Benzyl alcohol, mass spectrum

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably a-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. We suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H20 (I), might also contain a contribution from benzocyclobutene from the interaction of a-bromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-o-xylene were run separately under the same conditions. 

Peaks arising from these fragment ions can be observed in the mass spectrum of benzyl alcohol (Fig. 8.25). 

A second common mode of fragmentation involves dehydration. The importance of dehydration increases as the chain length of the alcohol increases. While the fragment ion peak resulting from dehydration (m/z = 70) is very intense in the mass spectrum of 1-pentanol, it is quite weak in the other pentanol isomers. Dehydration may occur by either thermal dehydration prior to ionization or by fragmentation of the molecular ion. Thermal dehydration is especially troublesome for alcohol samples analyzed by GC-MS. The injection port of the gas chromatograph is usually maintained at more than 200°C, and many alcohols, especially tertiary or allylic/benzylic, will dehydrate before the sample molecules even reach the GC column and certainly before the molecules reach the ion source of the mass spectrometer. Thermal dehydration is a 1,2-eUmination of water. If the alcohol molecules reach the ion source intact, however, dehydration of the molecular ion can still occur, but in this case it is a 1,4-elimination of water via a cyclic mechanism ... 

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