Benzylic bond cleavage phenylalkanes

Note Overall, the former two reaction lead to the typical appearance of the ion series m/z 39, 51, 65, 77, 91 in the El mass spectra of phenylalkanes. However, one needs to be aware of the two competing reaction sequences leading to this series i) IVT 91 65 - 39 and ii) IVT 77 - 51 the latter usually being of lower intensity, which is in accordance with the preference for benzylic bond cleavage. ... 

In addition to the already described benzylic and phenylic cleavages (Chap. 6.4), phenylalkanes may undergo alkene loss by a mechanism that is perfectly analogous to the true McLafferty rearrangement, provided the alkyl substituent fulfills all requirements. The y-hydrogen is transferred to the ortto-position where the aromatic ring serves as the accepting double bond ... 

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