BETA-DEUTERIUM KINETIC ISOTOPE EFFECTS FOR IDENTITY PROCESSES - BROMIDE ION SUBSTITUTION AT 1-BROMO-1-ARYLETHANES AND 2-BROMOOCTANE

While deuterium kinetic isotope effects for solvolyses have been exten sively studied, other nucleophilic substitutions have received less at tention, and identity processes, that is, substitutions where the nucl eophile and leaving group are the same, have rarely been examined. Ide ntity reactions must pass through a truly symmetrical stage, a transit ion 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 ArCHBrC H3/CD3 (Ar = C6H5, 4-Br- and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in ace tone, 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 Bu(4)NBr decreases. S olvolyses are either first order or pseudo first order whereas plottin g observed racemization rate versus [Bu(4)NBr] allows separation of fi rst- and second-order components; those species giving more stable car bocations 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 degrees C for all substrates in the three solvents, very analogous to the values seen for racemizatio n of 1,1,1-d(3)-2-bromooctane or solvolysis of ethyl substrates but co nsiderably lower than the typical solvolysis values of 1.15-1.25 for s econdary, 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 d iscussed.