Halogenation and deuterium exchange in ethyl cyanoacetate. Enolisation mechanism and enol reactivity

Reactions of ethyl cyanoacetate (EGA) in 30% dioxane-water with bromine, iodine and chlorine (with [ECA](0) much greater than [Halogen](0)) are all first-order processes and are not acid catalysed over the range studied (0.02-1.10 mol dm(-3)). Values of the observed first-order rate constant divided by [EGA] are very similar for the three halogens, suggesting that all three react with the enol form of ECA at or near to the diffusion limit, This enables the determination of a pK(E) value of 9.6 and a pK(a)(E) value for the acid dissociation of the enol of 2.1, The rate constant for enolisation is obtained by NMR measurements of the H-D exchange reaction and is shown to be independent of the acidity in the range 0.08-0.39 mol dm(-3) D+, The results are consistent with the enolisation mechanism involving rate-limiting water-catalysed proton removal from the methylene group, followed by rapid protonation of the enolate, The results are discussed in terms of the enolisation of other carboxylic esters and acids.