Benzyl cations rearrangement

The mass spectrum of w-butylbenzene has its base peak at m/z 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The benzyl cation rearranges to the tropylium ion, detected at m/z 91. 

The benzylic cation rearranges to form cycloheptatrienyl cation, which is even more stable. Note that the cycloheptatrienyl carbocation is aromatic 6 7T electrons) and has the positive charge delocalized around the entire ring. 

Although this cleavage is probably driven by the stability of benzyl cation evidence has been obtained suggesting that tropyhum cation formed by rearrangement of benzyl cation IS actually the species responsible for the peak... 

This reflects the greater stability of the benzylic cation (32), though only secondary, compared with the tertiary cation (33) that would be—but in fact is not—obtained by its rearrangement (c/. p. 105). 

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

Cone, C. Dewar, M.J.S. Landman, D. Gaseous Ions. MINDO/3 Study of tiie Rearrangement of Benzyl Cation to Tropy-lium. J. Am. Chem. Soc. 1977, 99, 372-376. 

It has been proposed that, in most cases, the ion of mass 91 is a tropylium rather than a benzylic cation. This explains the ready loss of a methyl group from xylenes, although toluene does not easily lose a methyl group. The incipient molecular radical ion of xylene rearranges to the methylcycloheptatriene radical ion, which then cleaves to the tropylium ion (QH/). 

No rearrangement of benzyl cations in acid solutions to tropylium ions has been found, although this rearrangement is observed in the gas phase (mass... 

Step 3b Allylic rearrangement via allylic benzylic cation. 

Macro cation (6), which can be formed from (1), (4), or (5), can undergo P-scission to form styrene, rearrange to form a tertiary benzyl cation (7), or may abstract a hydride to form (4). 

The tertiary benzyl cation (7), which would be preferentially formed by hydride abstractions in addition to rearrangement of (6), may be a precursor for chain unsaturation and for the formation of indenes. 

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