Intramolecular nucleophilic assistance in the solvolyses of benzyl derivatives: solvolyses of o-nitrobenzyl bromide and tosylate

Abstract

The specific rates of solvolysis of o-nitrobenzy] p-toluenesulfonate (1) have been measured in a wide range of solvents. Comparison with the previously studied para-isomer (2) indicates very similar behaviour (dominant SN2 pathway) in solvents without fluoroalcohol content. With an appreciable fluoroalcohol component, the specific rates are considerably higher than those predicted based on the comparison with the solvolysis of 2 and intramolecular participation by the ortho-nitro group is implicated. Consistent with this postulate, in 97% 2,2,2-trifluoroethanol, the entropy of activation for the solvolysis is considerably less negative for 1 than for 2. Measurements in four representative solvents of the specific rates of solvolysis of the bromides led to k(oTs/)k(Br) ratios of 10 to 30, consistent with appreciable nucleophilic assistance (by either solvent or nitro group) in the rate-determining step. For the solvents with an appreciable fluoroalcohol content, an analysis using the extended Grunwald-Winstein equation shows a negligible sensitivity towards changes in solvent nucleophilicity and a low (m = 0.27 +/- 0.02) sensitivity towards changes in solvent ionising power. The low m value may result from a favourable solvation of the leaving group being partially counterbalanced by an unfavourable solvation of the nucleophilic nitro group.