Activation of tritium gas by irradiated silica gel and consequent exchange reactions with organic substrates

Abstract

The reactions between tritium gas and organic substrates sorbed on irradiated silica gel have been studied. It has been found that tritium gas, when admitted to gamma-irradiated silica gel, is adsorbed on radiation-induced sites on the solid and bleaches the blue colour of the solid. The sorbed tritium becomes activated to at least two forms; one capable of exchange with alkanes such as 2 methylbutane and both capable of exchange with toluene and 2 chloro2methylbutane. High levels of tritium were incorporated into 2 methylbutane, 2 chloro2methylbutane and toluene, up to 32±2%, 95% and 100% of the total tritium present, respectively, being incorporated (using 2 g SiO2, irradiated 15h), with high standards of radiochemical purity, (97±1% for 2 methylbutane). Yields of labelled hydrocarbons were found to be sensitive to the conditions under which the silica gel was degassed before irradiation; optimum degassing conditions being evacuation for one hour at 973 K. Yields were also found to be dependent on the quantities of silica gel and tritium used and on the radiation dose given the silica gel, all three of these being increased concurrently for maxim yields to be obtained. In a study of the labelling of hydrocarbons of different structures it was found that alkanes containing a tertiary carbon atom were labeled heavily compared to straight-chain hydrocarbons; activity yields of the substrate hydrocarbon decreasing with increasing molecular size, to insignificant values with hydrocarbons heavier than 2 methylhexane. Unsaturated hydrocarbons such as hex-1-ene were found to undergo hydrogenation on irradiated silica gel to produce labelled alkanes; branched-chain alkenes undergoing double-bond isomerization. Reaction mechanisms for tritium activation and exchange have been proposed, involving ionization of tritium by radiation-induced positive holes in the solid and exchange of the ionic species with carbonium ion intermediates. It has also been established that gamma-irradiated alkali halides do not promote exchange of tritium gas with alkanes.