A comparison of homogeneous and heterogeneous platinum as catalysts for the deuteration of polyphenyl aromatic hydrocarbons

Abstract

Polyphenyl aromatic compounds exchange with heavy water in the presence of both homogeneous and heterogeneous platinum with high M values. Diphenyl, m- and p-terphenyls exhibit similar isotope orientations during initial rates in both catalytic systems whereas o-terphenyl does not. The first three compounds show ortho steric effects and appear to exchange by n-dissociative processes. In m- and p-terphenyls, the deactivated ortho protons exchange heterogeneously at rates which may be directly related to their degrees of steric hindrance. With o- terphenyl, all ortho hydrogens are deactivated in homogeneous exchange whereas heterogeneously, all centre ring ortho hydrogens exchange readily. A novel catalytic intermediate has been proposed to explain this orientation. These results show that o-terphenyl is a unique compound for differentiating between homogeneous and heterogeneous metal catalysed exchange processes. The data from the polyphenyls and the condensed polycyclics support the π-complex theory of catalysis.