McLafferty rearrangement of aldehydes and ketones

The loss of alkenes from molecular ions of carbonyl compounds has early been noted. [23,82] Soon, a mechanism involving y-H shift and P-cleavage has been proposed and studied in detail. [24-26,83,84] Strictly speaking, the term McLafferty rearrangement only describes an alkene loss from molecular ions of satu- 

Example The El mass spectmm of butanal mainly shows carbenium fragment ions due to simple bond cleavage that can be easily recognized from their odd-numbered m/z values. However, the base peak is represented by a [M-28] ion, m/z 44, obviously resulting from rearrangement. A closer look reveals that the charge-migration product, m/z 28, is also present in the spectrum. 

Requirements for the McLafferty rearrangement in a broad sense i) the atoms A, B, and D can be carbons or heteroatoms, ii) A and B must be connected by a double bond, hi) at least one y-hydrogen is available that iv) is selectively transferred to B via a six-membered transition state, v) causing alkene loss upon cleavage of the p-bond. 

In addition to these general requirements, the distance between the y-hydrogen and the double-bonded atom must be less than 1.8 x 10 m [87,88] and the Cy-H bond must be in plane with the acceptor group. [89] 

The McLafferty rearrangement itself proceeds via charge retention, i.e., as alkene loss from the molecular ion, but depending on the relative ionization energies of the respective enol and alkene products, the charge migration product, i.e., the corresponding alkene molecular ion is also observed. This is in accordance with Stevenson s rule (Chap. 6.2.2). 

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