How the McLafferty Rearrangement Affects Carbonyl Fragmentation in Mass Spectrometry
After molecules are ionized in a mass spectrometer, they become radical cations that can then be sorted and weighed. Some of these radical cations can also rearrange after being ionized. The most famous rearrangement is called the McLafferty rearrangement.
A radical cation is a cation with an unpaired electron. After ionization, a radical cation breaks apart into a cationic piece (which is seen by the mass spectrometer’s detector) plus a neutral radical piece (not seen by the detector).
The McLafferty rearrangement can occur on carbonyl compounds (such as ketones and aldehydes) that have a hydrogen on a carbon that’s three carbons away from the carbonyl group. This third carbon position is called the gamma
position. The rearrangement involves a six-membered ring transition state in which the carbonyl group pulls off the gamma proton, breaking the molecule into two pieces. These pieces consist of an enol radical cation (recall that an enol is a combination of an alkene and an alcohol) and a neutral alkene fragment (a fragment with a carbon double bond). The enol radical cation is observed in the mass spectrum, while the neutral alkene fragment is not observed. Any carbonyl compound that has a hydrogen in the
position is likely to have a peak in the mass spectrum corresponding to the enol radical cation that’s formed by the McLafferty rearrangement of that carbonyl compound. An example of the McLafferty rearrangement is shown here.