Alpha cleavage

Alpha cleavage, (α-cleavage) in organic chemistry, refers to the act of breaking the carbon-carbon bond, next to the carbon bearing a specified functional group.

Mass spectrometry
In mass spectrometry, an α-cleavage occurs when molecules break apart within a mass spectrometer. A mass spectrum tells the size and elements found in each molecule fragment.  Chemists use their understanding of how molecules break apart to guess the structure of the original molecule.  Many different molecules break apart with α-cleavage inside the mass spectrometer. Many times, α-cleavage explains the resulting mass spectra.

As an example of a mechanism of alpha cleavage, an electron is knocked off an atom (usually by electron collision) to form a radical cation. Electrons are most likely to be removed 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 that lost the two electrons that were removed.


In molecules containing carbonyl groups, alpha cleavage often competes with a McLafferty rearrangement.

Photochemistry
In photochemistry, it is the homolytic cleavage of a bond close to a specified group.

References

Category:Organic reactions
Category:Mass spectrometry