single or double.As shown in Figure 1.15, the fourth electron of each carbon atom ina benzene ring appears to be equally shared by the carbon atoms oneither side, causing an even spread or delocalisation of electronsover the ring. See Phenols in Chapter 4 for more details.

Chemical names

The names of carbon-based molecules convey useful informationabout their structure. Table 1.3 is a key to the meanings of somechemical names. Some of them, such as methane (a greenhouse gas),propane and butane (BBQ and cigarette lighter fuels), may already befamiliar.


OH

C

CC

C

CC

C

CC

OH

HH

H

H

H

H

H

HHHH

H

H

H

H

H

H

C

FFiigguurree 11..1144 TTwwoo wwaayyss ooff rreepprreesseennttiinngg aa lliinnaallooooll ((CC1100HH1177OOHH)) mmoolleeccuullee,,sshhoowwiinngg eeaassee ooff tthhee ssiimmpplliiffiieedd ssttiicckk mmeetthhoodd

C

CCC

C CH

H

H

H

H

H

or

FFiigguurree 11..1155 AA bbeennzzeennee rriinngg ((CC66HH66)),, sshhoowwiinngg tthhee wwaayy eelleeccttrroonnss aarreesshhaarreedd bbeettwweeeenn ccaarrbboonn aattoommss iinn tthhee rriinngg


Functional group names

A functional group is a chemical entity that gives a molecule its par-ticular characteristics, or its function. Functional groups can affectthe odour, solubility, toxicity and therapeutic properties of molecules.A number of essential oil molecules have one or more functionalgroups attached to the carbon skeleton of the molecule. Table 1.4summarises the functional groups found in essential oil molecules.Chapter 4 considers the research literature about the therapeuticproperties of molecules with different types of functional groups.

Further reading

For an interactive periodic table of elements outlining the characteristicsof each chemical try http://www.webelements.com/

P. Strathern (2001), Mendeleyevs Dream: The Quest for the Elements,Penguin Putnam Inc, Berkley, describes the history of chemistry fromancient Greek times in an accessible and, at times, humorous way.




TTaabbllee 11..33 SSiimmppllee ccaarrbboonn ccoommppoouunnddss

Name and Structure Part of Meaningformula name

Methane CH4 Meth- One carbon atom

Ethane C2H6 Eth- Two carbonatoms

Propane C3H8 Prop- Three carbon atoms in a chain

Butane C4H10 But- Four carbon atoms in a chain

-ane Only singlebonds between the

carbon atoms

Ethene C2H4 -ene A double bond present betweentwo carbonatoms

H

CH

H

H

H

CH

H

C

H

H

H

H

CH

H

C

H

H

C

H

H

H

H

CH

H

C

H

H

C

H

H

C

H

H

H

C CH

H

H

H



TTaabbllee 11..44 FFuunnccttiioonnaall ggrroouuppss ffoouunndd iinn eesssseennttiiaall ooiill mmoolleeccuulleess

Molecule Functional Name Structure Locationdescription group ending

Alcohol Hydroxyl -ol -OH Primary = at endof chain;secondary = onC joined to twoother Cs; tertiary= on C joined tothree other Cs.

Aldehyde Carbonyl -al; -aldehyde At end of Cchain

Ketone Carbonyl -one In middle of Cchain

Acid Carboxyl -ic acid At end of Cchain

Ester Ester -yl + -ate

Phenol Phenol -ol

Ether Ether -ole C-O-C

Phenyl Phenyl -olemethyl methylether ether

Furanoid Furan -furan- O atom part of 5-membered ring

C

O

H

C

O

C

C

C

O

OH

C

O

OC

OH

OCH3

O



Pyranoid Pyran -pyran- O atom part of6-memberedring

Oxide Cyclic ether -ole O atom part ofclosed ring(number of Catoms varies)

Lactone Lactone -one; -in O atom includedin closed ring(number of Catoms varies)

Coumarin Benzo- -in; -en; Benzene ring + alpha- -one lactone ringpyrone

O

O

O

O

OO


2PLANTS AND ESSENTIAL OILS

Plant classification

In order to understand how plants make essential oils, it is probablyhelpful to take a step back from the molecular level and look atplants as living organisms. The study of plants is known as botany.Botany uses a taxonomic classification system to arrange the hugevariety of plants into groups that share similar anatomical character-istics.

The different groups can be arranged in a branching manner like afamily tree, showing the degree of relatedness between different plantfamilies. The names start at the top with Kingdom, Sub-kingdom,Division, Class, Order, Family, Genus, Species and Sub-species.Figure 2.1 shows a diagram of how the levels are arranged. There are4 sub-kingdoms, 5 superdivisions, 5 divisions, 1 class, 2 orders, 6families, 7 genera and 50 species of Pinus, so not all the groups areshown in the diagram.

There are at least four classification systems in use, but the differ-ences between them occur mainly at higher levels of classification anddo not affect the genus and species names too much. The system usedin Figure 2.1 is the Cronquist system (see Further reading for refer-ences to other systems). Table 2.1 shows common plant families andexamples of plant species in each that yield commercially availableessential oils.

How to write botanic names

Botanic names are generally derived from Latin or Greek words, asthese were the academic languages when taxonomic systems werefirst invented. The eighteenth-century Swedish naturalist Linnaeus (orLinn) was the first to use two names (the Latin binomial system) to


FFiigguurree 22..11 TTaaxxoonnoommiicc lleevveellss uusseedd iinn bboottaannyy,, wwiitthh eexxaammppllee ooff nnaammeess ffoorrSSccoottcchh ppiinnee ((PPiinnuuss ssyyllvveessttrriiss)).. NNoott aallll ggrroouuppss aarree sshhoowwnn ffoorr eeaacchh lleevveell..


KINGDOMPlantae

SUB-KINGDOMTracheobionta

(Vascular plants)

SUB-KINGDOMHepatophyta(Liverworts)

SUB-KINGDOMBryophyta(Mosses)

SUPERDIVISIONSpermatophyta

(Seed-bearing plants)

SUPERDIVISIONPteridophyta

(Ferns)

SUPERDIVISIONEquisitophyta(Horsetails)

DIVISIONConiferophyta

(Conifers)

DIVISIONCycadophyta

(Cycads)

DIVISIONMagnoliophyta

(Flowering plants)

CLASSPinopsida

ORDERPinales

ORDERTaxales

FAMILYPinaceae

(Pine family)

FAMILYTaxodiaceae

(Redwood family)

FAMILYCupressaceae

(Cypress family)

GENUSCedrus Trew

GENUSAbies P. Mill

GENUSPinus L

GENUSPseudotsuga Carr.

GENUSPicea A. Dietr

SPECIESpinaster Soland, non Ait.

SPECIESglabra Walt.

SPECIESsylvestris L.

SPECIESradiata D. Don

Source: Adapted from United States Department of Agriculture, Plants Database,http://plants.usda.gov/ [accessed 16-05-2003].


classify plants and animals. A large number of genera and specieshave a capital L. following their names, indicating that it wasLinnaeus who first classified them. As new species are discovered, orold ones re-classified, the name of the botanist who most recentlycreated a name gets added in abbreviated form.

The most commonly used botanic names of any specimen are thelast two, the genus and the species. They are normally written initalics, with only the genus name having a capital letter, for examplePinus sylvestris. A small x between genus and species indicates ahybrid, for example Mentha x piperita. Above the genus level, thenames are written in normal type. Sub-species are minor variations ofa species, which differ in colour or structure of a plant part.

Another addition to the botanical name that often occurs onbottles of essential oils is the chemotype. Some plants from anatomi-cally identical species produce essential oils of different odour andcomposition (see Variation in essential oil composition on page 34).The chemotype name usually appears after the italic genus-speciesbinomial: Rosmarinus officinalis CT camphor (CT, chemotype;camphor, the distinctive compound in this chemotype).

In an ideal world, all essential oils should be extracted from onlyone species or one chemotype of a species, from one geographicallocation and labelled with their botanic name and chemotype.However, the multi-level nature of the industry tends to preclude suchstringent quality assurance. Suppliers that source their oils directlyfrom the growers are more likely to be able to determine the exactbotanical species of their oils. Chemotypic variation can usually bedetected by odour, but should preferably be confirmed by GCMSanalysis (see Chapter 6).

Plant anatomy

Plant anatomy is the study of the physical features of plants, such asleaves, flowers, fruits, shape and size of the plant. Differences in plantanatomical features form the basis for plant classification. Todetermine which family a plant belongs to, you need to examine theshapes, orientation, structure and numbers of its anatomical features.Variations in leaf width and length, presence or absence of hairsand different flower colours give the genus of a plant. Figure 2.2shows some of these features as they appear in a sage plant (Salviaofficinalis).

PLANTS AND ESSENTIAL OILS 23



FFiigguurree 22..22 AAnnaattoommiiccaall ssttrruuccttuurreess ooff SSaallvviiaa ooffffiicciinnaalliiss ((ffaammiillyy LLaammiiaacceeaaee))

2

4 3

15

1. Salvia officinalis;2. its corolla laid open;3. its pistil;4. the pistil and lower part of the flower cut open;5. perpendicular section of a nut.

Petals


TTaabbllee 22..11 CCoommmmoonn ppllaanntt ffaammiilliieess aanndd eexxaammpplleess ooff eesssseennttiiaall ooiillpprroodduucciinngg ppllaannttss ffrroomm eeaacchh ffaammiillyy

Plant family Plant part Common name Botanic nameextracted foressential oil

Annonaceae Flowers Ylang Ylang Cananga odorata

Apiaceae Fruits and roots Angelica Angelica(also known archangelicaas Umbelliferae)

Fruits Aniseed Pimpinella anisum

Fruits and Dill Anethumleaves graveolens

Fruits Sweet Fennel Foeniculum vulgarevar. dulce

Asteraceae Flowering tops German Chamomilla(also known Chamomile recutitaas Compositae)

Flowering tops Roman Chamomile Anthemis nobilis

Burseraceae Resin Frankincense Boswellia carterii

Resin Myrrh Commiphoramyrrha

Resin Elemi Canariumluzonicum

Cupressaceae Needles Cypress Cupressussempervirens

Berries and Juniper Juniperusleaves communis

Geraniaceae Leaves Geranium Pelargoniumgraveolens

Lamiaceae Leaves and Basil Ocimum basilicumflowering tops

Leaves and Clary Sage Salvia sclareaflowering tops

Leaves and Lavender Lavandulaflowering tops angustifolia

Leaves Oregano Origanum vulgare

Leaves Peppermint Mentha x piperita

Leaves Rosemary Rosmarinusofficinalis



TTaabbllee 22..11 continuedLamiaceae Leaves Patchouli Pogostemon

cablin

Lauraceae Heartwood Rosewood Aniba rosaeodora

Wood, bark Camphor Cinnamomumand leaves camphora

Bark and Cinnamon Cinnamomumleaves zeylanicum

Leaves Bay Laurus nobilis

Myristicaceae Nuts Nutmeg Myristica fragrans

Myrtaceae Leaves Eucalyptus Eucalyptus sp.

Leaves Myrtle Myrtus communis

Leaves Tea-tree Melaleuca sp.

Leaves and Clove Syzygiumdried buds aromaticum

Oleaceae Flowers Jasmine Jasminum sp.

Piperaceae Dried fruits Black Pepper Piper nigrum

Pinaceae Needles Pine Pinus sp.

Wood Cedarwood Cedrus sp.,Juniperus sp.

Poaceae Grass leaves Lemongrass, Cymbopogon sp.Citronella,Palmarosa

Roots Vetiver Vetiverazizanoides

Rosaceae Flowers Rose Rosa damascena

Rutaceae Fruit peel Lemon, Citrus sp.Mandarin,Grapefruit,Lime,Orange, Bergamot

Flowers Neroli Citrus aurantiumvar. amara

Leaves and Petitgrain Citrus aurantium stems var. amara

Styraceae Resin Benzoin Styrax benzoin

Zingiberaceae Rhizome Ginger Zingiber officinale

Santalaceae Heart wood Sandalwood Santalum sp.



Essential oil storage structures

Plants that produce essential oils usually store their essential oils inspecial structures. Examples of these include secretory hairs, secretorycells within the epidermis (outer layer of cells), special sacs made fromseveral secretory cells surrounding an oil-filled space and secretoryducts (tubes lined with secretory cells). The secretory hairs pointoutwards from the surface of the leaves and stems, quickly releasingtheir essential oil when the surface is brushed. Oils produced insecretory hairs are usually designed to repel would-be predators.

Secretory sacs and ducts are more often located inside the leaves orin the heartwood or roots. The essential oils in these structurespossibly protect the plant against bacteria, fungi and pests liketermites. Svoboda et al. (2000) have collected some excellent micro-scopic photographs of secretory essential oil structures (see Furtherreading).

The secretory cells manufacture essential oils according to theplants needs. Predation is likely to increase production of repellentessential oils, which may partly account for the extra deliciousness oforganically grown herbs. If essential oil is produced as an attractantfor pollinators, it may only be produced at certain times of day ornight when the pollinators are most likely to be about.

Plant physiology

Let us now return to the molecular level and see how plant moleculesare made. Plant tissues, like those in all living things, are made up ofcells. Cells are made up of thousands of different types of molecules,each with a specific function in the cells life. Millions of chemicalreactions take place every minute inside each cell and most moleculesin living cells exist for only a very short period before they arechanged by some reaction or rearrangement of structure. You couldsay that life is the sum of all the chemical reactions happening in thecells of an organism at any one moment.

Plant cells have many substructures or organelles, for example anucleus, mitochondria and chloroplasts. Each organelle has a specificfunction. The nucleus contains the genetic instructions for makingproteins. Mitochondria are responsible for energy production.Chloroplasts containing the green pigment chlorophyll harnessenergy from sunlight for photosynthesis.



Photosynthesis

Photosynthesis is an important chemical reaction that takes place inplants and other organisms that dont survive by eating othercreatures. It is the means by which plants obtain the carbon atomsthat they need to create their molecules. Carbon dioxide from the airprovides the carbon atoms, which join together with hydrogen andoxygen atoms (from water molecules) to make molecules known assugars. Sunlight provides the energy needed for the reaction, whichtakes place in the chloroplasts found in leaves and green stems. Plantsalso need nitrogen atoms to make proteins and nucleic acids.Nitrogen from the air is mainly trapped or fixed by bacteria in thesoil. It can then be absorbed by plant roots in the form of water-soluble nitrogen compounds.

The chemical reaction for photosynthesis can be written asfollows, showing glucose as the final product. Light and chlorophyllare required before the reaction can proceed:

Light and chlorophyll6CO2 + 6H2O C6H12O6 + 6O2

Glucose might be the end product of photosynthesis, but thereaction initially forms more simple sugars. These simple sugars arethen changed into many different types of molecules, includingglucose. Glucose is a 6-carbon sugar used by every plant cell as anenergy source. Each time a glucose molecule is broken down itreleases the chemical energy it derived from sunlight during photo-synthesis.

Secondary products

Plants can manufacture or absorb all the molecules they need forcellular functioning but they still have to solve another problem: howto defend themselves against predators. Some make defensive struc-tures, such as thorns or spikes, but a large number prefer to usechemical weapons. The defensive chemicals are known assecondary products, as they are not necessary for primary cellularfunctions, and essential oils form part of this chemical arsenal.Examples of secondary plant chemicals found in essential oils are:



terpenoid molecules most essential oil molecules are ter-penoids and are used by plants to repel predators and preventbacterial and fungal infections. Terpenoid molecules in flowersare also used to attract insect pollinators.

phenolic and phenyl propanoid molecules a few phenoliccompounds are found in essential oils. They are most likely usedby the plant to repel predators.

non-terpenoid aliphatic molecules these are also likely to repelpredators, as they are found in citrus peel oils.

heterocyclic molecules these molecules contain atoms otherthan carbon in closed rings. They are found in only a few oils,and include molecules such as the nitrogen-containing indoleand methyl anthranilate, and the oxygen-containing lactones,coumarins and furanoid compounds.

Other types of secondary compounds found in plants includealkaloids, flavonoids and saponins. Several of these have notablepharmacological effects on humans, for example, the well-knownalkaloid caffeine found in tea and coffee. However, such compoundsare not extracted into essential oils by steam distillation.

How plants make essential oil molecules

Terpenoid molecules

Terpenoid molecules are made via the mevalonic acid biosyntheticpathway. This is the same pathway for the formation of carotenoidand sterol molecules (30 or more carbon atoms) and is also thepathway in animals for the formation of cholesterol and steroidhormones. The first step is to make mevalonic acid molecules (withsix carbon atoms), hence the name of the pathway. Mevalonic acidmolecules are then rearranged by a series of enzymatic transfor-mations to form molecules known as isopentenyl pyrophosphate.Isopentenyl pyrophosphate consists of a branched 5-carbon molecule(known as an isoprene unit) joined to two phosphate groups. Theisoprene unit is the starting point for manufacture of terpenoidcompounds, the main type of molecule found in essential oils. Figure2.3 shows the structure of an isoprene molecule and an isoprene unit.The illustration of the unit demonstrates the branching structure ofthe 5-carbon atom chain, but does not specify double bonds. The



double bonds rearrange during the formation of terpenoid molecules,but isoprene molecules usually contain at least one C=C bond.

FFiigguurree 22..33 SSttrruuccttuurree ooff aann iissoopprreennee mmoolleeccuullee aanndd aann iissoopprreennee uunniitt

Isoprene molecules are easily linked together to form long,branched chains of carbon atoms. When rubber was first analysed inthe late 1800s, it was found to contain hundreds of isoprene units alllinked together. Rubber molecules are called polymer molecules(from the Greek poly, many and mer, unit).

Terpenoid molecules in essential oils have either two or threeisoprene units. The name terpene derives from turpentine, a liquidsolvent derived from the resin of Pinaceae species. When the structureof terpenoid molecules was first discovered, researchers thought thesimplest molecule was one containing ten carbon atoms and startedthe naming system at this point, calling these molecules monoter-penes (from the Latin mono, one). The later discovery of isopreneunits (an isoprene molecule has five carbon atoms) has not changeduse of the earlier naming system, so monoterpene still means twoisoprene units joined together. Terpenoid molecules with threeisoprene units are known as sesquiterpenes (from the Latin sesqui,one and a half). There are sometimes trace amounts of diterpenes inessential oils with four isoprene units, but the triterpenes with sixisoprene units are waxes and are not found in steam-distilledessential oils because they are not volatile enough.

Monoterpenes

Monoterpenes contain two isoprene units, that is 10 carbon atoms.Figure 2.4 shows an example of myrcene, a common monoterpene inessential oils. Myrcene molecules follow the isoprene rule which saysthat isoprene units are usually joined head to tail to make up the


C CH

CH2

head tailisoprene molecule

CH2 CH3

isoprene unit


carbon skeletons of terpenoid molecules (Morrison & Boyd, 1987).1

The number of C=C double bonds in terpenes varies from the exactnumber in each isoprene molecule as rearrangements occur duringformation of the bonds between the isoprene units.

FFiigguurree 22..44 TThhee mmoonnootteerrppeennee aallpphhaa--mmyyrrcceennee.. TThhee ddootttteedd lliinnee rreepprreesseennttsstthhee bboonndd ffoorrmmeedd bbeettwweeeenn ttwwoo iissoopprreennee uunniittss..

Sesquiterpenes

Sesquiterpenes contain three isoprene units, that is, 15 carbon atoms.Figure 2.5 shows the sesquiterpene (E)-beta-farnesene, an open chainsesquiterpene. However, one of the features of sesquiterpenes is thatthey readily undergo reactions that create molecular structures withclosed rings. The molecular shapes formed by the closed rings can beclassified into sub-groups of sesquiterpenes, for example eudesmanesand azulenes, but it is beyond the scope of this book to investigatethese sub-groups beyond the detail found in Chapter 3.

FFiigguurree 22..55 TThhee sseessqquuiitteerrppeennee ((EE))--bbeettaa--ffaarrnneesseennee.. TThhee ddootttteedd lliinneessrreepprreesseenntt tthhee bboonnddss ffoorrmmeedd bbeettwweeeenn tthhrreeee iissoopprreennee uunniittss..


head

head

tail

tail


Both monoterpenes and sesquiterpenes can be modified by theaddition of functional groups (see Table 1.4 on page 18 for a summary,and Chapter 4 for full descriptions). When a terpene has a functionalgroup added to it, it is known as a terpenoid, either monoterpenoid orsesquiterpenoid. The oid ending means like, or derived from, henceterpenoid can refer to all molecules with a terpene-like structure.

Some authors refer to all terpenoid molecules by the general termterpenes. In this book, terpenes are mono- and sesquiterpenehydrocarbons, molecules with only hydrogen and carbon atoms.Some citrus essential oils which have had their monoterpenesextracted by fractional distillation are sold as terpeneless oils. Ter-peneless oils are made up mainly of oxygenated terpenoid, aliphaticand coumarin molecules, and generally have a much stronger odourthan the original essential oil. Not much research has been carriedout yet on the physiological effects of terpeneless oils.

Phenols and phenyl propanoids

Phenols and phenyl propanoids are made via the shikimic acidpathway in plants which also make compounds like the tannins foundin tea. As we have seen in Chapter 1, they have a distinctive benzene oraromatic ring. Phenols have a hydroxyl (OH) group attached to thering, and usually an isopropyl tail (3-carbon chain bonded to the ringat the middle carbon atom). Phenyl propanoids usually have a methylether functional group attached to the ring (see Phenyl methyl ethersin Chapter 4), and a propenyl tail (3-carbon chain with one C=Cbonded to the ring by one end). Figure 2.6 illustrates these differences.


OH

CH3

O

thymol anethole

FFiigguurree 22..66 SSttrruuccttuurree ooff pphheennoollss aanndd pphheennyyll pprrooppaannooiiddss.. TThhyymmooll iiss aapphheennooll,, aanneetthhoollee iiss aa pphheennyyll pprrooppaannooiidd..


Non-terpenoid aliphatic molecules

The word aliphatic describes molecules made up of carbon chainsthat are in a straight line and do not have a closed or aromatic ring.Examples of aliphatic molecules are the acrid-smelling C8 (8-carbon),C9 and C10 aldehydes found in small amounts in citrus oils, and thegreen-leafy smelling C6 compounds found in some floral oils likeJasmine and Rose. Figure 2.7 shows a molecule of the aldehydeoctanal (C8H16O), found in Sweet orange oil. Aliphatic molecules areusually found only in trace amounts in essential oils, but if they haveoxygenated functional groups attached their odours are usuallynoticeable despite this.

Heterocyclic compounds

Heterocyclic compounds are made up of carbon atoms arranged in aring, with either a nitrogen or oxygen atom included as part of thering. These molecules are uncommon in essential oils, occurringmainly in heady floral oils like Jasmine, Neroli and Narcissus. Figure2.8 shows the structure of indole, found in Jasmine oil. Alkaloids areheterocyclic compounds that feature a nitrogen atom as part of theclosed ring. However, alkaloid molecules are seldom found in steam-distilled essential oils as they are soluble in water.


C

H

O

FFiigguurree 22..77 SSttrruuccttuurree ooff tthhee aalliipphhaattiicc aallddeehhyyddee ooccttaannaall ((CC88HH1166OO))

FFiigguurree 22..88 SSttrruuccttuurree ooff tthhee hheetteerrooccyycclliicc mmoolleeccuullee iinnddoollee,, sshhoowwiinngg tthheeppoossiittiioonn ooff tthhee nniittrrooggeenn aattoomm

N

H


Variation in essential oil composition

While there are obvious variations in essential oil compositionbetween oils from different species of the same genus (e.g. Pepper-mint and Spearmint), there are many factors that influence the com-position of oils from different specimens of the same species. Themost influential is that of geo-climatic location, as it gives rise todifferent chemotypes of essential oils. Other factors that affect theratios of essential oil molecules made by the plant include soil type,life-stage of the plant (pre- or post-flowering) and even the time ofday when harvesting is done.

Chemotype and effects of geo-climatic location

The term chemotype is usually used to describe essential oils thatvary in composition but are from the same species of plant. Differentgeographical locations are often associated with different chemo-types. For example, Rosemary oil from Spain is known as CT1 (CT,chemotype), and has higher levels of camphor, whereas Rosemaryoil from Tunisia is known as CT2 and contains higher levels of1,8-cineole. A third rosemary chemotype, CT3 from France, hashigher levels of verbenone, which is thought to be a less toxic ketonethan camphor. Not surprisingly, due to the similarity of its climatewith Tunisia, Moroccan Rosemary oil is usually the CT2 chemotype.Table 2.2 shows the varying ratios of major constituents in rosemaryCT1 and rosemary CT2. It is thought that CT1s higher percentageof camphor and alpha-pinene makes it more effective for muscularaches, whereas CT2s higher percentage of 1,8-cineole is better forrespiratory ailments.

Thyme (Thymus vulgaris) plants also produce at least five differentchemotypes, although in this case chemotypical variation is not asrelated to geographical source.

The importance of knowing which chemotype you are using iseven greater with thyme oils. Most aromatherapy books caution theuse of Thyme oil on the skin, without specifying which chemotype.While this is a valid caution for thyme chemotypes with high per-centages of phenols (thymol and carvacrol), the thyme chemotypescontaining linalool and geraniol can be safely used on the skin at theusual aromatherapy concentrations.

Thyme oil harvested from wild populations (Thymus vulgaris Pop-ulation) is often a mixture of different chemotypes, sometimes even



of different species. The odour of each batch should be carefullyassessed to gauge the proportions of phenols it contains. Table 2.3illustrates the differences in odour and therapeutic propertiesbetween the different chemotypes.

Another example of chemical variation is between Geranium(Pelargonium graveolens LHerit. ex Ait) essential oil produced in theReunion Islands (known as Bourbon geranium) and Chinesegeranium oil (see Table 2.4). Other countries also produce geraniumoils with varying levels of constituents, but so far there have been noofficial chemotype numbers assigned as for the rosemary oils. This isperhaps because the oils from different areas consistently produceoils of the same chemotypic profile.

Other plant species that show chemotypical variation include:


TTaabbllee 22..22 VVaarriiaattiioonn iinn tthhee ccoommppoossiittiioonn ooff RRoosseemmaarryy eesssseennttiiaall ooiill,,cchheemmoottyyppeess 11 aanndd 22

Rosemary CT1 (Spain)1 Rosemary CT2 (Tunisia)2

% %

Alpha-pinene 22 10

Beta-caryophyllene 2.5 1

Beta-pinene 5 4

Borneol 2 8

Bornyl acetate 1.5 0.8

Camphene 9 3

Camphor 17 11

1,8-cineole 17 51

Limonene 4 2

Myrcene 4 1

Terpinen-4-ol 1.5 1

Verbenone 4 0.05

1 M.H. Boelens (1985), The Essential Oils from Rosmarinus officinalis L.,Perfumer & Flavorist Oct./Nov. 10, pp. 2137.

2 G. Fournier, J. Habib, A. Reguigui, F. Safta, S. Guetari and R. Chemli (1989),Etude de divers echantillons dhuile essentielle de Romarin de Tunisie, PlantesMedicinales et Phytotherapie 23, pp. 1805.


basil, Salvia sp., Cinnamomum camphora and many Eucalyptus andMelaleuca species (see Brophy & Doran, 1996 and Webb, 2000 inFurther reading for more details on Australian essential oil chemo-types).

Simon et al. (1984) studied variations in essential oil compositiondue to soil type and rain-fall. They noted that clary sage (Salviasclarea) plants require a dry chalky soil for maximum essential oilproduction, yielding very little oil if the soil is too rich. Lemon balm(Melissa officinalis) plants require deep soil and just the right amountof water. Too much or too little water resulted in reduced oil quantityand quality.2

Harvesting time

As all farmers know, there is an optimal time for harvesting crops.The same goes for essential oil producing plants. In a study doneby Basker and Putievsky (1978) plants from the Lamiacae family


TTaabbllee 22..33 CCoommppaarriissoonn ooff ffoouurr TThhyymmuuss vvuullggaarriiss cchheemmoottyyppeess*

Chemotype Major Description Propertiesconstituents

CT thymol Thymol Pungent, camphoraceous, Strong anti-para-cymene herbaceous, clear to bacterial,

yellowish in colour. possible irritant.

CT carvacrol Carvacrol Pungent, camphoraceous, Strong anti-Thymol red in colour (due to the bacterial,borneol carvacrol). possible irritant.

CT linalool Linalool Similar odour to Lavender Mild anti-terpinen-4-ol oil (Lavandula angustifolia), bacterial, linalyl acetate clear to yellowish colour. possible sedative

due to linaloolcontent, non-irritant.

CT geraniol Geraniol Similar odour to Geranium Mild anti-geranyl acetate oil (Pelargonium graveolens) bacterial, non-

but more herbaceous, clear irritant.to yellowish colour.

* The list of constituents is taken from D. Pnol and P. Franchomme (1990),Laromathrapie exactement, Roger Jollois, Limoges, pp. 4024.


were tested to discover the optimum time for harvesting.3 Theirconclusions were that:

the volatile oil content of the leaves increases with time, and alsowith the size of the leaf;

maximum leaf yield was late summer for most Lamiaceae herbspecies, but the season for maximum oil yield and compositionvaried from species to species.

For example Sage oil (Salvia officinalis) contains different amounts ofthe neurotoxic ketone, alpha-thujone, depending on when it isharvested. Alpha-thujone taken internally can have neurotoxic effectson the central nervous system (GABA-antagonism), causing halluci-nations in low doses and convulsions in larger doses (Tisserand &Balacs, 1995).4 Sage leaves contain more alpha-thujone after theplant has flowered, so sage is usually harvested before flowering.

Flower oils like Jasmine are also affected by choice of harvestingtime. Ahmad et al. (1998) examined the ratio and type of con-stituents produced by jasmine over a 24-hour cycle. Oil from flowersharvested in the morning contained the preferred combination of


TTaabbllee 22..44 CCoommppaarriissoonn ooff GGeerraanniiuumm ooiillss ffrroomm CChhiinnaa aanndd RReeuunniioonn*

Geranium, China Geranium, Reunion% (Bourbon) %

Cis-rose oxide 1.9 0.6

Citronellol 40 22

Citronellyl formate 11 8

Geraniol 6 17

Geranyl formate 2 7.5

Iso-menthone 6 7

Linalool 4 13

Menthone 1.4 1.5

Trans-rose oxide 0.6 0.2

* G. Vernin, J. Metzger, D. Fraisse and C. Sharf (1983), tude des huiles essen-tielles CG-SM Banque SPECMA: Essences de Geranium (Bourbon), Parfum.Cosmet. Arom., 52, pp. 5161.


linalool, benzyl alcohol, cis-jasmone and indole, whereas thoseharvested in the evening contained higher levels of eugenol, benzylbenzoate and methyl salicylate. Eugenol and methyl salicylateintroduce unpleasant odour notes which would not be acceptable ineither the perfumery or aromatherapy industries.5

A brief overview of the essential oil industry

Over 3000 plant species produce essential oils, but only about 300of these are available commercially. According to RIRDC (RuralIndustries Research and Development Council, Australia), worldtrade in essential oils is growing hugely. In 1986, world imports andexports of essential oils and related perfumes and flavours werevalued at a total of US$4157 million. In 1998 it was US$14 246million. Australia currently accounts for only 1 to 2 per cent of theworld trade in essential oils. The combined value of Australianimports and exports of essential oils has increased fromUS$34.3 million in 1986 to US$88.1 million in 1998.6 According toB.J. Lawrence, the industry values of the top three essential oils in1993 were: Sweet orange (US$58.5 million), Cornmint (Menthaarvensis) (US$34.4 million) and Eucalyptus (cineole type)(US$29.8 million).7

While the flavour and fragrance industry largely dominates thedemand for essential oils (and hence controls their availability), noveloils produced on a small scale can still find their niche in the aroma-therapy market. As we saw in Table 2.3, a knowledge of essential oilchemistry can equip aromatherapists to experiment safely with newoils and maybe even to estimate their possible therapeutic actions.

Further reading

For more details on plant classification systems see: A. Cronquist (1988),The Evolution and Classification of Flowering Plants, New YorkBotanical Garden, Bronx, New York; and A. Takhtajan (1997),Diversity and Classification of Flowering Plants, Columbia UniversityPress, New York.

Photosynthesisan in-depth introduction to university-level photosynthesis.http://photoscience.la.asu.edu/photosyn/education/photointro.html.

Plant secondary metabolitesFunctions of Plant Secondary Metabolites



and Their Exploitation in Biotechnology (1999), ed. Michael Wink, vols2 and 3, Culinary and Hospitality Industry Publications Services,Weimar, Texas.

Details on therapeutic properties of alkaloids and other plant productsused in herbal medicine are well explained in S. Mills and K. Bone(2000), Principles and Practice of Phytotherapy, Churchill Livingstone,Edinburgh.

A succinct university-level description of the formation of terpenes fromisoprene units showing all the molecular details can be found at:http: / /www.usm.maine.edu/~newton/Chy251_253/Lectures /BiopolymersI/TerpenesFS.html [accessed 25 May 2003].

For useful information on variations in plant growth that happen underdifferent cultivation methods, see Plants in Action: Adaptation inNature, Performance in Cultivation (1999), ed. Brian Atwell,Macmillan, Australia.

For some lovely microscopic images of essential oil secretory structuresin plants see K.P. Svoboda, T.G. Svoboda and A.D. Syred (2000),Secretory Structures of Aromatic and Medicinal Plants. A review andatlas of micrographs, Microscopix Publications Knighton, Powys, UK.

For details of chemical composition and chemotypic variation in Aus-tralian plants see J.J. Brophy and J.C. Doran (1996), Essential Oils ofTropical Asteromyrtus, Callistemon and Melaleuca species, ACIARMonograph No. 40, Australian Centre for International AgriculturalResearch, Canberra; and M.A. Webb (2000), Bush Sense: AustralianEssential Oils and Aromatic Compounds, self-published, Australia,ISBN 0646 40353 2.

For further details of global essential oil production see B.M. Lawrence(1993), A planning scheme to evaluate new aromatic plants for theflavor and fragrance industries, in J. Janick and J.E. Simon (eds), NewCrops, Wiley, New York, pp. 6207, http://www.hort.purdue.edu/newcrop/proceedings1993/v2-620.html [accessed 25 May 2003].

For information on essential oil production in Australia see ESSENTIALOILS & PLANT EXTRACTS Research Program, Rural IndustriesResearch & Development Corporation (RIRDC), Canberra, Australia,http://www.rirdc.gov.au/programs/eop.html [accessed 25 May 2003].The New Crops newsletter can be found at http://www.newcrops.uq.edu.au/newslett/ncnl1218i.htm.



3TERPENES

This chapter looks at terpenoids, which form the largest class ofmolecules found in essential oils. Terpenoid chemistry is a branch oforganic chemistry. As we have seen, monoterpene and sesquiterpenemolecules are made up of hydrogen and carbon atoms, with carbonatoms forming the skeleton or structural shape of the molecule. Whenan oxygen atom or oxygen-containing functional group is added toeither of these types of terpene molecule, we call it a terpenoid molecule.

Our goal is to come to an understanding of what sort of inter-actions terpenoid molecules might have with molecules in the humanbody. The concepts of polarity, solubility and emulsification are usefulwhen considering how terpenoids might react once inside the body.

Physical characteristics of terpenoid molecules

To start with, lets look at what sort of substances terpenoidmolecules are. On a physical level, they have the following character-istics (which are shared by non-terpenoid compounds in essential oilslike phenylpropanoids):

VolatilityThey evaporate easily, mostly below 100C. Thismeans that when they are applied to the skin, they will to someextent evaporate with the warmth of the skin unless the site ofapplication is covered. It also means that inhalation is aneffective application method.

FlammabilityTerpenoids in bulk are labelled flammableliquids, though they are not nearly as flammable as otherorganic fuels such as octane.

DensityMono- and sesquiterpenoids are nearly all less densethan water, meaning they will float on the surface of water.


OdourMost terpenoid molecules in essential oils can beperceived by the olfactory system. Some are pleasant, othersunpleasant. There is a great range in thresholds of odour per-ception for different molecules. Some substances, like rose oxide(found in Rosa damascena), may be detected at a concentrationof only 0.5 parts per billion (ppb). Others, like nerol, also foundin Rosa damascena, can be detected at 300 ppb (Dodd, 1988).1

It is of interest to aromatherapists how terpenoid molecules behavewhen they come in contact with the substances found in humanbodies, such as skin oils, cell membranes and the variety of sub-stances in blood and other body fluids. The types of environmentfound in the body can be identified as either hydrophilic (water-loving) or lipophilic (fat-loving). The blood, inter- and intra-cellularfluids and urine are all mainly composed of water and thereforehydrophilic. Skin oils and cell membranes are made of lipids and arelipophilic. To understand how essential oils might interact withhydrophilic or lipophilic molecules, lets examine the concepts ofpolarity, solubility and emulsification.

Polarity

Polar moleculesIf we go back to Chapter 1, where we were looking at molecularstructure, we notice that molecules are usually neutrally charged.This is because the numbers of protons and electrons are equal ineach component atom, and there is no imbalance of charge.However, molecules of some substances seem to behave as thoughthey are actually charged, and undergo attraction and repulsion witheach other.

Water is one of these substances. Instruments which can measurethe overall charge over a molecule of water detect that near theoxygen atom there is an area of slight negative charge, and nearthe two hydrogen atoms there are areas of slight positive charge.Figure 3.1 shows a water molecule with its different areas of charge.

The imbalance in charge is ascribed to the presence of the oxygenatom. Due to the size of oxygen atoms and the way their electronsare arranged, oxygen atoms are strongly electronegative. Elec-tronegativity is the relative ability of an atom to attract electronstowards its nucleus. So when oxygen atoms form bonds with otheratoms with low electronegativity, like hydrogen, the electrons in the



bonds are drawn more closely towards the oxygen atom, rather thanbeing equally shared between the two atoms. Other types of highlyelectronegative atoms are fluorine, nitrogen and chlorine. Bothcarbon and hydrogen have low electronegativity.

Most molecules containing electronegative atoms are polarmolecules. The electronegative atom creates an area of relativenegative charge, or negative pole, and the other atoms attached,being relatively depleted of electrons, function a

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40202092 the Chemistry of Aroma Therapeutic Oils