Carbanion and enol intermediates in c-nitrosation and halogenation
A kinetic study of the mtrosation of ethyl cyanoacetate, diethyl malonate and malononitrile, in acidic water/dioxan solution, by nitrous acid, at 25ºC, was under taken. Catalysis of this reaction was obtained by the addition of nucleophilic catalysts; chloride ion, bromide ion, thiocyanate ion and thiourea. The results were consistent with a mechanism where malononitrile reacted exclusively via the carbanion intermediate. Within the pH range used, pH 0∙7 to pH 3∙3, ethyl cyano acetate and diethyl malonate reacted either through a carbanion intermediate, at higher acidity, or an enol intermediate, at lower acidity. Values of the second order rate constant for the attack of the nitrosating species upon the carbanions were obtained. The carbanions of malononitrile and diethyl malonate reacted at the diffusion limit, in the presence of catalysts. Nitrosation of ethyl cyanoacetate, via its carbanion, showed an already established trend in the reactivity of the nitro sating species, NOSC(NH(_2))(_2) < NOSCN < NOBr < NOCl. A kinetic study of the nitrosation of malonic and methylmalonic acids, and of the iodination and bromination of these two acids as well as ethylmalonic and phenylmalonic acids, in aqueous acidic solutions, at 25ºC, was also undertaken. At high acidity nitrosation was shown to proceed via an enol intermediate and at lower acidities via a carbanion. Nitrosation of the intermediate was rate determining. Under certain conditions, in nitrosation, it was possible to make the enolisation rate limiting. lodination and bromination, by the halogen molecules, involved rate determining enol formation. lodination by triiodide ion involved rate determining iodination of the enol. Values of the enolisation rate constant, kg, were obtained for all four of the acid substrates, these were in reasonable agreement for the different electrophilic processes. Between pH 0 and pH 2 the results fitted an intramolecular acid catalysed enolisation mechanism. At higher pH values (2 to 4) the results fitted a change in mechanism to include, additionally, base catalysed enolisation and enol carboxylate formation pathways.