1

TerpenesTerpenes

Compounds with multiples of a 5carbon structural unit

(Isoprene)

Terpenes

•Terpenes are natural products thatform from a pyrophosphate and arestructurally related to isoprene.

HH22CC CC

CHCH33

CHCH CHCH22

IsopreneIsoprene(2-methyl-1,3-butadiene)(2-methyl-1,3-butadiene)

OP

OPO

OO

O

O

Isopentenyl pyrophosphate Isoprene

The Isoprene Unit

• An isoprene unit is the carbon skeletonof isoprene (ignoring the double bonds)

Two isoprene units:Two isoprene units:

Question

• How many isoprene units does geraniolcontain?

• A) 2

• B) 3

• C) 4

• D) 5

The Isoprene Unit

• The isoprene units are joined "head-to-tail."

headhead tailtail

tailtail headhead

Isoprene LinksHeads or Tails?

2

Common Carbon Backbone

Terpenes

•Class Number of carbon atoms

•Monoterpene 10

•Sesquiterpene 15

•Diterpene 20

•Sesterpene 25

•Triterpene 30

•Tetraterpene 40

Classification of Classification of TerpenesTerpenes

Question

• Farnesol is classified as a

• A) monoterpene.

• B) sesquiterpene.

• C) diterpene.

• D) triterpene.

TerpenesRepresentative Representative MonoterpenesMonoterpenes

αα--PhellandrenePhellandrene(eucalyptus)(eucalyptus)

MentholMenthol(peppermint)(peppermint)

CitralCitral(lemon grass)(lemon grass)

OO

HH

OHOH

TerpenesRepresentative Representative MonoterpenesMonoterpenes

αα--PhellandrenePhellandrene(eucalyptus)(eucalyptus)

MentholMenthol(peppermint)(peppermint)

CitralCitral(lemon grass)(lemon grass)

OO

HH

OHOH

3

TerpenesRepresentative Representative MonoterpenesMonoterpenes

αα--PhellandrenePhellandrene(eucalyptus)(eucalyptus)

MentholMenthol(peppermint)(peppermint)

CitralCitral(lemon grass)(lemon grass)

TerpenesRepresentative Representative SesquiterpenesSesquiterpenes

αα--SelineneSelinene(celery)(celery)

HH

TerpenesRepresentative Representative SesquiterpenesSesquiterpenes

αα--SelineneSelinene(celery)(celery)

HH

TerpenesRepresentative Representative SesquiterpenesSesquiterpenes

αα--SelineneSelinene(celery)(celery)

TerpenesRepresentative Representative DiterpenesDiterpenes

Vitamin AVitamin A

OHOH

TerpenesRepresentative Representative DiterpenesDiterpenes

Vitamin AVitamin A

OHOH

4

TerpenesRepresentative Representative DiterpenesDiterpenes

Vitamin AVitamin A

Common Terpenes

OH

Geraniol C10H18O

OH

Nerol C10H18O

OH

Terpineol C10H18O

OH

Linalool C10H18O

Limonene

CH2

CH2

Limonene

CH2

Ultraviolet-Visible (UV-VIS) Ultraviolet-Visible (UV-VIS)

SpectroscopySpectroscopy

Gives information about conjugated π electronsystems

5

gaps between electron energy gaps between electron energy levels are greater than thoselevels are greater than thosebetween between vibrational vibrational levelslevels

gap corresponds to wavelengthsgap corresponds to wavelengthsbetween 200 and 800 nmbetween 200 and 800 nm

Transitions between electron energy statesTransitions between electron energy states

ΔΔEE = = hhνν

X-axis is wavelength in nm (high energy at left, X-axis is wavelength in nm (high energy at left, low energy at right)low energy at right)

λλmaxmax is the wavelength of maximum absorption is the wavelength of maximum absorption and is related to electronic makeup of moleculeand is related to electronic makeup of molecule—— especially especially ππ electron system electron system

Y axis is a measure of absorption of electromagnetic Y axis is a measure of absorption of electromagnetic radiation expressed as radiation expressed as Absorbance or molar Absorbance or molar absorptivity absorptivity ((εε))

Conventions in UV-VISConventions in UV-VIS

UV and Visible light cause only two kinds of electronic transitions

• Only organic compounds with π electrons can absorb energy in the UV/Visible region• A visible spectrum is obtained if visible light is absorbed

• A UV spectrum is obtained if UV light is absorbed

Most stable Most stable ππ-electron -electron

configurationconfiguration

ππ-Electron -Electron configuration of configuration of

excited stateexcited state

ψψ11

ψψ22

ψψ33∗∗

ψψ44∗∗

ψψ11

ψψ22

ψψ33∗∗

ψψ44∗∗

ππ→→ππ* Transition in cis,trans-1,3-* Transition in cis,trans-1,3-cyclooctadienecyclooctadiene

HOMOHOMO

LUMOLUMO

ΔΔEE = = hhνν

ππ→→ππ* Transition in Alkenes* Transition in Alkenes

HOMO-LUMO energy gap is affected byHOMO-LUMO energy gap is affected bysubstituents substituents on double bondon double bond

as HOMO-LUMO energy difference decreasesas HOMO-LUMO energy difference decreases(smaller (smaller ΔΔEE), ), λλmaxmax shifts to longer wavelengthsshifts to longer wavelengths

Substituent Substituent EffectsEffects

Methyl groups on double bond cause Methyl groups on double bond cause λλmaxmax

to shift to longer wavelengthsto shift to longer wavelengths

CC CC

HH

HH

HH

HH

CC CC

HH

HH CHCH33

λλmaxmax 170 nm170 nm

CHCH33

λλmaxmax 188 nm188 nm

6

A chromophore is the part of a molecule which absorbsUV or visible light

Effect of Conjugation on λmax

Substituent Substituent EffectsEffects

Extending conjugation has a larger effectExtending conjugation has a larger effecton on λλmaxmax; shift is again to longer; shift is again to longerwavelengthswavelengths

CC CC

HH

HH

HH

HH

CC CC

HH

HH

λλmaxmax 170 nm170 nm λλmaxmax 217 nm217 nm

HH

CC CC

HH

HHHH

UV Spectrum: (+)-carvone

0

0.5

1

1.5

2

2.5

3

3.5

200 220 240 260 280

Wavelength (nm)

Series1

O

C

H

3

C

H

3

C

H

2

What is What is carvonecarvone’’s s λλmaxmax??

236 nm236 nm

The Beer–Lambert Law

The molar absorptivity of a compound is a constant thatis characteristic of the compound at a particular

wavelength

A = log(I / I0)c = concentration of substancein solutionl = length of the cell in cmε = molar absorptivity

A = clε

7

200200 220220 240240 260260 280280

10001000

20002000

Wavelength, nmWavelength, nm

λλmaxmax 230 nm230 nm

εεmaxmax 26302630

MolarMolar

absorptivity absorptivity ((εε))

UV Spectrum of cis,trans-1,3-UV Spectrum of cis,trans-1,3-cyclooctadienecyclooctadiene Substituent Substituent EffectsEffects

λλmaxmax 217 nm217 nm

(conjugated (conjugated dienediene))

HH

CC CC

HH

HH CC CC

HH

HHHH

CC CC

HH CHCH33

HH

HH

CC CC

HH33CC

HH CC CC

HH

HH

λλmaxmax 263 nm263 nmconjugated conjugated trienetriene plusplus

two methyl groupstwo methyl groups

Both the λmax and ε increase as the number of conjugated double bonds increases

An auxochrome is a substituent in a chromphore thatalters the λmax and the intensity of the absorption

Uses of UV/Vis Spectroscopy

•Identifying molecular structures

•Measuring rates of a reaction

• Determining the pKa of a weak acid

•(Quantitative analysis: Beer’s Law)

Spectroscopic Analysis:Infrared:

IR Peak(s) (cm-1) Chemical Function(s)

Ultraviolet:

Structural feature(s):

aromatic ring � ; oxygen atom � ; diene (conjugated) �

none of the above �

Unknown UV max: _______

Features of conjugated system (if present) Value (nm)

Base value 210 – 215 nm

Homoannular diene +39-69 nm

Extended conjugation +30 nm

Alkyl group(s) +5-12 nm

Exocyclic double bond +5 nm

TOTAL (Calculated)

8

Rate of Reaction Rate of Reaction

pKa of a Weak Acid