The ion-pair mechanism and bimolecular displacement at saturated carbon. V. Racemization and bromide – radio-bromide exchange for substituted 1-phenylbromoethanes in acetone

Abstract

Results of kinetic studies of the racemizations and radio-bromide exchanges with 1 -arylbromoethanes in acetone with added LiBr are reported. The results are direct empirical evidence supporting the ion-pair mechanism for bimolecular nucleophilic displacement at saturated carbon. For the unsubstituted and less electron rich substrates, behaviour expected for a classical Sn2 mechanism is found, but for 1-(4-methylphenyl)bromoethane, the racemization rate is greater than twice the exchange rate, indicating internal return with racemization from a solvated ion pair. For 1-(3,4-dimethylphenyl)bromoethane, the carbocation is sufficiently stabilized that the reaction shows intermediate behaviour; the racemization rate is significantly less than twice the exchange rate, for example. Through the series from the electron withdrawer to electron donor substituted substrate, the activation parameter ΔH≠ progressively decreases and ΔS≠ becomes rapidly more negative.