Kinetics and Mechanism of bromination of 1-Hetera-4-Cyclohexanones by N-Bromobarbitone

Abstract

Kinetics of bromination of substituted 1-hetera-4-cyclohexanones

by N-bromobarbitone [NBB] has been studied in aqueous

acetic acid medium in the presence of HCIO₄ and mercuric acetate.

The reaction is acid and mercuric acetate catalysed exhibiting first

order dependence each in [acid], [Hg(OAc)₂] and [substrate] and

zero order in [NBB]. This supports the acid catalysed enolisation

of ketone as the rate determining step and reaction between enol

and NBB as the first step. The decrease in dielectric constant of the

medium enhances the rate of reaction. Arrhenius activation parameters

have been computed. A plausible mechanism based on these

observations is proposed. The effect of the various substituents on

the rates of bromination has been rationalized on the basis of their

inductive and steric effects,