Mc Lafferty Rearrangement
Mc Lafferty Rearrangement
McLafferty rearrangement is a reaction observed in mass spectrometry during the fragmentation of organic molecules.
Mc Lafferty Rearrangement involves the migration of γ- H atom followed by cleavage of a β-bond. The rearrangement lead to the elimination of neutral molecules from aldehydes, ketones, amines, unsaturated compounds, substituted aromatic compounds etc. The rearrangement proceeds through a sterically hindered six membered transition state.
Generally a molecular ion with an even mass value cleaves to give fragment ions with odd mass values and vice-versa. But if the even mass molecular ion yields fragments of even mass and odd mass molecular ions yields fragments of odd mass, it indicates that a rearrangement has taken place.
In case the mass difference between expected and the observed fragment ion is of one unit, rearrangement involving migration of hydrogen atom can be expected.
Mc Lafferty Rearrangement Mechanism
Mc Lafferty Rearrangement involves a six-membered ring transition state in which the carbonyl group pulls off the gamma proton, splitting the molecule into two pieces. These pieces consist of an enol radical cation and a neutral alkene fragment. The enol radical cation is observed in the mass spectrum while neutral alkene fragment is not observed.
Double Mc Lafferty Rearrangement
A double Mc Lafferty rearrangement is also known in certain ketones. The second hydrogen atom originates exclusively from the γ-position. A secondary hydrogen is preferred to a primary hydrogen atom in this process.
