Nucleophilic cleavage of diaryl disulphides

Abstract

The rates of cleavage of 14 symmetrically substituted diaryl disulphides by cyanide ion have been measured in 60% aqueous t-butyl alcohol at pH 9.2. A plot of log k against σ° shows that while the reaction rate is accelerated by inductive electron withdrawal from the benzene rings, substituents capable of conjugative interaction are not correlated by their σ° parameters. +R substituents cause reaction to occur much faster than predicted on the basis of their σ° values, while -R substituents react more slowly than predicted. Measurement of rates of cleavage of three series of unsymmetrically substituted disulphides by cyanide or hydroxide shows that these unusual substituent effects arise from substituents in the thiocyanate-forming aryl ring. This behaviour is explained in terms of a change in the electronic behaviour of the thio- cyanate-forming sulphur atom from -I, + R in the disulphide to -I,-R in the rate-determining transition state for the reaction. The study does not show whether the cleavage involves an S_N2 process or rapid equilibrium formation of a pentacovalent intermediate.