Effects of Bromine Substituents on Rates of Acetate-promoted Enolization of Ketones

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Abstract
Effects of bromine substituents on rates of acetate-catalyzed bromination of ketones are reported. Approximate polar contributions to the rates were obtained, based on the approximation that the steric effects of bromine and methyl are identical. Such polar substituent effects are quite large at the α′-position of an α-bromo ketone, leading to the conclusion that the carbonyl group is much better than the methylene group in transmitting polar effects.Rate enhancements caused by α-bromine substituents are largely entropic in origin and rates correlate well with ionization constants of α-bromo acids. These results are interpreted as indicative of an enolate-like transition state for acetate-catalyzed enolization.

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