Solvent dependence of the primary deuterium kinetic isotope effect in the sodium hydroxide-catalysed E2 elimination of hydrogen bromide from p-acetyl- and p-nitro-phenethyl bromide

Abstract

Rate constants have been determined from the NaOH-catalysed 1,2-elimination of HBr from p-acetyl- and p-nitro-phenethyl bromide over a range of dimethyl sulphoxide concentrations. The calculated K₂^H/K₂^D ratios show no dependence on the para-substituent or the dimethyl sulphoxide concentration except for p-nitrophenethyl bromide at 59.7 and 69.7 mole %. The change in the isotope effect for the p-nitro-compound cannot be satisfactorily explained in terms of a change in transition-state symmetry. The previously reported substituent-induced maximum in the isotope effect for phenethyl bromides is shown to be due to a specific solvent effect rather than indicative of a ‘Westheimer’ maximum. The implication of the overall lack of variation is briefly discussed.