McL of Aromatic Hydrocarbons

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

Independent of the alkyl substituent, [C7Hg], 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 [ 7117] species, m/z 91, which then produces 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 ortto-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. 6.27). As long as pentene loss may occur, there is not much difference to spectra of isomers like 2-methylpentyl-, 4-methylpentyl, or n-hexyl. Reference spectra are needed for dis- 

In case of alkyl benzylethers, aldehyde loss can occur following the same reaction pathway. For exanple, acetaldehyde is eliminated from the molecular ions of benzylethylether, thus producing [CvHg] fragment ions. Again, evidence has been presented for a stepwise mechanism involving a distonic intermediate [88]. 

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