The McLafferty rearrangement is a fragmentation reaction observed in mass spectrometry of organic molecules in which the parent radical cation splits into daughter radical cation and neutral spin-paired molecule. It is sometimes found that a molecule containing a keto-group undergoes ò-cleavage, with the gain of the ó-hydrogen atom, as first reported by Anthony Nicholson working in the Division of Chemical Physics at the CSIRO in Australia. This rearrangement is a retro-ene reaction, and a member of the sigmatropic family of reactions.
A description of the reaction was later published by the American chemist Fred McLafferty in 1959 leading to his name being associated with the process.