Implementation
One set of Woodward–Fieser rules for dienes is outlined in table 1. A diene is either homoannular with both double bonds contained in one ring or heteroannular with two double bonds distributed between two rings.
Structural feature | λmax effect (in nanometers) |
---|---|
Base value for heteroannular diene | 214 |
Base value for homoannular diene | 253 |
Increments | |
Double bond extending conjugation | + 30 |
Alkyl substituent or ring residue | + 5 |
Exocyclic double bond | + 5 |
acetate group | + 0 |
Ether group | + 6 |
Thioether group | + 30 |
bromine, chlorine | + 5 |
secondary amine group | + 60 |
With the aid of these rules the UV absorption maximum can be predicted, for example in these two compounds:
In the compound on the left, the base value is 214 nm (a heteroannular diene). This diene group has 4 alkyl substituents (labeled 1,2,3,4) and the double bond in one ring is exocyclic to the other (adding 5 nm for an exocyclic double bond). In the compound on the right, the diene is homoannular with 4 alkyl substituents. Both double bonds in the central B ring are exocyclic with respect to rings A and C.
For polyenes having more than 4 conjugated double bonds one must use Fieser-Kuhn rules.
Read more about this topic: Woodward's Rules