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

Abstract

Reactions of ethyl cyanoacetate (ECA) in 30% dioxane–water with bromine, iodine and chlorine (with [ECA]₀[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 [ECA] 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.