Obtention of Aldehydes by Jones Oxidation

Jones oxidation is generally not useful for the transformation of primary alcohols into aldehydes. This is due to the equilibrium of the aldehydes with the corresponding hydrates in the aqueous media, leading to the subsequent oxidation of the aldehyde hydrates into carboxylic acids. In fact, kinetic studies support the assumption that chromic acid oxidizes aldehydes into carboxylic acids via the corresponding aldehyde hydrates.5 

Nevertheless, in those cases in which the proportion of hydrate in equilibrium with the aldehyde is low, it is possible to obtain a useful yield of aldehyde.60,61 Electron donating groups,68,69 conjugation with alkenes and aromatic rings5 and steric hindrance69 decrease the proportion of hydrates in equilibrium with aldehydes. This explains the fact that alcohols successfully transformed into aldehydes by Jones oxidation, normally belong to the allyl,70 benzyl71 or neopentyl kind.72 

In simple molecules, it is possible to obtain a good yield of aldehyde— including examples possessing an important proportion of hydrate in equilibrium—by continuous distillation of the aldehyde from the reaction mixture.73 This procedure only succeeds in the preparation of simple volatile aldehydes. 

The obtention of aldehydes can be facilitated by the use of ethyl methyl ketone,74 instead of acetone, due to the lower polarity of the former, leading to a decreased concentration of aldehyde hydrate. 

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