Woodward's rules

Woodward's rules are a set of rules about how organic chemical compounds absorb ultraviolet light.

They give information about the wavelength of the absorption maximum (symbol λ_max ) in an ultraviolet-visible (UV) spectrum of a compound. The rules are named after Robert Burns Woodward. He was a Harvard University professor who won the 1965 Nobel Prize in chemistry. The rules are sometimes called the Woodward-Fieser rules, to also honor Louis Fieser.

The rules build the prediction on the type of chromophores present, the substituents on the chromophores, and changes due to the solvent. Examples are conjugated carbonyl compounds, conjugated dienes, and polyenes.

Implementation

One set of Woodward-Fieser rules for dienes is shown in table one. A diene is either homoannular with both double bonds contained in one ring or heteroannular with two double bonds distributed between two rings.

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
Table 1. Rules for wavelength of maximum diene absorption (in nanometers)

• These rules predict the UV absorption maximum of compounds. Here are two examples:

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.