The behaviour of some α,α′-dibromoketones on electron impact. An analogy to the reductive removal of bromine in solution

The relative importance of the rearrangement ions M ? Br ? CO]⁺, M ? Br₂ ? CO]⁺ and M ? HBr₂ ? CO]⁺ in the mass spectra of the title compounds is compared with the amounts of α-methoxyketone formed on reduction of these compounds with a Zn/Cu couple in methanol. It is suggested that the quantitative correlation found reflects the electron releasing powers of the substituents on the α carbons.