Protonation acidity constants for some benzamides, acetamides, and lactams

Abstract

, 1568 (1981).Protonation acidity constants for 27 benzamides, acetamides, and lactams have been determined. Ultraviolet, and I3C and 'H nmr, spectroscopic methods were used; almost all the spectra were found to be subject to medium effects, some quite severe, as the sulfuric acid concentration was increased. Despite this, the excess acidity (X-function) method gave accurate pKB,+ and m * values.In all cases protonation, followed by a medium shift in the spectrum of the protonated form, was the only process taking place; protonation was essentially complete in 60% H,S04. The results for the different bases are interpreted in terms of the resonance, inductive, and steric effects in operation in the acid and base forms. On a determine les constantes d'acidite de la protonation de 27 benzamides, acetamides et lactames. A cette fin on a utilise la rmn du 'H et du "C ainsi que la spectroscopie ultraviolette; lorsque la concentration en acide sulfurique augmente, presque tous les spectres subissent des effets de milieu qui sont quelquefois trbs importants. En depit decette influence, la methode d'excts d'acidite (fonction X ) donne des valeurs prkcises de pKBH+ et de m*. Dans tousles cas, la protonation, suivie d'un deplacement moyen dans le spectre de la forme protonee, est le seul processus qui a lieu; la protonation est tout a fait compl'ete dans du H,S04 a 60%. On interprbte les resultats des differentes bases en fonction de la resonance ainsi que des effets inductifs et steriques presents dans les formes acide et basique.[Traduit par le journal]

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

“…Product analysis shows that amide is the ko slow I exclusive hydrolysis product for 7 (32); presumably the eq. [5] and eq. [6] mechanisms leading to ester/carboxylic acid products are greatly retarded because of steric hindrance to the approach of water at the carbonyl carbon by the ring methyl groups (32).…”

“…The observed very small extent of 1 T+, in eq. [5], were formed directly in the ratedetermining step, rather than subsequently as in eq.…”

“…Collapse of T+, to product would then be very much faster than the reverse of reaction [5]. Cyclic mechanisms of this type are not unknown; one involving four water molecules has recently been proposed for the water-catalyzed hydrolysis of 1-acetyl-l,2,4-triazole, following a proton inventory study (39).…”

“…These were obtained using eq. [4] (I), listed values of X and log CH+ (1) and the recently obtained accurate [4] log I -log CH+ = m*X + pKsH+ m* and pKsH+ values for 1-3 and 8-12 (5). X and log CH+ were corrected to the reaction temperature as previously described (8).…”

“…Amides are protonated in dilute acid (4,5), and so come under Case 2 in ref. 6; the appropriate kinetic equation for an A-2 reaction is reproduced here as eq.…”