Woodward's rules, named after Robert Burns Woodward and also known as Woodward-Fieser rules (for Louis Fieser) are several sets of empirically derived rules which attempt to predict the wavelength of the absorption maximum ( λmax ) in an ultraviolet-visible spectrum of a given compound. Inputs used in the calculation are the type of chromophores present, the substituents on the chromophores, and shifts due to the solvent.[1][2] Examples are conjugated carbonyl compounds [3][4] conjugated dienes,[5] and polyenes.[4]
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.
| 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) [6] |
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With the aid of these rules the UV absorption maximum can be predicted, for example in these two compounds [7]:
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.