REDUCTION AND OXIDATION
Reduction of an aldehyde with sodium borohydride or lithium aluminum hydride gives a primary alcohol. Similar reduction of a ketone gives a secondary alcohol.
There are three methods of deoxygenating aldehydes and ketones. The method used depends on whether the compound is sensitive to acid or base. If sensitive to acid, reduction is carried out under basic conditions by the Wolff–Kishner reduction. If sensitive to base, the reaction is carried out under acid conditions – the Clemmenson reduction. If sensitive to both acid and base, the carbonyl group is converted to a dithioacetal or dithioketal then reduced with Raney nickel.
Aldehydes can be oxidized to carboxylic acids, but ketones are resistant to oxidation.
Aldehydes and ketones can be reduced to alcohols with a hydride ion – provided by reducing reagents such as sodium borohydride or lithium borohydride. Primary alcohols are obtained from aldehydes, and secondary alcohols from ketones.
Aldehydes and ketones can be reduced to alkanes by three different methods which are complementary to each other. The Wolff–Kishner reduction is carried out under basic conditions and is suitable for compounds that might be sensitive to acid (Fig. 1). The reaction involves the nucleophilic addition of hydrazine followed by elimination of water to form a hydrazone. The mechanism is the same as that described for the synthesis of 2,4-dinitrophenylhydrazones.
However, the simple hydrazone formed under these reaction conditions spontaneously decomposes with the loss of nitrogen gas.
The Clemmenson reduction (Fig. 2) gives a similar product but is carried out under acid conditions and so this is a suitable method for compounds which are unstable to basic conditions.
Compounds which are sensitive to both acid and base can be reduced under neutral conditions by forming the thioacetal or thioketal, then reducing with Raney nickel (Fig. 3).
Aromatic aldehydes and ketones can also be deoxygenated with hydrogen over a palladium charcoal catalyst. The reaction takes place because the aromatic ring activates the carbonyl group towards reduction. Aliphatic aldehydes and ketones are not reduced.
Ketones are resistant to oxidation whereas aldehydes are easily oxidized. Treatment of an aldehyde with an oxidizing agent results in the formation of a carboxylic acid (Fig. 4a). Some compounds may be sensitive to the acid conditions used in this reaction and an alternative way of carrying out the oxidation is to use a basic solution of silver oxide (Fig. 4b).
Both reactions involve the nucleophilic addition of water to form a 1,1-diol or hydrate which is then oxidized in the same way as an alcohol (Fig. 5);.
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