HETEROGENEOUS CATALYTIC ISOTOPIC EXCHANGE OF BENZYLIC COMPOUNDS IN SOLUTION

Isotopic exchange reactions of bibenzyl and benzylic derivatives with deuterium (D2) and tritium (T2) gas, catalyzed by Pd/C, have been perf ormed in solution. Catalyst pre-washed with the reacting gas showed im proved H/D or H/T exchange. In aprotic solvents such as dioxane, ethyl acetate, and cyclohexane, 3.1 to 3.5 D atoms were exchanged under sta ndard conditions in 1 h, while in benzene 1 D atom was exchanged. D at oms adsorbed on the catalyst surface were rapidly replaced by H from m ethanol, diluting the D2 gas phase. Compounds containing 0 or N atoms enhanced the rate of exchange, while S atoms inhibited the catalyst ac tivity. The H/D exchange obeyed pseudo-first order kinetics when D2 ga s was used in large excess. When T2 gas replaced D2, a substantial dec rease in rate of exchange was observed, due to the slower dissociative chemisorption of T2 on the catalyst surface. The results indicated th at this exchange process occurred through two simultaneous mechanisms. The breaking of the C-H bond appeared to be the rate-determining step . (C) 1994 Academic Press, Inc.