9.3 Conjugated Alkenes
447
9.3 Conjugated Alkenes
9.3.1 Dienes and Polyenes
The π   π* transition of conjugated double bonds is above ≈200 nm with typical intensities of the order of log ε ≈ 4. Its position can be estimated with the Woodward–
Fieser rule. For cross-conjugated systems, the value for the chromophore absorbing 
at the longest wavelength has to be calculated.
Woodward–Fieser rule for estimating the position of the π π * transition
(λ max in nm)
Parent system
acyclic
217
heteroannular
214
homoannular
253
Increments
for each additional conjugated double bond
+30
for each exocyclic double bond
C C
+5
for each substituent
C-substituent
+5
Cl
+5
Br
+5
O–alkyl
+6
OCOCH 3
0
N(alkyl) 2
+60
S–alkyl
+30
Solvent correction
≈0