Norrish reaction

The Norrish reaction in organic chemistry describes the taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions. The reaction is named after Ronald George Wreyford Norrish.

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 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.

A Norrish type II reaction is the photochemical intramolecular abstraction of a γ-hydrogen (a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct. Norrish first reported the reaction in 1937.

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