The effect of the nature of the amine leaving group on the nature of the E2 transition state for the reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol

Abstract

To investigate the effect of the leaving group on the elimination reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol at 60 °C, the reaction of seven different quaternary ammonium salts and their β-deuterated analogues with trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, N,N-dimethylbenzylamine, tripropylamine, and N,N-diethylbenzylamine as leaving groups has been studied. In all cases the elimination, which was shown to proceed via the concerted E2 process, was accompanied by competing substitution reactions. Although a significant dependence of the rate of the elimination process on the nature of the leaving group was noted, there was not any linear correlation with the basicity of the amine leaving group. The primary hydrogen–deuterium kinetic isotope effect for the elimination process, (kH/kD)E, was found to increase initially with an increase of reaction rate, [Formula: see text] for substrates containing the leaving groups trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, and N,N-dimethylbenzylamine; i.e., (kH/kD)E = 5.03, 5.26, 5.40, 5.83, and 5.85, respectively. A further increase in rate, using substrates with tripropylamine and N,N-diethylbenzylamine as leaving groups resulted in a decrease of the magnitude of the hydrogen–deuterium isotope effect; i. e., (kH/kD)E = 5.42 and 4.67, respectively. It is concluded that steric effects mainly determine leaving group ability. As well, it is concluded that the leaving group ability of the amine determines the structure of the E2 transition state. For the reaction of the poorer leaving groups, trimethylamine, N-methylpiperidine, and N-methyldiethylamine, the proton is morethan one-half transferred at the transition state while for reaction involving the two best leaving groups, tripropylamine and N,N-diethylbenzylamine, the Cβ—H bond is lessthan one-half broken at the transition state. The conclusions are considered in the light of the More O'Ferrall – Jencks potential energy surface diagram. Keywords: elimination mechanism, transition state, isotope effects, leaving group, quaternary salts.