Butylbenzene mass spectrum

Example In the mass spectrum of /er/-butylbenzene, C10H14, M = 134, the [CvHv]" ion is formed by C2H4 loss from the product of benzylic cleavage, m/z 119, despite such a rearrangement would not be expected at first sight (Fig. 6.15). 

Alkyl benzenes, upon Cl with (C2HS)+ and (CHS)+, yielded (M + C2H5)+ as well as (M + H)+ ions, which decompose yielding (CgHu)+ and the previously mentioned (C6H7)+, respectively. For butylbenzene with the alkyl side-chain deuterated, isotope effects of 1.3 and 1.5 were obtained from the Cl mass spectrum for the former and latter reaction, respectively, while for pentylbenzene respective isotope effects of 1.5 and 1.3 were obtained [903]. 

A secondary hydrogen isotope effect has been observed in the loss of methyl radical from the -butylbenzene ion. In the El mass spectrum, the isotope effect IchJIct>3 was 1.1 and for metastable ions the isotope effect ranged from 1.5 to 1.9 [642], The isotope effect in the mass spectrum is equal to the ratio of the stretching frequencies of the dissociating bonds. The isotope effect on the metastable ions has been discussed in terms of an excited electronic state and specific radiationless transitions. The isotope effect is, however, explicable within QET. If the critical energy for CHj loss is 3.4 kJ mole-1 less than that for CDj loss, an isotope effect of about 2 is predicted by QET for metastable ions [853]. 

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. 

Mass Spectrometry The most common mass spectral fragmentation of alkylben-zene derivatives is the cleavage of a benzylic bond to give a resonance-stabilized benzylic cation. For example, in the mass spectrum of -butylbenzene (Figure 16-18),... 

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