Nucleophilic Substitution at a Carbonyl Carbon Atom (). Solvolysis of Methylchloroformate and Its Thioanalogues in Methanol, Ethanol and Ethanol-Water Mixtures

[摘要] Solvolysis rate constants for methylchloroformate, methylthionochloroformate, methylthiolchloroformate and methyldithiochloroformate have been determined conductometrically in methanol, ethanol and ethanol-water mixtures and activation parameters have been derived. Results show that methylchloroformate solvolyzes through SN2 process while methyldithiochloroformate solvolyzes by SN1 process in all the solvent systems.The rate of hydrolysis decreased in the order, CH3S(CS)Cl > CH3S(CO)Cl > CH3O(CS)Cl > CH3O(CO)Cl which corresponds to the order of decreasing SN1 character. In methanol, CH3S(CS)Cl solvolyzed via the SN1 mechanism while the others solvolyzed via the SN2 process. In ethanol, however, SN2 character was dominant for all the compounds, except methyldithiochloroformate, for which SN1 character was still strong enough to accelerate the rate of ethanolysis. In ethanol-water mixtures, CH3S(CO)Cl and CH3O(CS)Cl solvolyzed via SN2 process in ethanol-rich region while the SN1 character increased greatly in water-rich region for the solvolysis of these compounds. The order of SNl character for solvolysis in water-rich region was the same as the order of hydrolysis rate.

[学科分类] 化学（综合）