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The Norrish reaction in organic chemistry describes the photochemical reactions taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions.〔''Named Organic Reactions'', 2nd Edition, Thomas Laue and Andreas Plagens, John Wiley & Sons: Chichester, England, New York, 2005. 320 pp. ISBN 0-470-01041-X〕 The reaction is named after Ronald George Wreyford Norrish. == Type I == The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state. Through intersystem crossing the triplet state can be obtained. On cleavage of the α-carbon bond from either state, two radical fragments are obtained. The size and nature of these fragments depends upon the stability of the generated radicals; for instance, the cleavage of 2-butanone largely yields ethyl radicals in favor of less stable methyl radicals. Several secondary reaction modes are open to these fragments depending on the exact molecular structure. * The fragments can simply recombine to the original carbonyl compound, with racemisation at the α-carbon. * Two organic residues can recombine with formation of a new carbon-carbon bond, with the loss of carbon monoxide.〔 The rate and yield of this product depends upon the bond-dissociation energy of the ketone's α substituents. Typically the more α substituted a ketone is, the more likely the reaction will yield products in this way. * The abstraction of an α-proton from the carbonyl fragment may form a ketene and an alkane. * The abstraction of a β-proton from the alkyl fragment may form an aldehyde and an alkene. The synthetic utility of this reaction type is limited, for instance it often is a side reaction in the Paternò–Büchi reaction. One organic synthesis based on this reaction is that of bicyclohexylidene.〔''Bicyclohexylidene'' Nicholas J. Turro, Peter A. Leermakers, and George F. Vesley Organic Syntheses, Coll. Vol. 5, p.297 (1973); Vol. 47, p.34 (1967) (Online article ).〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Norrish reaction」の詳細全文を読む スポンサード リンク
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