McL of Aldehydes and Ketones

Example The El mass spectrum of butanal mainly shows carbenium fragment ions due to simple bond cleavage that can be easily recognized from their odd-numbered m/z values. Only the base peak is represented by a [M-28] ion, m/z 44, obviously resulting from rearrangement. 

Following the above general description of the McLafferty rearrangement, the peak at m/z 44 in the mass spectrum of butanal can be explained by C2H4 loss from the molecular ion. The process may either be formulated in a concerted manner (a) or as a stepwise process (b)  

While the concerted pathway has been preferred in early publications on the subject, evidence for a stepwise mechanism involving distonic ion intermediates is presented in more recent work taking kinetic isotope effects into account [82]. This is also in agreement with the postulation that reactions involving multiple bonds are generally stepwise processes [34,83]. Nevertheless, this question is still a matter of debate [82]. 

In principle, the enolic fragment ion may or may not tautomerize to the keto form before further fragmentation takes place  

The gas-phase heats of formation of several enol positive ions of aliphatic aldehydes, ketones, acids, and esters were measured and compared with those of the corresponding keto ions. The enolic ions were found to be thermodynamically more stable by 58-129 kJ mol This is in marked contrast to the neutral tautomers, in which the keto forms are generally more stable [84]. The experimental findings also are in good agreement with MNDO calculations [85], and support the hypothesis that reketonization does not play a major role for further fragmentation. Anyway, it is helpful to consider tautomerization when seeking for subsequent decomposition pathways. 

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