Catalytic exchange with deuterium of monohalogen substituted benzenes and associated reactions on evaporated metal films

Abstract

Exchange reactions of fluorobenzene and chlorobenzene with deuterium have been investigated over evaporated films of palladium, platinum, nickel and tungsten and reactions of bromobenzene have been studied on platinum and tungsten. Rates of exchange were much slower than rate for the exchange of benzene on the same metals and simultaneous reaction of the halogeno-benzenes to form other compounds occurred in every case. All five hydrogen atoms in each of the molecules were replaced at about the same rate, which showed that neither steric nor electronic effects of the halogen atoms were playing a substantial part in the metal-catalyzed reactions with deuterium.

The main products from the reaction of fluorobenzene with hydrogen or deuterium were benzene, cyclohexane and fluorocyclohexane but others were formed at higher temperatures. The types of reaction varied with the metal and there was a correlation between the extent of the rupture of the C—F bond, the formation of unsaturated compounds and the influence of the decomposition on the rate of exchange. The order of the metals for all these effects was W > Ni > Pd > Pt. With chlorobenzene and bromobenzene, the decomposition gave benzene as the major product and no exchange occurred with chlorobenzene on palladium or nickel.