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Oxidation of Alcohols

Oxidation of Alcohols to Aldehydes or Ketones: Introduction

The outcome of the oxidation of an alcohol depends on the type of oxidizing agent used and on the substituents at the carbon atom bearing the OH group.

  • Primary alcohols are first converted to aldehydes which are frequently oxidized further to carboxylic acids in a fast reaction.
  • Secondary alcohols yield the corresponding ketones.
  • Tertiary alcohols cannot be oxidized unless harsh conditions are used that cause complete structural degradation.

All reactions are reversible under reductive conditions.

Fig.1

Selective oxidation of primary alcohols to aldehydes is the most difficult preparation to be carried out. In most cases, further oxidation to carboxylic acid is being observed even under mild conditions and the use of only one equivalent of oxidizing agent. Since the oxidation of primary alcohols to aldehydes is a synthetically important reaction, many attemps have been made to develop reagents for just this purpose. Unfortunately, no generally applicable method is available; however many different ones have been used. These are very often named after their discoverers (Jones, Swern, Collins, Dess-Martin, PCC, PDC, etc.). Usually, several methods have to be tried to find the best reaction conditions for the conversion. Commonly used and easily carried out is the reaction with dipyridine Cr(VI) oxide (Collins reagent).

Fig.2

The use of chromium(VI) reagents in aqueous media most often leads directly to carboxylic acids or in the case of secondary alcohols to ketones.

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