Kinetics and mechanism of chromic acid oxidation of chalcones (phenyl styryl ketones)

Abstract

The kinetics of the oxidation by chromic acid of chalcone (phenyl styryl ketone) and several substituted chalcones have been investigated in 95%(v/v) acetic acid. The reaction is overall of the second order, first order in each reactant. The parameters of Arrhenius and transition state theories have been computed. The enthalpies of activation are in the range 12·3–19·4 kcal mol^–1 and the entropies of activation range from –5 to –22 cal mol^–1 K^–1. The effect of substituents on the rate of oxidation has been evaluated. A Hammett-type correlation with Brown's σ⁺ values has been established with ρ⁺–1·2. The data are consistent with a mechanism which involves electrophilic attack of chromic acid at the carbon–carbon double bond resulting in the formation of an epoxide.