Carbocation-like isomerisations at radical sites precede methyl losses from ionised butanoic acid

Abstract

The low-energy losses of methyl from ionised butanoic acid and isomeric ions have been further characterised utilising isotopic labelling, field ionisation kinetics, and translational energy releases. Long lived butanoic acid ions lose methyl groups containing C-2, C-3, and C-4. The loss of methyl containing C-2 follows migration of the protonated carboxy group to the radical site of CH₃ĊHCH₂C(OH)₂⁺. Methyl containing C-3 is lost following a 1,2-shift of the terminal methyl from C-3 to C-2. 1,2- and 1,4-hydrogen shifts take place in all of the pathways to methyl loss. The 1,2-shifts of methyl and hydrogen may be permitted by a reduced electron density on the α carbon atoms of the intermediate ions CH₃CH₂ĊHC(OH)₂⁺ and (CH₃)₂ĊC(OH)₂⁺. The isomerisations of ionised butanoic acid can be rationalised as a mixture of free-radical-like and carbocation-like reactions.