Alpha cleavage

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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.

Photochemistry[edit]

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

References[edit]

  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.