Fieser Kuhn Rule for Calculating 位max in Polyenes

Fieser Kuhn Rule for Calculating 位max in Polyenes

Fieser Kuhn Rule for Calculating 位max in Polyenes

Fieser-Kuhn Rules to Calculate Wavelength of Maximum Absorption (位max) of Polyenes

Woodward-Fieser rule work well for conjugated dienes and polyenes with upto four-double bonds or less but not hold good for more than four double bonds. If a conjugated polyene contains more than four double bonds, then Fieser-Kuhn rules are used.
According to this approach, both 位max and 蔚max are related to the number of conjugated double bonds as well as other structural units by the following equation-
max = 114 + 5M + n (48.0 x 1.7 n) − 16.5 Rendo − 10 Rexo
max = (1.74 x 104)n
Where, M is the number of alkyl substituents / ring residues in the conjugated system
n is the number of conjugated double bonds
Rendo is the number of rings with endocyclic double bonds in the conjugated system
Rexo is the number of rings with exocyclic double bonds in the conjugated system.
Calculation of max wavelength of beta Carotene
Base Value: 114 nm
M (number of alkyl substituents): 10
n (number of conjugated double bonds): 11
Rendo (number of endocyclic double bonds): 2
Rexo (number of exocyclic double bonds): 0
max = 114 + 5M + n (48.0 – 1.7 n) – 16.5 Rendo – 10 Rexo
= 114 + (5 x 10) + 11 [48.0 - (1.7 x 11)] – (16.5 x 2) – (10 x 0)
= 114 + 50 + (11 x 29.3) – 33 – 0
= 114 + 50 + 322.3 – 33
Calculated 位max value = 453.30 nm
max observed value = 452nm
max = (1.74 x 104)n
= (1.74 x 104) x 11 nm
Calculated 蔚max value = 19.14 x 104
Observed 蔚max value = 15.2 x 104

Similarly you can also calculate for-
Retinol
max observed: 325 nm
max = (1.74 x 104)n

Lycophene
max observed: 474 nm
max = (18.6 x 104)n

Woodward Fieser Rule for Calculating 位max in Dienes

Woodward Fieser Rule for Calculating 位max in 伪, 尾 Unsaturated Carbonyl Compounds