Four-centred rearrangements in the mass spectra of aliphatic ethers

The CH₃O?CHCH₃]⁺ ions observed in the mass spectra of ethers of formula CH₃OCH (CH₃)R(R = H or alkyl) undergo two rearrangement fragmentation reactions to form C₂H₅]⁺ and CH₂OH]⁺. The scope of the rearrangements has been investigated and it is shown that enlargement of the alkyl group on either side of the ether linkage leads to alternative fragmentation routes. From a study of metastable intensities it is concluded that the fragmentations probably occur directly from the CH₃O?CHCH₃]⁺ structure through four centred rearrangements rather than through the intermediacy of the C₂H₅O?CH₂]⁺ ion.