Abstract:
The exchange reaction between tritium gas and toluene on both γ-irradiated silica gel and Raney nickel has been studied. In addition, the study has included the exchange of a variety of other aromatic compounds on γ-irradiated silica gel.
It has been found that on irradiated silica gel, tritium adsorbs to become activated so that it may exchange with the organic substrate. This exchange takes place almost entirely in the aromatic ring; in addition, classical ‘activating’ groups of electrophilic aromatic substitution, bring about enhanced exchange, while ‘deactivating’ groups bring about the opposite. A mechanism analogous to that of electrophilic substitution has been postulated. Up to 40% of the surface-adsorbed tritium could be incorporated into some compounds, giving specific activities of the order of 5 Ci mol-1. The radiochemical purity of the labelled compound has been shown to depend on the structure of the substrate, varying from 20% to > 99.9%.
On Raney nickel, toluene was found to exchange rapidly with tritium, to give a product of high radiochemical purity, and of specific activity in the range 5 - 10 Ci mol-1. The incorporated activity was found to be almost entirely in the methyl group of the molecule. With CCl4 adsorbed on the catalyst prior to toluene, poisoning of methyl-exchange sites took place, to give a product labelled almost entirely in the ring. With CH2Cl2 on the surface, however, a reverse poisoning effect was shown to take place. The exchange mechanism has been interpreted in terms of π-complex exchange theories.