Alpha cleavage

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Alpha cleavage (α-cleavage) in organic chemistry refers to the act of breaking the carbon-carbon bond[1] adjacent to the carbon bearing a specified functional group.[2]

Mass spectrometry[edit]

Generally this topic is discussed when covering tandem mass spectrometry fragmentation and occurs generally by the same mechanisms.[3][4]

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.

Example of alpha cleavage

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


In photochemistry, it is the homolytic cleavage of a bond adjacent to a specified group.[4][5]


  1. ^ Hathaway, Bruce A. (2005). Organic chemistry the easy way. Woodbury, N.Y: Barron's Educational Series. p. 315. ISBN 0-7641-2794-2. 
  2. ^ "α-cleavage (alpha-cleavage)". IUPAC Compendium of Chemical Terminology (Gold Book). IUPAC. Archived from the original on 20 July 2008. Retrieved 2008-07-08. 
  3. ^ Todd, J. F. J. (1991). "Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)". Pure and Applied Chemistry. 63 (10): 1541. doi:10.1351/pac199163101541. 
  4. ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "α-cleavage (alpha-cleavage)".
  5. ^ Verhoeven, J. W. (1996). "Glossary of terms used in photochemistry (IUPAC Recommendations 1996)". Pure and Applied Chemistry. 68 (12): 2223. doi:10.1351/pac199668122223.