Volatile Oils (or Essential Oils)

Volatile oils are the odorous and volatile products of various plant and animal species. As

they have a tendency to undergo evaporation on being exposed to the air even at an ambient

temperature, they are invariably termed as volatile oils, essential oil or ethereal oils. There

were specialized secretary structures within the plants which are primarily responsible for the

generation of volatile constituents, as shown in the following.

These volatile oils are usually formed by two modes namely; first, by hydrolysis of some glycosides; and secondly, by the protoplasm directly. It has been observed that the volatile oils are present in different parts of a plant as given in the bellow table:

Plant Organs Containing Volatile Oils

Differences between volatile and fixed oils:

Volatile oils Fixed oils

Volatile non-volatile

Terpenoids Triglycerides

Non-greasy Greasy

Resinify (O2 Rancid (O2

In nature, essential oils play very important roles in plant defense and signaling processes

for example, essential oils are involved in plant defense against microorganisms, insects, and

herbivores, attraction of pollinating insects and fruit-dispersing animals, water regulation and

allelopathic interactions . Also, they are valuable natural products used as raw materials in

many fields, such as pharmaceutical, agronomic, food, sanitary, cosmetic, and perfume

industries

Physical properties of Essential Oils 1. Liquid at ambient temperature

2. Volatile, (c.f. fixed oils)

3. Colorless liquids with the exception of chamomile oil

4. Their density is lower than water, with the exception clove or cinnamon heavier than water)

5.They have a high refractive index and most of them rotate

6. They are soluble in common organic solvent and liposoluble (lipophilic)

Therapeutic action:1- Antispasmodic. Ex: Rosmarinus officinalis, Mentha piperita Matricaria chamomilla.

2- Food preservation and cosmetic preparations

3- Flavors for foods and confections and in the spice, perfume, and cosmetic trades.

BASIC CHEMISTRY OF ESSENTIAL OILS Essential oils consist of various chemical constituents that contribute to their aroma and

therapeutic properties. They are made by plants in order to protect them from predators, pests,

bacteria and fungi, and to attract pollinators. Generally, the chemical constituents contained in

essential oils are made up of blocks of hydrogen, carbon and oxygen molecules. Several

different classes of molecules can be found in essential oils; each varying in structure, solubility,

evaporation rate and therapeutic properties.

Essential oils that are extracted from the same species of plant but vary in chemical

composition are known as chemotypes. It is important to know the chemotypes of certain

essential oils used in aromatherapy. For example, there are two chemotypes of basil essential oil.

The main constituent of the first – known as exotic basil – is methyl chavicol, a compound that

is regarded as toxic and possibly carcinogenic, and should not be used in aromatherapy. Instead,

it is preferred to use the safer linalol chemotype, known as French or true sweet basil. Essential

oil suppliers should clearly state which chemotype they are selling, but always check with them

if in any doubt.

Biosynthetic Pathways

In nature, two main groups of metabolites can be found: primary and secondary metabolites.

Primary metabolites are universal compounds, present in all living organisms, and include

proteins, carbohydrates, lipids, and nucleic acids. Secondary metabolites are found only in some

species and are classified as terpenoids, shikimates, polyketides, and alkaloids, the first two

being the most relevant in essential oils. Although terpenoids are more frequent and abundant in

essential oils, certain species contain high quantities of shikimates, namely phenylpropanoids

and when these compounds are present, they provide specific odor and flavor to the plants.

Terpenes

Terpenes result from the condensation of a pentacarbonate unit with two unsaturated bonds,

isoprene (2-methyl-1,3-butadiene), and therefore are many times called isoprenoides. The

designation “terpenes” was first used by Kekulé in 1880 to name C10H16 compounds found in

turpentine. In 1887, his assistant Otto Wallace formulated the “isoprene rule” suggesting that

terpenes were formed by two or more isoprene units. Later, Robinson suggested that the isoprene

units were connected in a head-to-tail way. In summary, terpenoids are derived from aliphatic

precursors such as geraniol for the formation of monoterpenes, farnesol for sesquiterpenes, and

geranylgeraniol for diterpenes . Terpenes are classified into different structural and functional

classes. According to the number of isoprene units in their structure, terpenes can be classified

into hemiterpenes (1 unit), monoterpenes (2 units), sesquiterpenes (3 units), diterpenes (4 units),

and so on. The terpenes most often found in essential oils are monoterpenes (C10H16) and

sesquiterpenes (C15H24). These compounds have many isomeric cyclic or linear structures,

various degrees of unsaturations, substitutions, and oxygenated derivatives, being generally

called terpenoids.

The biosynthesis of terpenes involves two universal precursors: isopentenyl pyrophosphate (IPP)

and dimethylallyl pyrophosphate (DMAPP). In higher plants, IPP is biosynthesized through two

pathways: the mevalonate pathway (MVA) and the non-mevalonate (mevalonate independent) or

deoxyxylulose phosphate pathway

GPP: Geranyl pyrlphosphate

FP: Farnesyl pyrophosphate

GGPP: Geranyl Geranyl pyrophosphate

Terpenes:

Classification: General Formula – (C5H8)n

n=1 Hemiterpenes

n=2 Monoterpenes

n=3 Sesquiterpenes

n>4 Polyterpenes C40 carotenoids occur in chromoplasts of plants.

Monoterpenes Sesquiterpenes

Diterpene triterpene

Common terpenes include:

limonene – found in 90 percent of citrus essential oils pinene – found in pine, juniper, frankincense and nutmeg chamazulene – found in German chamomile.

Alcohols

Alcohols all have the ending -ol. They are often less volatile than terpenes and may be middle or base notes. Alcohols possess antibacterial and antifungal properties and are generally non-toxic. They also have uplifting and sedative properties and are considered to be one of the most useful

groups of chemical constituents in aromatherapy. Some alcohols may also have analgesic and antispasmodic effects.

Common alcohols include:

linalol – found in lavender and basil menthol – found in peppermint geraniol – found in palmarosa, citronella and geranium.

Phenols

Phenols also have names ending in -ol. The four most common phenols are listed below; nearly all other constituents ending in -ol are alcohols. Phenols do not evaporate very quickly and usually have a very strong aroma. They generally have bactericidal properties and produce a warming, stimulating effect. Phenols are also the most irritating group of constituents, therefore essential oils containing phenols should only be used in moderation on the skin.

Common phenols include:

eugenol – found in clove and cinnamon thymol – found in thyme carvacrol – found in marjoram and thyme chavicol – found in West Indian bay.

Aldehydes

Aldehydes will either end in -al, or feature the word aldehyde in their name. They are derived from primary alcohols but often smell stronger than their equivalent alcohol. Aldehydes generally have calming and sedative effects, as well as antibacterial properties. They can cause irritation to the skin and mucous membranes, therefore essential oils containing high percentages of aldehydes should only be used in moderation.

Common aldehydes include:

geranial – found in lemongrass, may chang and melissa citronellal – found in lemon-scented eucalyptus and citronella neral – found in melissa, lemongrass and may chang.

Ketones

Ketones are derived from alcohols and normally end in -one, with the exception of camphor. They have fairly high boiling points and often share a potent, minty-camphoraceous odour. Ketones typically ease nasal congestion and benefit the respiratory system. They may also possess wound-healing properties. Some of the most toxic essential oils contain ketones, such as mugwort, tansy and wormwood, which all contain thujone. However, not all ketones are toxic.

Other ketones include:

jasmone – found in jasmine fenchone – found in fennel camphor – found in rosemary.

Esters

Esters are a widespread group of constituents that often have a fruity aroma. They are made from a combination of alcohols and acids and are subsequently named after both groups of molecules. Esters typically have antispasmodic, anti-inflammatory, analgesic and sedative properties and are generally non-toxic.

Common esters include:

linalyl acetate – found in lavender, clary sage and bergamot isobutyl angelate – found in Roman chamomile benzyl benzoate – found in jasmine and ylang ylang.

OxidesWith a few exceptions, oxides usually keep the name of the molecule they were derived from, followed by the word oxide. One such exception is cineol, also known as eucalyptol, the most prevalent oxide found in essential oils. Oxides probably have the strongest odours of all the classes of molecules, and they give essential oils their characteristic aromas even at low percentages. With the exception of cineol, little is known about the therapeutic effects of oxides. Cineol is beneficial for the respiratory system due to its expectorant and anti-inflammatory properties. However, due to its strong odour, cineol may cause irritation of the mucous membranes in some individuals.

Common oxides include:

cineol – found in eucalyptus, rosemary and spike lavender rose oxide – found in rose and geranium menthofuran – found in peppermint.