1 Terpenes Terpenes Compounds with multiples of a 5 carbon structural unit (Isoprene) Terpenes •Terpenes are natural products that form from a pyrophosphate and are structurally related to isoprene. H 2 C C CH CH 3 CH CH CH CH 2 Isoprene Isoprene (2-methyl-1,3-butadiene) (2-methyl-1,3-butadiene) O P O P O O O O O Isopentenyl pyrophosphate Isoprene The Isoprene Unit • An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds) Two isoprene units: Two isoprene units: Question • How many isoprene units does geraniol contain? • A) 2 • B) 3 • C) 4 • D) 5 The Isoprene Unit • The isoprene units are joined "head-to- tail." head head tail tail tail tail head head Isoprene Links Heads or Tails?
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TerpenesTerpenes
Compounds with multiples of a 5carbon structural unit
(Isoprene)
Terpenes
•Terpenes are natural products thatform from a pyrophosphate and arestructurally related to isoprene.
Gives information about conjugated π electronsystems
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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
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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
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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