WOODWARD & FIESER RULES
The Woodward & Fieser rules allow to predict the absorption maximum’s wavelength of conjugated systems.
They apply in systems where π and σ bonds alternate.
To calculate the value of the absorption maximum’s wavelength, the bases are:
215 nm for heteroannular dienes:
252 nm for homoannular dienes (or cissoids):
A certain number of increments are added according to the substituents of the molecule:
| -R (alkyl radical) | + 5 nm | |
| -OR (alcoxy radical) | + 6 nm | |
| -X (-Cl, -Br, -halogen) | + 10 nm | |
| -OC(O)R (acyloxy, ether) | + 0 nm | |
| -SR (sulfur) | + 30 nm | |
| -NR2 (amine) | + 60 nm | |
| – C=C (double conjugated bond) | + 30 nm | |
| – C6H5 (phenyl radical) | + 60 nm | |
| -C=C (exocyclical double bond) | + 5 nm |