β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane

Abstract

While deuterium kinetic isotope effects for solvolyses have been extensively studied, other nucleophilic substitutions have received less attention, and identity processes, that is, substitutions where the nucleophile and leaving group are the same, have rarely been examined. Identity reactions must pass through a truly symmetrical stage, a transition state or an intermediate, so that data will be of interest to both theoretical and experimental chemists. Values of kH/kD have been determined by polarimetry for bromide exchange– racemization at ArCHBrCH3/CD3 (Ar = C6H5,4-Br- and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in acetone, acetonitrile, and nitromethane. Observed values are analogous to values seen in solvolyses. They range from 1.01 to 1.35 and, in some cases, increase markedly as the concentration of Bu4NBr decreases. Solvolyses are either first order or pseudo first order whereas plotting observed racemization rate versus [Bu4NBr] allows separation of first- and second-order components; those species giving more stable carbocations in the more dipolar solvents, the systems showing kH/kD variation with Br− concentration, alone show an appreciable first-order component. The second-order kH/kD ratio averages 1.062 ± −0.018 at temperatures ranging from 25 to 50 °C for all substrates in the three solvents, very analogous to the values seen for racemization of 1,1,1-d3-2-bromooctane or solvolysis of ethyl substrates but considerably lower than the typical solvolysis values of 1.15–1.25 for secondary, and 1.35–1.5 for tertiary substrates. The first-order kH/kD values obtained are higher, 1.1–1.5. These and other results are discussed.