A Norrish reaction in organic chemistry is a photochemical reaction taking place with ketones and aldehydes. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions.[1] The reaction is named after Ronald George Wreyford Norrish. While of limited synthetic utility these reactions are important in the photo-oxidation of polymers such as polyolefins,[2] certain polycarbonates and polyketones.
. . . Norrish reaction . . .
The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates (α-scission). 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.[3] 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.[4]
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.
- The acyl radical can lose a molecule of carbon monoxide, forming a new carbon radical at the other α-carbon, followed by formation of a new carbon–carbon bond between the radicals.[3] The ultimate effect is simple extraction of the carbonyl unit from the carbon chain. The rate and yield of this product depends upon the bond-dissociation energy of the ketone’s αsubstituents. Typically the more α substituted a ketone is, the more likely the reaction will yield products in this way.[5][6]
- 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.[7]
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