Mechanism of Solvolysis of Substituted Benzoyl Halides

Abstract

Most substituted benzoyl fluorides undergo hydrolysis in aqueous solution through an associative mechanism with = 1.7, IHOH/kDOD = 2.3 ± 0.2, little dependence on the leaving group (kCl/kF= 1.2), and general-base catalysis by fluoride ion. There is an abrupt change to a dissociative mechanism through an acylium ion intermediate for the hydrolysis of p-(dimethylamino)benzoyl and (in part) p-anisoyl fluorides, with p+≤ -1.2, kCl/kF = 106-107, and kHOH/kDOD= 1.1 for p-Me2NPhCOF. Common ion inhibition by fluoride ion traps the p-(dimethylamino)benzoyl acylium ion, which undergoes hydration with kh ~ 109—1010 s-1. The increase in the solvent isotope effect for hydrolysis of p-(dimethylamino)benzoyl fluoride to kHOH/kDOD = 1.9 in the presence of concentrated potassium fluoride is attributed to general-base-catalyzed hydration of the acylium ion intermediate. The large yield of trifluoroethyl ester from the solvolysis of p-anisoyl fluoride in TFE/EtOH/HOH suggests that the acylium ion reacts in a solvent-separated ion pair, with kh~ 1012s-1; extrapolation predicts rate constants of > 1013 s-1 for the hydration of less stable acylium ions. A change in sensitivity to solvent ionizing power from m-= 1.4 in water to m-= 0 in 60% ethanol for p-(dimethylamino)benzoyl fluoride suggests a change to an associative mechanism. Benzoyl fluorides and acylium ions show selectivity toward alcohols, with βnuc ~ 0.2. The absence of common ion inhibition for the solvolysis of several benzoyl chlorides in water or 90% TFE/HOH is consistent with kh> 1011 s-1 for the acylium ions. Solvolysis occurs through the dissociative reaction channel, with p+ = -3.0, even when the estimated lifetimes of the acylium ion species suggest that there is no chemical barrier for their hydration. However, there is a change to an associative mechanism for the solvolysis of p-nitrobenzoyl chloride in water. © 1989, American Chemical Society. All rights reserved.