Eliminations promoted by thiolate ions. Part I. Kinetics and mechanism of the reaction of DDT with sodium benzenethiolate and other nucleophiles

The kinetics of the dehydrochlorination of 1,1,1-trichloro-2,2-di-p-chlorophenylethane (DDT) by sodium methoxide and sodium ethoxide in the corresponding alcohols have been investigated. Some measurements have also been made with sodium ethanethiolate in these alcohols and with sodium t-butoxide and sodium ethoxide in t-butyl alcohol. The main part of the work is concerned with the kinetics and mechanism of the reaction with sodium benzenethiolate and evidence, including the use of tritiated ethanol (EtOT) as solvent, is presented to show that the mechanism is E2 rather than E1cB which, at first sight, might appear to be the case, especially in methanol. It is shown, furthermore, that the real reagent is not alkoxide ion formed by alcoholysis of benzenethiolate. The rate of dehydrochlorination with benzenethiolate is remarkably slow compared with that for sodium ethoxide, which reacts some 14,000 times faster. The reaction has also been studied with a series of p-substituted benzenethiolates and phenoxides in order to test the applicability of the Bronsted equation to the reaction and determine the β-parameters. A sample of DDT with the aliphatic hydrogen replaced by deuterium has been prepared and studied kinetically to measure the isotope effect in the reaction. It is concluded that, although the dehydrochlorination mechanism is E2, the transfer of the proton to the base in the transition state is well in advance of the heterolysis of the C–Cl bond.