Is the McLafferty Rearrangement of Ketones Concerted or Stepwise? The Application of Kinetic Isotope Effects

Article abstract of DOI:10.1002/oms.1210270319

Intramolecular 13C and 2H isotope effects have been measured for unimolecular losses of ethene (the McLafferty rearrangement) from metastable molecular ions of 2-ethyl-1-phenylbutan-1-one, 3-ethylpentan-2-one and heptan-4-one.Primary and secondary deuterium isotope effects are observed at the γ-(terminal) and β-positions, respectively.Large primary 13C isotope effects occur at β-positions and for the γ positions of 3-ethylpentan-2-one and heptan-4-one.The carbon isotope effects in the cases of the doubly isotopically labelled CH3COCH(C2H5)((13)CH2CH3) and CD3COCD(C2D5)((13)CD2CD3) are 1.17 (+/-0.01) and 1.04 (+/-0.01), respectively.All of these isotope effects are consistent with a stepwise mechanism in which more than one step is rate determining.