Mechanism of acid bromate oxidation of trans mono-substituted cinnamic acids: structure reactivity correlation - a non-linear hammett's plot

Abstract

Oxidation of cinnamic and trans mono-substituted cinnamic acids with Br(V), unmixed by generated molecular bromine addition, has been studied from the kinetic and mechanistic aspects. Acid bromate, in the presence of mercury(II), oxidises cinnamic acids smoothly and quantitatively to the respective benzaldehydes. Reactions are catalysed by mineral acids and they exhibit first, fractional orders in [Br(V)] and [substrate] respectively. An inverse secondary deuterium kinetic isotope effect characterises the oxidation and reaction rates increase when deuterium replaces either α or β-proton of cinnamic acids. Oxidation rate is not much affected by electron attractors whereas it is susceptible to electron demand at the seat of the reaction. The free energy relationship is characterised by two distinct lines in the Hammett's plot with ϱ values of -3.7 and -0.8 for the two limbs. Two alternative mechanistic pathways have been proposed involving (i) a partially and a fully developed benzylic carbonium ion like activated complex for electron attractors and donors respectively in the slow and the rate determining step, or (ii) two discrete centres viz. Cα and Cβ passing via C+α and C+β carbonium ion intermediate for electron attractors and donors respectively. Mercury(II) role is only in scavenging bromide ions.