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Concept
Carbonyl reduction
Summary
In organic chemistry, carbonyl reduction is the organic reduction of any carbonyl group by a reducing agent.
Typical carbonyl compounds are ketones, aldehydes, carboxylic acids, esters, and acid halides. Carboxylic acids, esters, and acid halides can be reduced to either aldehydes or a step further to primary alcohols, depending on the strength of the reducing agent; aldehydes and ketones can be reduced respectively to primary and secondary alcohols. In deoxygenation, the alcohol can be further reduced and removed altogether.
Metal hydrides based on boron and aluminum are common reducing agents; catalytic hydrogenation is also an important method of reducing carbonyls. Before the discovery of soluble hydride reagents, esters were reduced by the Bouveault–Blanc reduction, employing a mixture of sodium metal in the presence of alcohols.
The reaction mechanism for metal hydride reduction is based on nucleophilic addition of hydride to the carbonyl carbon. In some cases, the alkali metal cation, especially Li+, activates the carbonyl group by coordinating to the carbonyl oxygen, thereby enhancing the electrophilicity of the carbonyl.
For reductions of carboxylic acid derivatives, after reduction by an aluminium hydride ion, an elimination leads to the aldehyde product (which can be reduced a second time to an alcohol):
For reductions of aldehydes and ketones, an aluminium hydride ion reduces the compound to form an alkoxide salt. After the complete reduction, the alkoxide is protonated to give the alcohol product:
Ketones are less reactive than aldehydes, because of greater steric effects, and because the extra alkyl group can donate electron density to the partial positive charge of the polar C=O bond. Therefore, aldehydes reduce more easily than ketones and require milder reagents and milder conditions. Carboxylic acids and esters are further stabilized by the presence of a second oxygen atom which can donate a lone pair into the already polar C=O bond. Acyl halides are the least stable of the carbonyls since halides are poor electron donors, as well as great leaving groups.
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