The kinetics of hydrolysis of N-trimethylammonioacetamide and of substituted N-trimethylammoniobenzamides in concentrated sulphuric acid

Abstract

The hydrolysis of N-ammonioacetamide and a number of substituted N-ammoniobenzamides has been studied in sulphuric acid solutions of concentration greater than 75%, using ¹H n.m.r. and u.v. spectroscopic methods. Good first-order behaviour is found in all cases. The logarithms of the observed rate constants are linearly related to the H₀(or H₊) acidity function, with slopes –1 to –1.2₈. This is consistent with a unimolecular mechanism of decomposition of the N-protonated substrates. Substituent effects on the observed rate constants are best correlated with Hammett substituent constants by a Yukawa–Tsuno equation, with ρ–3.9 ± 0.2 (r 0.986). The large negative value of this constant indicates the dominant effect of the rate-limiting unimolecular formation of the acyl cation intermediates upon the overall rate of the reaction.