Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/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 mass spectrometry 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 C-C-(O::)-H > C-C-(O:.+)-H > C' + (C=O:+)-H where : is a lone pair + is a positive charge and ' is a radical/free electron

In molecules containing carbonyl groups, 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. http://books.google.com/books?id=fjy2oJuJ_0kC&pg=PA315&dq=%22Alpha+cleavage%22+%22carbon-carbon+bond%22&lr=lang_en&num=20&as_brr=3&ei=LsVySOaJNZOkiwGD_LgO&sig=ACfU3U3IlpMzSiZN4SPDARQjeuWcUVZLyA. 
  2. "α-cleavage (alpha-cleavage)". IUPAC Compendium of Chemical Terminology (Gold Book). IUPAC. http://goldbook.iupac.org/A00004.html. 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: 1541. doi:10.1351/pac199163101541. 
  4. a b International Union of Pure and Applied Chemistry. "α-cleavage (alpha-cleavage)". Compendium of Chemical Terminology Internet edition.
  5. Verhoeven, J. W. (1996). "Glossary of terms used in photochemistry (IUPAC Recommendations 1996)". Pure and Applied Chemistry 68: 2223. doi:10.1351/pac199668122223.