Cleavage of Non-Symmetrical 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 alkyls at the carbonyl, Stevenson s rule may also be applied to decide which of them will dominantly be detected as part of the acylium ion and which should preferably give rise to a carbenium ion. Overall, a nonsymmetrical ketone will yield four primary fragment ions in its El mass spectrum 

Example The 70 eV El mass spectrum of butanone shows an ethyl loss, m/z 43, which is largely preferred over methyl loss, m/z 57 (Fig. 6.5). Furthermore, the C2Hs ion, in/z 29, is more abundant than the less stable CHs ion. 

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