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.