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

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ABSTRACT

The reactions between tritium gas and organic s u b s t r a t e s sorbed on i r r a d i a t e d s i l i c a g e l have been studied.

It has been found t h a t tritium gas, when admitted t o gamma-irradiated s i l i c a gel, is adsorbed on radiation-induced sites

an

t h e s o l i d and

I bleaches t h e blue c o l m of t h e solid. The sorbed tritium becomes

1

a c t i v a t e d t o a t l e a s t two forms; one capable of exchange with alkanes such

1

^{as 2} methylbutane and both capable of exchange with toluene and

2 chloro2methylbutane. High l e v e l s of tritium w e r e incorporated i n t o 2 methylbutane, 2 chloro2methylbutane and toluene, up t o 3222%. 95% and 100

%

of t h e t o t a l tritium present, respectively, being incorporated (using 2 g SiOp, i r r a d i a t e d 15h), with high standards of radiochemical purity, (97+1%

-

f o r 2 methylbutane), Yields of l a b e l l e d hydrocarbons w e r e found t o be s e n s i t i v e t o the conditions under which t h e s i l i c a g e l was degassed before i r r a d i a t i o n ; optimum degassing conditions being evacuation f o r one hour a t 973 K. Yields were a l s o found t o be dependent on t h e

1

q u a n t i t i e s of s i l i c a g e l and tritium used and on t h e r a d i a t i o n dose given t h e s i l i c a g e l , a l l t h r e e of these being increased concurrently f o r

m a x i m y i e l d s t o be obtained.

I n a ^study of t h e l a b e l l i n g of hydrocarbons of d i f f e r e n t s t r u c t u r e s it was found t h a t alkanes containing a t e r t i a r y carbon atom w e r e l a b e l l e d heavily compared t o straight-chain hydrocarbons; a c t i v i t y y i e l d s of t h e s u b s t r a t e hydrocarbon decreasing with increasing molecular s i z e , t o i n s i g n i f i c a n t values with hydrocarbons heavier than 2 methylhexane.

Unsaturated hydrocarbons such a s hex-1-ene were found t o undergo hydrogenation on i r r a d i a t e d s i l i c a g e l t o produce l a b e l l e d alkanes; branched-chain alkenes undergoing double-bond isomerization.

Reaction mechanisms f o r tritium activation and exchange have been proposed, involving ionization of tritiwn by radiation-induced p o s i t i v e holes i n the s o l i d and exchange of the i o n i c species with carbonium ion intermediates.

I t has a l s o been established t h a t gamma-irradiated a l k a l i halides do not promote exchange of tritiwn gas v i t h alkanes.