Woodward – Fieser rule for calculating absorption maximum in α,β – unsaturated carbonyl compounds: Woodward and Fieser framed certain empirical rules for estimating the absorption maximum for α,β – unsaturated carbonyl compounds. The rules were later modified by Seott and are as follows a) The basic value of α,β – unsaturated ketone is taken as 215 nm. The α,β – unsaturated ketone may be acyclic or six membered. For a compound – CH – COX, the basic value is 215 nm, if X is an alkyl group. If X – H , basic value becomes 207 nm. The basic value is 193 nm if X is OH or OR. b) If the double bond and the carbonyl group are contained in a five membered ring ( cyclopentaneone ), then for such α,β – unsaturated ketone , the basic value becomes 202 nm. The ε max for such compounds are generally above 10,000.
Example 1: Calculate λ max for the given structure Solution) The basic value for acyclic α,β – unsaturated ketone is 215 nm. In this structure, we see two β- alkyl substituents. The value of absorption maximum is thus calculated as Basic value = 215 nm 2β – alkyl substituents ( 2× 12) = 24 nm Calculated value = 239 nm The observed value is found to be 237 nm.

Example 2: Calculate λ max for the given structure Solution) Basic value = 215 nm 2 β – ring residues ( 2×12 ) = 24 nm 1 exocyclic double bond = 5 nm Calculated value = 244 nm The observed value is found to be 241 nm.
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