A comparative study of the pyrolytic and electron impact-induced fragmentations of 4-phenylbutanoic acid and some analogues

By use of deuterium labelling it is shown that the mechanisms of fragmentation of the title compounds upon Curie-point pyrolysis and electron impact are essentially different. For example, the pyrolytic expulsion of water from 4-phenylbutanoic acid is a 1,2 eliminations, whereas upon electron impact water is expelled via a 1,4 elimination; the generation of styrene from this acid proceeds via a concerted mechanism upon pyrolysis, but via a stepwise mechanism upon electron impact.