Expulsion of Stable Neutral Fragments

Enol acetates have a particular propensity for this fragmentation, which may in these cases proceed through a six-membered transition state. Such a reaction could be placed in the category of McLafferty rearrangements (Equation 2.86). 

Even more striking is the spectrum of 3-bromo-3-phenylpropionic acid, which first loses a bromine radical, then ketene. The rearrangement fragmentation is supported by a metastable peak at m/z 76.8 (107 /149 = 76.84). The stability of the benzylic product ion and the stability of the neutral ketene molecule provide the impetus for this rare rearrangement of a hydroxy group (Equation 2.87). 

Without a further elaboration of the mechanisms (mainly because not much detail is known) or the scope of these reactions, let it suffice to say that fragmentations that 

Phthalic anhydride, when heated under vacuum in a flow tube to several hundred degrees, yields benzyne, which is detected by isolating its dimer (Equation 2.90). The mass spectral behavior of this compound is analogous, with an initial loss of CO2 followed by loss of CO. 

The reactions outlined in this chapter form the basis for interpreting almost any mass spectrum. Various modifications or combinations of them, however, may tend to obscure the processes that occur in some complicated molecules. It is with practice that one will eventually be able to recognize these complications and actually 

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