Hydrogen migrations in mass spectrometry. IV—formation of [C 6 H 7 ] + in the chemical ionization mass spectra of alkylbenzenes

Using specific deuterium labelling the mechanisms of the olefin elimination reactions leading to formation of [C 6 H 7 ]+ in the H 2 and CH 4 chemical ionizatin mass spectra of ethylbenzene and n ‐propylbenzene (and to [C 2 H 5 C 6 H 6 ]+ in the CH 4 chemical ionization mass spectra) have been investigated. The results show that the reaction does not occur by specific migration of H from the β position of the alkyl group to the benzene ring. For ethylbenzene 23–29% of the migrating H originates from the α‐position, while for n ‐propylbenzene H migration from all propyl positions is observed in the approximate ratio, position 1:position 2:position 3=0.30:0.22:0.48. It is proposed that the results can be explained on the basis of competing H migration from each alkyl position involving cyclic transition states of different ring sizes, rather than by H randomization within the alkyl chain.