McLafferty rearrangement of aromatic hydrocarbons

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  

Independent of the alkyl substituent, [CvHs] , m/z 92, is obtained as the product ion, provided there are no other substituents at the ring. The product is an isomer of toluene molecular ion, and as such it readily stabilizes by H loss to yield the even-electron [CvHv] species, m/z 91, which then gives rise to the well-known characteristic fragments m/z 65, 39). Provided that there is no prior isomerization of the molecular ion, this dissociation is prohibited if both or//io-positions are substituted and/or if there is no y-hydrogen in the alkyl group. 

Example The base peak in the El mass spectrum of (3-methylpentyl)-benzene is formed by McLafferty rearrangement of the molecular ion (Fig. 6.28). As long as pentene loss may occur, there is not much difference to spectra of other isomers such as 2-methylpentyl-, 4-methylpentyl, or n-hexyl. Reference spectra are needed to distinguish those isomers, because differences are mainly due to peak intensities, whereas only minor peaks might appear or vanish depending on the isomer. 

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