Chemistry of recoil tritium atoms. Part 4.—Reactions with olefins in the presence of helium

Abstract

The reactions of recoil tritium atoms with ethylene, propylene isobutene and buta-1,3-diene in the presence of oxygen scavenger and helium moderator are described. The decomposition reactions of excited radicals formed by “hot” tritium atom to the double bond are discussed; it is estimated that the probability of C—C to C—H rupture is about 17:1. However, when the yields of products from radicals that can decompose easily only by C—C rupture are compared with those from radicals that can only break C—H bonds the ratio is 1.4:1. The increase in the ratio, yield of labelled parent/yield of labelled hydrogen, for small olefins by comparison with the corresponding alkane, is rationalised as partly due to certain excited radical decomposition reactions and partly due to the interaction of the double bond with the orbitals of the recoiling tritium atom and of the impacted C—H, causing a reduction in the activation energy for the displacement reaction. Small yields of some minor labelled products are indifferent to moderator concentration; it is thought they are formed by radiation induced decomposition reactions.