Norrish Reaction - Norrish I

Norrish 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 carbon bond from either state, two radical fragments are obtained.

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 (path A).
• By extrusion of carbon monoxide in path B, two organic residues can recombine with formation of a new carbon carbon bond
• When the carbon fragment has an α-proton available it gets abstracted forming a ketene and a saturated hydrocarbon in path C
• When the alkyl fragment contains a β-proton it gets abstracted with formation of an aldehyde and an alkene.

The synthetic utility of this reaction type is limited. It often is a side reaction for instance in the Paternò–Büchi reaction. One organic synthesis based on this reaction is that of bicyclohexylidene.