McLafferty Rearrangement

Abstract

Abstract This reaction is explained as the rearrangement of monounsaturated molecular ion or radical ion with the cleavage of α,β‐bond of the unsaturated system along with the concomitant transfer of a γ hydrogen via a six‐membered transition state by the formation of a pair of unsaturated fragments, regardless of which fragment holds the charge is generally referred to as the McLafferty rearrangement. During the fragmentation, the structural, steric, and electronic factors determine the actual fragmentation paths. The mechanistic feature for carbonyl compounds has been discussed and it is explicated that if the distance between γ‐hydrogen and oxygen are >1.8A°, then McLafferty rearrangement do not occur. It has been found almost all molecular ions of aldehydes, ketones, thioketones, carboxylic acids, and acetates undergo a silyl group migration. Mechanistically, the McLafferty rearrangement can be either concerted with “simultaneous hydrogen transfer and β‐cleavage,” or stepwise with the “initial hydrogen transfer being followed by β‐cleavage”. This rearrangement is useful in structural elucidation using mass spectroscopy as the tool.