The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. Photochemical excitation of ketones usually causes the homolytic fission of the α-carbon-carbon bonds
Norrish type I reaction example
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 depend 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.