Enol and enolate (carbanion) intermediates in nitrosation. Reactions of ethyl cyanoacetate, diethyl malonate and malononitrile
Rate constants have been obtained for the reactions of ethyl cyanoacetate (ECA) and diethyl malonate (DEM) with nitrous acid in acid solution, over a range of acid concentrations and also in the presence of chloride, bromide, thiocyanate ions or thiourea. The reaction products are the corresponding oximes, and all of the results are consistent with a reaction mechanism involving rate-limiting attack by the nitrosating species XNO at both the enol and enolate (or carbanion) forms of the substrate. At low acidities reactions via the enolate are dominant. Many of these reactions, particularly those of DEM, occur at or near the diffusion-controlled limit. The findings are compared with those obtained earlier for the nitrosation reactions of malononitrile (MN) and with the kinetic results of the halogenation of DEM, and are discussed in terms of the involvement of enol and enolate intermediates and their relative reactivities towards electrophilic species. Under our experimental conditions there was no evidence of rate-limiting enol formation.