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The enolic [C5H10O]+· and [C3H6O]+· ions formed from aliphatic ketones

✍ Scribed by J. H. Beynon; R. M. Caprioli; R. G. Cooks

Publisher
John Wiley and Sons
Year
1974
Tongue
English
Weight
743 KB
Volume
9
Category
Article
ISSN
1076-5174

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✦ Synopsis

The structures of reactive and nonreactive ions formed by McLafferty rearrangement in a variety of ketones are studied using metastable ion abundances and the kinetic energy released in metastable ion fragmentations. The loss of methyl radicals is studied in [C,H,,O]+. ions and the mechanism delineated for ions formed in an initial configuration corresponding both to ketonic and enolic forms; the major mechanism in both cases appears to involve a common intermediate. Partial scrambling of hydrogen atoms on the C-1 and C-5 carbons and on the oxygen precedes methyl elimination. Loss of neutral ethylene from [CSH,,O]+. ions is also studied and again the major mechanism occurs via a common species, intermediate in structure between a ketone and an enol; scrambling of hydrogen is much faster for ions that lose ethylene than for those which lose a methyl radical. Keto ions of formula [C,H,O]+. show no unimolecular loss of methyl. The stable [C,H,O]+. ions generated in the keto and enol forms are shown to preserve their initial structures, confirming the results of ion cyclotron resonance experiments. Reactive [C3H,0]+. ions formed from 2-pentanone molecular ions by McLafferty rearrangement are shown to have the same structures as reactive ions originally generated as enolic structures from 1-methylcyclobutanol.

The paper highlights the relationship between hydrogen scrambling and keto-enol isomerization reactions in aliphatic ketones.

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