Reactions of hydroxycarbonium ions in strong acids IV: Kinetics of the rearrangements of protonated aldehydes and ketones

Abstract

AbstractA systematic study has been made of the kinetics of the rearrangements of protonated dialkyl ketones, aldehydes and α‐deuterated aldehydes. The rearrangements can be divided into four groups, the relative rates of which, at 25°, are approximately 107: 102: 102: 1, viz. (i) of α‐dibranched aldehydes to α‐mono‐branched ketones, (ii) of α‐monobranched aldehydes to di‐n‐alkyl ketones, (iii) of α‐dibranched ketones, (iv) of α‐monobranched ketones.Whereas the first three groups of reactions proceed by a two‐step process (1,2‐alkyl and/or hydrogen shifts via an α‐hydroxyalkyl carbonium ion), the interconversion of α‐monobranched ketones takes place by a four‐step process via tertiary α‐hydroxyalkyl carbonium ions and a protonated α‐dibranched aldehyde.The intramolecular exchange of α‐D and carbonyl‐H in deuterated isobutyraldehyde and propionaldehyde is about ten times slower than the rearrangements under (i) and (ii), respectively. This is due to the lower stability of the intermediate α‐hydroxyalkyl carbonium ions in the former two reactions; intrinsically, hydrogen shifts are faster than alkyl shifts. Other things being equal, the ethyl shift is about four times faster than the methyl shift.Protonated aldehydes are about 9 kcal.mole−1 less stable than the isomeric protonated ketones, which is mainly due to the difference in stability of the unprotonated parent compounds. Contrary to recently published results (ref. 13), the difference in pKa between aliphatic aldehydes and ketones is indicated to be only ∼ 1 unit.