Kinetics and Mechanism of the Hydrolysis of 2-Chloroethyl Sulfides

Article abstract of DOI:10.1021/jo00249a029

The hydrolysis of 2-chloroethyl sulfides approaches an S_N1 mechanism only under limiting conditions where the substrate is predissolved in an organic solvent prior to addition to water and the concentration is kept below 0.001 M.At greater substrate concentrations the kinetics are complicated by the formation of dimeric sulfonium chloride salts.These salts are formed in pure water, in buffered aqueous solution at pH 10, and in binary acetone-water solutions.The sulfonium chlorides are relatively stable and decompose via an S_N2 mechanism.One pathway includes the reversible formation of the initial sulfide substrate.This reaction presumably accounts for the recurring toxicity of mustard (2,2'-dichlorodiethyl sulfide) in human and natural environment.The presence of a powerful nucleophile such as thiosulfate can effectively capture the transient ethylenesulfonium ion intermediate formed during the initial step of hydrolysis.All dimeric sulfonium chloride salts are eliminated from the solution in the presence of thiosulfate anion, and the observed rate of displacement reaction at 0.1-O.2 M substrate is equivalent to that measured for the limiting S_N1 mechanism at a substrate concentration below 0.001 M.