Stability of α-sulphur- and α-oxygen-substituted carbonium ions
Abstract
The rates of hydrolysis of dimethoxymethane and methoxymethylthiomethane have been measured in aqueous sulphuric acid at 25 °C. The rate constant ratios for the oxygen and sulphur compounds (kO/kS) vary between 0.12 and 0.08 depending on the acid concentration. The solvolysis of chloromethyl methyl ether and sulphide gives, in dioxan–water, a kO/kS ratio of ca. 115. Comparison with other published data on reactions leading to α-sulphuror α-oxygen-substituted carbonium ions shows that kO/kS ratios may range between ca. 0.1 and ca. 440. This is explained by implying different degrees of carbonium ion character in the transition states and the greater ability of sulphur to stabilize a fully developed carbonium ion as opposed to the greater overall π stabilization for the oxygen derivative in the ground state.