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

Article abstract of DOI:10.1139/v94-227

While deuterian 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 k_H/k_D 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 k_H/k_D variation with Br^- concentration, alone show an appreciable first-order component. The second-order k_H/k_D 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 k_H/k_D values obtained are higher, 1.1-1.5. These and other results are discussed.