| Acid-catalyzed reactions of acetamide and N-benzyl-, N-(p-methylbenzyl)-, N-(p-methoxybenzyl)-, N-(p-chlorobenzyl)-, N-(p-nitrobenzyl)-, and N-t-butylacetamides have been investigated over a range of acidity (4%-91% w/w aqueous H(,2)SO(,4)) and temperature (50(DEGREES)-92(DEGREES)C), using nmr spectroscopy. Pseudo first order kinetics were obtained in every case.;Extensive product studies indicated the important processes: (1) N-benzyl-, N-(p-methylbenzyl)-, and N-(p-methoxybenzyl)acetamides react via normal bimolecular hydrolysis, unimolecular N-alkyl cleavage and sulfonation; (2) N-(p-chlorobenzyl)acetamide undergoes hydrolysis and sulfonation; (3) N-t-butylacetamide undergoes hydrolysis and N-alkyl cleavage, and (4) acetamide and N-(p-nitrobenzyl)acetamide undergo exclusive hydrolysis.;The relative magnitudes of the acid dissociation constants, determined from nmr spectral data, were generally consistent with inductive and steric effects on basicity.;The rate-acidity dependences were analyzed using the modified r-hydration parameter, the transition state activity coefficient, and the excess acidity treatments. The results confirmed the mechanistic proposals and demonstrated that, for a given substrate, k(,(psi)) values in different areas of the profile were frequently representative of different processes. Activation parameters, which were calculated at constant water activities by standard methods, and also calculated for the standard state by the excess acidity analysis, confirmed these results. Thus, for N-benzyl-, N-(p-methylbenzyl)-, and N-(p-methoxybenzyl)acetamides in moderately concentrated aqueous acids, the measured K(,(psi)) values represent rates of hydrolysis of the cleavage product (acetamide), rates of N-alkyl cleavage, and rates of substrate hydrolysis.;Four types of acidity dependence were observed: (i) a rate maximum at intermediate acidities, followed by very slow hydrolysis in more concentrated acids; (ii) a rate maximum at intermediate acidities, followed by a sharp increase in rate in more concentrated acids; (iii) a continuous increase in reaction rate with increasing acid concentration, and (iv) a rate maximum at intermediate acidities followed first by a local minimum, second by a second rate maximum at higher acidities, third by very slow reaction in more concentrated acids, and fourth by a sharp increase in rate in the most concentrated acids. |
