ACYLATION MECHANISMS IN APROTIC SOLVENTS .I. METHANOLYSIS OF P-NITROBENZOYL CHLORIDE IN ACETONITRILE

Contrary to previous proposals, the third-order kinetic term frequently observed for alcoholyses of acyl halides in aprotic solvents does not result from a push-pull mechanism. The methanolysis of p-nitrobenzoyl chloride (PNBC) in acetonitrile obeys the kinetic formulation: d[HCl]/dt = k2[PNBC][MeOH] + k3[PNBC][MeOH]2. The reaction is not catalyzed by addition of phenol, a more electrophilic but less nucleophilic species than methanol, and an alternative mechanism is proposed in which, for overall reaction, a first-formed tetrahedral intermediate is deprotonated by either a solvent molecule (second-order kinetics) or a second methanol molecule (third-order kinetics) to give a new tetrahedral intermediate which collapses to products. Consistent with this formulation, addition of chloride ion (a strong base in acetonitrile) leads to a tremendous acceleration and, for a given methanol concentration, this has the formulation: d[HCl]/dtt = k3ʹ [PNBC][MeOH][Cl¯]° With increase in [MeOH], there is an appreciable decrease in k3ʹ, consistent with a reduction in chloride ion basicity due to specific solvation. The claim by Briody and Satchell to have excluded the possibility of a tetrahedral intermediate mechanism for acetonitrile reactions of chloroacyl chlorides with phenols, and to have established a synchronous Sn2 type mechanism, is questioned and an alternative explanation of their kinetic data in terms of conventional ionization and tetrahedral intermediate mechanisms is presented. © 1969, American Chemical Society. All rights reserved.