CH437 CLASS 20
ULTRAVIOLET-VISIBLE SPECTROSCOPY 3
Synopsis. Effect of conjugation on spectra continued: empirical rules for
conjugated polyenes, , -unsaturated carbonyl compounds and aromatic
carbonyl compounds.
The Woodward-Fieser Rules for Conjugated Dienes
Although the max values of conjugated systems often requires difficult
calculations using MO theory, they can be readily estimated using a set of
empirical rules, depending on whether the basic chromophore is a conjugated
diene or an ,-unsaturated carbonyl compound or a benzoyl derivative. These
rules are considered next.
For1,3-butadiene itself, the s-trans conformer is rather more stable than the s-cis
conformer, but for other substituted dienes the reverse is usually the case,
because of steric hindrance between substituents. In general, the extent of porbital interaction is rather greater in s-cis conformers than in corresponding strans conformers, leading to higher max values for the former, as indicated in
diagram below.
1,3-Butadiene
s-trans (major)
s-cis (minor)
 max = 253 nm
 max = 217 nm
2-Methyl-1,3-pentadiene
H
H
C
C
CH3
C
H
C
CH3
H
H
C
CH3
C
CH3
C
H
C
H
H
s-trans (minor)
s-cis (major)
 max = 224 nm
 max = 263 nm
WOODWARD-FIESER RULES:
Conjugated dienes and trienes (in ethanol)
max for   * transitions. max6000-35000 /10-1m2mol-1
Base values
acyclic and heteroannular dienes 215 nm
homoannular dienes
253 nm
acyclic trienes 245 nm
Addition for each substituent
-R alkyl (including part of a carbocyclic ring)
-OR alkoxy
-SR thioether
-Cl, -Br
-OCOR acyloxy
-CH=CH- additional conjugation
5 nm
6 nm
30 nm
5 nm
0 nm
30 nm
if one double bond is exocyclic to one ring
if exocyclic to two rings simultaneously
5 nm
10 nm
solvent shifts are minimal
The following serve to illustrate the application of the Woodward-Fieser rules for
conjugated dienes, etc.
(1)
CH3
CH3
C C
CH3
H
C
(i.e. the s-trans form)
C
H
H
Base value
215 nm
Alkyl groups: 3 x 5 = 15 nm
Total
230 nm (Observed: 228 nm)
CH3
(2)
Base value
215 nm
Ring residues: 3 x 5 = 15 nm
Exocyclic double bond:
Total
5nm
235 nm
(Observed: 235 nm)
CH(CH3)2
Base value
(3)
CH3
253 nm
Ring residues: 3 x 5 = 15 nm
Alkyl group:
5 nm
Exocyclic double bond: 5 nm
CH3
Total
CO2H
278 nm
(Observed: 275 nm)
WOODWARD-FIESER RULES:
,-unsaturated carbonyl compounds (in ethanol)
max for   * transitions. max 4500-20000 /10-1m2mol-1
Base values
 
ketones -C=C-CO- acyclic or 6-ring cyclic
5-ring cyclic
aldehydes -C=C-CHO
215 nm
202 nm
207 nm
acids and esters -C=C-CO2H(R)
197 nm
Additional conjugation
   
-C=C-C=C-CO- etc
add 30 nm
If an additional double bond is homoannular
with another
add a further 39 nm
(i.e. in addition to the 30 nm already added above)
Addition for each substituent (nm)
-R alkyl (including part
of a carbocyclic ring)
-OR alkoxy
-OH hydroxy
-SR thioether
-Cl chloro
-Br bromo
-OCOR acyloxy
-NH2, -NHR, -NR2




 (and
higher)
10
12
17
17
17
35
35
15
25
6
-
30
30
80
12
30
6
95
17
30
12
25
6
-
31
50
12
25
6
-
-
If one double bond is exocyclic to one ring
If exocyclic to two rings
5 nm
10 nm
Solvent shifts
Above values shifted to longer wavelength in water and to
shorter wavelength in ‘less polar’ solvents. For common
solvents, the following corrections (to value in ethanol)
should be applied in estimating max.
water
+8 nm
cyclohexane -11 nm
methanol
chloroform -1 nm
dioxan
-5 nm
diethyl ether -7 nm
hexane
-11 nm
The following serve to illustrate application of the Woodward-Fieser rules for ,unsaturated carbonyl compounds.
O
(1)

C
CH3

C
CH3
C
CH3
CH3
O
(2)
CH3

OCOCH3
Base value
(acyclic enone)
215 nm
 - CH3
 - CH3 x 2
10
24
Total
249 nm
(Observed: 249 nm)
Base value
(6-membered enone)
215 nm
Double bond
extending conjugation
30 nm
Total

CH3

O
18 nm
302 nm
(Observed: 300 nm
Base value
(6-membered enone)
(3)
CH3 CH3
39 nm
Homocyclic diene
 '-Ring residue
215 nm
Double bond
extending conjugation x 2
60 nm
Homocyclic diene
39 nm
5 nm
Exocyclic double bond
 -substituent
 ' -substituent
Total
12 nm
18 nm
349 nm
(Observed: 348 nm)
Empirical rules have also been devised (by A. E. Scott) for estimating max of the
* transition in the UV-visible of benzoyl derivatives (R-C6H4-COX).
SCOTT’S RULES:
benzoyl derivatives R-C6H4-COX (in ethanol)
max for major band
Orientation
 EtOH
calc (nm)
Parent Chromophore:
X = alkyl or ring residue
X=H
X = OH or O alkyl
246
250
230
Increment for each substituent:
R = alkyl or ring residue
R = OH, OMe, O alkyl/ring res.
R = O-
R = Cl
R = Br
R = NH2
R = NHCOMe
R = NHMe
R = NMe2
o-, mpo-, mpompo-, mpo-, mpo-, mpo-, mppo-, mp-
3
10
7
25
11
20
78
0
10
2
15
13
58
20
45
73
20
85
The following serve to illustrate the application of Scott’s rules to benzoyl
derivatives.
(1)
Br
Base value
(ketone)
O
246 nm
o-ring residue
3 nm
m-Br
2 nm
Total
251 nm
(observed: 253 nm)
CO2H
Base value
(carboxylic acid)
(2)
OH
HO
230 nm
m-OH x 2
14 nm
p-OH
25 nm
Total
269 nm
OH
(observed: 270 nm)
The Woodward-Fieser rules break down when applied to molecules that have
some kind of strain (steric or ring strain) around the chromophore. They also give
poor prediction of max for “cross-conjugated” molecules (molecules whose
conjugation is extended other than at either end of the chromophore). In most
cases, resonance (delocalization) is impeded due to distortion caused by strain,
and so the Woodward-Fieser rules predict max values that are too high.
Examples of molecules whose are not max are not satisfactorily predicted by the
empirical rules include:
O
O