Intrinsic alkyl radical properties inferred from the study of unimolecular dissociations of gaseous carboxylic acid cation radicals

Abstract

The study of metastable carboxylic acid cation radicals (lifetime 10 -5s) in the gas phase provides a detailed insight into a chemistry which is best described in terms of “free radical chemistry”. In addition to the well-known 1, n-hydrogen migrations ( n = 4,5) of radicals evidence is presented that even the longsought 1,2-hydrogen migration may take place in appropriate cases. Examples for 1, n-migrations of protonated carboxyl groups ( n = 2,4,5) are also discussed, while (l, n) alkyl migrations do not occur. For all n-alkyl elimination processes studied, ene-1,1-diol cation radicals were identified as the essential intermediates ; the mechanism of dissociation is the exact counterpart to the addition of nucleophilic alkyl radicals to deactivated double bonds. The study of metastable carboxylic acids in the gas phase provides the following ordering of relative leaving group abilities for n-alkyl radicals: C 2H 2 ≫ > C 3H 7 > C 4H 9 > C 5H 11 > C 6H 13 > C 7H 15 > C 8H 17 ⋙> CH 3