Deuterium and tritium labelling of aromatic hydrocarbons by zeolite-catalysed exchange with perdeuteriobenzene, tritiated benzene, and [p-3H]toluene

Abstract

Three hydrogen zeolites, HNaY, H-Mordenite, and HZSM-5, have been employed as catalysts for hydrogen isotope exchange reactions. The zeolites catalysed exchange between the isotope sources perdeuteriobenzene, tritiated benzene, and [p-³H]toluene, and a wide range of organic substrates, thus providing a useful labelling technique. The extent and orientation of exchange in reaction products were assessed through the techniques of radio-gas chromatography, mass spectrometry, and ³H n.m.r. spectroscopy. Substituted benzenoid compounds were non-specifically labelled in the aromatic centres at temperatures from 40 to 175 °C, but heterocyclic compounds and alkanes were not exchanged. A series of competitive exchange experiments gave an insight into the reasons for the lack of reactivity of some substrates. Inspection of the multiple exchange parameters and relative rates of exchange of substrates of different bulk suggested correlations between zeolite pore sizes, substrate reactivity, and labelling patterns. Consideration of these experimental results together with those from redistribution reactions with [p-³H]toluene led to interpretation of results in terms of acid exchange mechanisms, where these mechanisms are mediated by the constraints of zeolite pore geometry.