Reactions of helium tritiide ions with gaseous bicyclo[n,1,0]alkanes. Evidence for gaseous bicycloalkylium ions

Abstract

The reactivity of the HeT⁺ ion from the β-decay of molecular tritium towards bicyclo[2,1,0]pentane, bicyclo[3,1,0]hexane, and bicyclo[4,1,0]heptane has been investigated in the gas phase. The radio gas chromatographic analysis of the labelled reactions products revealed in all the systems studied the formation of a tritiated hydrocarbon retaining the bicyclic structure of the starting substrate. This observation can be rationalized by postulating the intermediacy of the corresponding gaseous bicycloalkylium ion, formed via the exothermic triton transfer from the HeT⁺ to the organic substrate, and suggests the occurrence of protonated bicycloalkanes in the gas phase. Owing to the considerable exothermicity of the electrophilic attack, a large fraction of the bicycloalkylium ions cannot be stabilized by collision in the pressure range investigated. 100–300 Torr, and undergoes various fragmentation and isomerization processes involving the cleavage of one or more C–C bonds. The mechanistic implications of these processes are discussed, and the gas-phase reactivity of the bicyclo[n,1,0]alkanes towards the strong Brønsted acid HeT⁺ is compared with their behaviour under the attack of milder electrophiles in solution.