Solvent and substituent effects in solvolyses of benzoyl chlorides. Variation of mechanisms from Grunwald–Winstein correlation analyses with YBnCl scales

Abstract

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), and 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 indicates a limiting S_N1 mechanism for the solvolysis of 1. An S_N1 mechanism with significant nucleophilic solvent participation was found in the solvolysis of 2–4, based on the apparent splitting of lines in log kvs. Y_BnCl plots. Different types of non-S_N1 reactions are probably involved in the solvolysis of 5–7, and are likely solvent dependent. Hammett-type plots against σ⁺ constants indicated a linear relationship for 3–6 only in solvents of high water content (60% aqueous methanol, 50% aqueous acetone and 50% aqueous ethanol) or with high ionising power (100% trifluoroethanol and 80% trifluoroethanol–20% ethanol), but for 3–5 in some solvents containing lower amounts of water or trifluoroethanol. Plots of log k in different solvents against ΔE_iso for the isodesmic reaction also suggested a solvent dependence of mechanisms. The advantage of using Y_BnCl rather than a combination of Y_Cl and aromatic ring parameter I in the correlation analysis is discussed.