Cleavage of Nonsymmetrical Aliphatic Ketones

When a ketone grows larger it does not necessarily imply that it has two identical alkyl groups at the carbonyl. In case of different alkyl groups, Stevenson s mle may help to decide which of them will mainly be detected as part of the acylium ion and which should preferably appear as carbenium ion. Overall, a nonsymmetrical ketone will yield four primary fragment ions in its El mass spectmm. 

Example At m/z 43, the El mass spectrum of butanone shows a fragment due to ethyl loss from the molecular ion, which is largely preferred over methyl loss leading to a minor peak at m/z 57 (Fig. 6.5). Furthermore, the C2H5 ion, tn/z 29, is more abundant than the less stable CHs ion, m/z 15. If the alkyls become larger than ethyl, an additional dissociation pathway will occur (Chap. 6.7). 

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