S N2 Reactions in Dipolar Aprotic Solvents. VII. Kinetic and Equilibrium Secondary α-Deuterium Isotope Effects in Chlorine Isotopic Exchange Reactions of Substituted Chloromethanes in Acetonitrile

Abstract Secondary α-deuterium isotope effects were studied in the symmetrical nucleophilic substitution of the substituted chloromethanes in acetonitrile. The apparent second order rate coefficients show a significant isotope effect for chloromethyl aryl ethers and sulfides (1.11–1.14 per D), and an intermediate value of 1.05 for cinnamyl chlorides. 2-Arylethyl chlorides were shown to give an appreciable isotope effect of 1.03–1.04, and arylchloromethanes to give varying amount of the isotope effect from 1.006 to 1.050 depending upon the ring substituent. The deuterium label also gave rise to a significant equilibrium isotope effect on the substrate-nucleophile association in the reaction mixture. The observation of an equilibrium isotope effect suggested a possibility that an apparent isotope effect was a composite of the thermodynamic and the kinetic isotope effects. Thus the dissection resulted in a uniform and a fairly large kinetic isotope effect for all the activated substrates studied and in a small kinetic isotope effect for 2-arylethyl chloride. The results are explained in a framework of loose–tight transition state.