Mechanism of Solvolysis of Substituted Benzoyl Halides

Article abstract of DOI:10.1021/ja00204a021

Most substituted benzoyl fluorides undergo hydrolysis in aqueous solution through an associative mechanism with ρ = 1.7, k_HOH/k_DOD = 2.3+/-0.2, little dependence on the leaving group (k_Cl/k_F = 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 ρ⁺ =/< -1.2, k_Cl/k_F = 10E6-10E7, and k_HOH/k_DOD = 1.1 for p-Me2NPhCOF.Common ion inhibition by fluoride ion traps the p-(dimethylamino)benzoyl acylium ion, which undergoes hydration with k_h ca. 10E9-10E10 s^-1.The increase in the solvent isotope effect for hydrolysis of p-(dimethylamino)benzoyl fluoride to k_HOH/k_DOD = 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 k_h ca. 10E12 s^-1; extrapolation predicts rate constants of =/> 10E13 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 60percent ethanol for p-(dimethylamino)benzoyl fluoride suggests a change to an associative mechanism.Benzoyl fluorides and acylium ions show selectivity toward alcohols, with β_nuc ca. 0.2.The absence of common ion inhibition for the solvolysis of several benzoyl chlorides in water or 90percent TFE/HOH is consistent with k_h >10E11 s^-1 for the acylium ions.Solvolysis occurs through the dissociative reaction channel, with ρ⁺ = -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.