Alpha cleavage, (α-cleavage) in organic chemistry, refers to the act of breaking the carbon-carbon bond, adjacent to the carbon bearing a specified functional group.
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Mass spectrometry
Generally this topic is discussed when covering tandem mass spectrometry fragmentation and occurs generally by the same mechanisms.
For example, of a mechanism of alpha cleavage, an electron is knocked off an atom (usually by electron collision) to form a radical cation. Electron removal generally happens in the following order: 1) lone pair electrons, 2) pi bond electrons, 3) sigma bond electrons.
One of the lone pair electrons moves down to form a pi bond with an electron from an adjacent (alpha) bond. The other electron from the bond moves to an adjacent atom (not one adjacent to the lone pair atom) creating a radical. This creates a double bond adjacent to the lone pair atom (oxygen is a good example) and breaks/cleaves the bond from which the two electrons were removed.
In molecules containing carbonyl groups, alpha cleavage often competes with McLafferty rearrangement.
Photochemistry
In photochemistry, it is the homolytic cleavage of a bond adjacent to a specified group.