HALOGENATION AND DEUTERIUM-EXCHANGE IN ETHYL CYANOACETATE - ENOLIZATION MECHANISM AND ENOL REACTIVITY

Reactions of ethyl cyanoacetate (EGA) in 30% dioxane-water with bromin e, iodine and chlorine (with [ECA](0) much greater than [Halogen](0)) are all first-order processes and are not acid catalysed over the rang e studied (0.02-1.10 mol dm(-3)). Values of the observed first-order r ate 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) valu e 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 measurement s of the H-D exchange reaction and is shown to be independent of the a cidity in the range 0.08-0.39 mol dm(-3) D+, The results are consisten t with the enolisation mechanism involving rate-limiting water-catalys ed proton removal from the methylene group, followed by rapid protonat ion of the enolate, The results are discussed in terms of the enolisat ion of other carboxylic esters and acids.