Aspects of mass spectra of organic compounds. Part X. Rationalization of a variety of electron-impact induced rearrangements of ions

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Abstract
A wide variety of electron-impact induced rearrangements can be rationalized by one four-centre reaction scheme, [AB–C–D]+˙→[D–A–BC]+˙. In amides and derivatives where A is oxygen, B is carbon, and C is nitrogen, the migrating group D can be aryl, aroyl, methylene or substituted methylene, anilino, sulphonyl, or phosphorus. The behaviour of cyclic compounds where B is joined to D, including the loss of CO2 from cyclic imides and the loss of SO3 from saccharin and derivatives, is discussed. Competition between rearrangement and ‘simple’ cleavage processes is shown by a series unsubstituted and nitro- and methoxy-substituted N-methylbenzanilides. Rearrangement and fragmentation of thionoesters and derivatives, where A is sulphur and C is oxygen, fits the general scheme and in this case the migrating group D can be alkyl. The corresponding rearrangement of thioloesters (A is oxygen and C is sulphur) was negligible, in contrast with a report in the literature. Some rearrangements of azoxy-compounds and thiocarbonates are incorporated into the scheme. From the results we make the generalisation that: For rearrangement according to the above equation: (i) when A is oxygen, C must be nitrogen; (ii) when A is sulphur, C can be nitrogen or oxygen; (iii) B can be carbon, phosphorus, or nitrogen. Rearrangement is negligible when C is sulphur. The generalisation can be used as an aide-mémoîre for known rearrangement processes and as an indication of possible rearrangements of unexamined compounds.

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