Polyenes Woodward-Fieser rules

Empirical rules based on thousands of laboratory observations have been developed over the years relating the wavelengths of the UV absorption maxima to the structures of molecules. R.B. Woodward in 1941 and then L. Fieser and M. Fieser developed rules for predicting the absorption maxima of dienes, polyenes, and conjugated ketones by studying terpenes, steroids, and other natural products. The rules are known as Woodward s Rules or the Woodward-Fieser Rules. 

The Woodward-Fieser rules for conjugated ketones and aldehydes appear in Table A3-3. Note that bathochromic effects of alkyl groups depend on their location 10 nm for groups a to the carbonyl and 12 nm for groups in )3 positions. Contributions from additional conjugated double bonds (30 nm) and exocyclic positions of double bonds (5 nm) are similar to those in dienes and polyenes. 

The following examples show how the Woodward-Fieser rules predict values of Amax variety of conjugated ketones and aldehydes. Notice that the molar absorp-tivities (s) for these transitions are quite large (>5000), as we also observed for tt — tt transitions in conjugated dienes and polyenes. 

The wavelengths of maximum absorption of conjugated dienes and polyenes and conjugated enones and dienones are given by the Woodward and Fieser Rules, Tables 1 and 2. 

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