Oxidation

The carbon atom of a carbonyl group has a relatively high oxidation state. This is reflected in the fact that most of the reactions described thus far either cause no change in the oxidation state (e.g. acetal and imine formation) or effect a reduction (e.g. organometallic additions and deoxygenations). The most common and characteristic oxidation reaction is the conversion of aldehydes to carboxylic acids. In the shorthand equation shown here the [O] symbol refers to unspecified oxidation conditions which effect the desired change. Several different methods of accomplishing this transformation will be described here.

RCH=O   +   [O]     RC(OH)=O

In discussing the oxidations of 1º and 2º-alcohols, we noted that Jones' reagent (aqueous chromic acid) converts aldehydes to carboxylic acids, presumably via the hydrate. Other reagents, among them aqueous potassium permanganate and dilute bromine, effect the same transformation. Even the oxygen in air will slowly oxidize aldehydes to acids or peracids, most likely by a radical mechanism. Useful tests for aldehydes, Tollens' test, Benedict's test & Fehling's test, take advantage of this ease of oxidation by using Ag⁽⁺⁾ and Cu⁽²⁺⁾ as oxidizing agents (oxidants).

RCH=O   +   2 [Ag⁽⁺⁾ OH^(–)]     RC(OH)=O   +   2 Ag (metallic mirror)   +   H₂O

When silver cation is the oxidant, as in the above equation, it is reduced to metallic silver in the course of the reaction, and this deposits as a beautiful mirror on the inner surface of the reaction vessel. The Fehling and Benedict tests use cupric cation as the oxidant. This deep blue reagent is reduced to cuprous oxide, which precipitates as a red to yellow solid. All these cation oxidations must be conducted under alkaline conditions. To avoid precipitation of the insoluble metal hydroxides, the cations must be stabilized as complexed ions. Silver is used as its ammonia complex, Ag(NH₃)₂⁽⁺⁾, and cupric ions are used as citrate or tartrate complexes.

Saturated ketones are generally inert to oxidation conditions that convert aldehydes to carboxylic acids. Nevertheless, under vigorous acid-catalyzed oxidations with nitric or chromic acids ketones may undergo carbon-carbon bond cleavage at the carbonyl group. The reason for the vulnerability of the alpha-carbon bond will become apparent in the following section.