Enol intermediates derived from carboxylic acids, esters and amides

Abstract

A kinetic investigation into the enolisation mechanism and enol contents of simple carboxylic acids, esters and amides has been undertaken. Measurement of the enolisation rate constants for malonic acid, ethylhydrogenmalonate and 2- carboxyacetamide were obtained. The halogenation reactions were carried out under conditions where the rate determining step was the enolisation and the reactions were zero order in halogen. The measurements were carried out using u.v./visible spectrophotometry. Results obtained for malonic acid and ethylhydrogenmalonate support the idea of an intramolecular acid catalysed mechanism involving a hydrogen bonded six membered transition state. The enolisation mechanisms were investigated in a number of buffer solutions and were found to be catalysed by general bases. Catalytic coefficients for the following bases were calculated, HO(_2)CCH(_2)CO(_2)-, EtO(_2)CCH(_2)CO(_2)-,ClCH(_2)CO(_2)-, and H(_2)O. Deuterium exchange reactions were monitored using (^1)H N.M.R. and enolisation rate constants for cyano-containing esters and amides were calculated. The results show that the enolisation of amides occurs via an acid catalysed process whereas the enolisation of carboxylic acids and esters proceeds by an intramolecular acid catalysed or a general base catalysed mechanism. Enol contents for simple carboxylic acid derivatives were determined from their reactions with halogens under conditions where the reaction of the halogen with the enol is rate limiting and the reaction is found to be first order in the halogen. It was demonstrated that chlorine, bromine and iodine react with carboxylic ester enols at or very close to the diffusion controlled limit (ca 5x10(^9) 1 mol(^-1) sec(^-1)). The enol contents measured were found to be greater than expected, with the enol content of malonic acid very similar to that of acetone. An estimate of the acidity of carboxylic acid enols has shown them to be strong acids with pK(_a) values in the region 2-5.