Cleavage of the Benzylic Bond in Phenylalkanes

Example In the mass spectrum of /er/-butylbenzene, C10H14, the [C7H7] ion is formed by C2H4 loss from the product of benzylic cleavage, m/z 119, though such a rearrangement would not be expected at first sight (Fig. 6.15). 

Note The illustrative term scrambling is used in mass spectrometry to describe rapid processes of (intramolecular) positional interchange of atoms. Scrambling may occur with hydrogens or may involve the complete carbon skeleton of an ion. Aryl radical ions and protonated aryl compounds are well known for their numerous scrambling processes [50]. 

Whatever the initial or preferred structure, the ions dissociate by loss of 

The phenyl ion [CeHs], m/z 77, also undergoes C2H2 loss, thereby generating the cyclobutadienyl ion, [ 4113] m/z 51  

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.  

Molecular ions of phenylalkanes are comparatively stable due to the good charge stabilizing properties of the aromatic ring and thus, they normally give rise to intense peaks. Those molecular ions, possessing a benzylic bond preferably show 

Note The mere occurrence of the [C7H7] ion - especially when it is of low intensity - is not sufficient to prove that the analyte belongs to the phenylal-kanes. This ion and its characteristic fragments may also be observed if there is some way at all to generate a [CvHv] fragment ion. 

As already indicated in the preceding schemes, the resonance-stabilized benzyl ion, [CvHv] , initially formed by benzylic bond cleavage reversibly isomerizes to the tropylium and tolyl ion isomers. The isomerization of [CvHv] ions has been the subject of numerous studies, [50] revealing the tropylium ion as the thermodynamically most stable isomer. [51,52] 

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