Abstract
In the electron impact mass spectra of a series of cycloalkylacetones with ring sizes varying from cyclopropane to cycloheptane, the intensity of the McLafferty rearrangement ion at m/z 58 increases with the larger, less strained rings. The larger ring ketones give surprisingly intense “McLafferty plus one” ions at m/z 59, which is the base peak in the mass spectrum of cyclohexylacetone. This double hydrogen McLafferty rearrangement is favored over the single hydrogen rearrangement at both high (70 eV) and low (14 eV) ionizing voltages. Deuterium labelling confirms the expected considerable hydrogen migration in the cycloalkyl ring, occurring following hydrogen transfer to the carbonyl oxygen and prior to C-C bond cleavage.
