The hydrolyses of benzamides, methylbenzimidatium ions, and lactams in aqueous sulfuric acid. The excess acidity method in the determination of reaction mechanisms

Abstract

The excess acidity method has been applied to hydrolysis rate data for a number of benzamides, methylbenzimidatium ions, and lactams, obtained as a function of sulfuric acid concentration and temperature. All of the substrates studied except β-propiolactam (8) and methyl-2,6-dimethylbenzimidatium ion (7) were found to follow the AOT2 mechanism at all acidities. The excess acidity method provided considerable mechanistic detail; in dilute acid the transition state contains O-protonated (or methylated) substrate and three water molecules (large negative ΔS≠), but in more concentrated solutions a one-water-molecule mechanism takes over (smaller negative ΔS≠). In strong acid bisulfate ion acts as the nucleophile (positive ΔS≠). N-protonated intermediates are not involved for "normal" substrates, being observed in this work only for 8, which follows the AND1 pathway. Observed differences between benzamide and methylbenzimidatium ion (4) hydrolyses are due to their differing activity coefficient behaviour, the mechanism being the same for both. The hydrolysis of 7 involves a one-water-molecule SN2 displacement at the O-methyl group. Comparison with 7 shows that this displacement is not likely to occur under the reaction conditions for 4; however, for the N-methyl and N,N-dimethyl derivatives studied it is probably an important reaction pathway. A comprehensive mechanistic framework for amide hydrolyses in strong acid media is given.

Hydrolysis mechanisms for acylhydrazines in aqueous sulfuric acid, determined using the excess acidity method

“…Several examples of the method in use are now available (1, 3,[10][11][12]. For the acylhydrazine hydrolyses under consideration here, a reasonable possibility is that given in Scheme 1 , which is based on the normal amide hydrolysis mechanism (3).…”

“…Diprotonated SH?' then undergoes nucleophilic attack; one water molecule is shown as the nucleophile in Scheme 1, but several water molecules or a bisulfate ion may be involved (3,(10)(11)(12). If the second protonation is fast this second stage will probably be the slow step.…”

“…and H,SO, (4), of Mashima et al, on a number of acylhydrazines in H2S04 (7), of Zarakhani et al, on benzoyl- [3] log (fSH+fH+/f+) = mo' log ( f s~+ f i l + / f s~~~+ ) = mo'm*x h)drazine in H2S04 (8), and of Jagdale and Nimbalkar, o n [4] log k h -log cH+ = log k, + m o + m *~ some substituted benzoylhydrazines in H2S0,, HC104, and HC1 (9). The consensus of their conclusions is that most compounds…”

“…Recently, the excess acidity method, developed as a tool for the investigation of reaction mechanisms in strong acid media ( I ) , was applied to rate data obtained for the hydrolysis of benzamides (2) and related compounds in sulfuric acid (3). It was found that nucleophilic attack upon oxygen-protonated amide was rate limiting, and that, as the acidity increased, the nucleophile changed from three water molecules acting in concert, in dilute acid, to one water molecule, to one bisulfate ion at high acidities (3).…”

“…It was found that nucleophilic attack upon oxygen-protonated amide was rate limiting, and that, as the acidity increased, the nucleophile changed from three water molecules acting in concert, in dilute acid, to one water molecule, to one bisulfate ion at high acidities (3).…”