Elimination reactions on the borderline between concerted and stepwise processes. Base-promoted dehydrohalogenation of 2-phenylsulphonylethyl bromide, chloride, and fluoride

Abstract

The reactions of 2-phenylsulphonylethyl bromide, chloride, and fluoride with methoxide ion in methanol and with triethylamine in acetonitrile and in benzene, have been followed kinetically. The reactivity order Br > Cl > F has been observed. The kinetic isotope effects were determined for the nine substrate–base–solvent combinations. The measured k_H/k_D values range from a maximum of 5·0 in the case of the bromo-derivative in alcohol to a minimum of 1·0 in the case of the fluoro-derivative in benzene. On the basis of isotope effects, leaving group effects, and related observations the position of the elimination mechanism in the E2–E1 cB spectrum is tentatively located.