Solvent and substituent effects in solvolyses of benzoyl chlorides. Variation of mechanisms from Grunwald-Winstein correlation analyses with Y-BnCl scales

The solvolysis of seven aromatic acid chlorides, namely 2,6-dimethyl- (1), 2-methyl- (2), 4-methoxy- (3), 4-methyl- (4), 4-chloro-(6), 4-nitro- (7), a nd unsubstituted benzoyl chloride (5), in a variety of solvents was studied . The observation of a linear correlation of log k against solvent ionizing power Y-BnCl using the single-parameter Grunwald-Winstein equation indicat es a limiting S(N)1 mechanism for the solvolysis of 1. An S(N)1 mechanism w ith significant nucleophilic solvent participation was found in the solvoly sis of 2-4, based on the apparent splitting of lines in log k vs. Y-BnCl pl ots. Different types of non-S(N)1 reactions are probably involved in the so lvolysis of 5-7, and are likely solvent dependent. Hammett-type plots again st sigma(+) constants indicated a linear relationship for 3-6 only in solve nts of high water content (60% aqueous methanol, 50% aqueous acetone and 50 % aqueous ethanol) or with high ionising power (100% trifluoroethanol and 8 0% trifluoroethanol-20% ethanol), but for 3-5 in some solvents containing l ower amounts of water or trifluoroethanol. Plots of log k in different solv ents against Delta E-iso for the isodesmic reaction also suggested a solven t dependence of mechanisms. The advantage of using Y-BnCl rather than a com bination of Y-Cl and aromatic ring parameter I in the correlation analysis is discussed.