Reduction of Aldehydes and Ketones to Alcohols: Hydride Reagents

One of the most common laboratory methods used to reduce aldehydes and ketones is using hydride reagents. These are used more commonly than catalytic hydrogenation since these reagents can be used in the ambient atmosphere. They can also selectively reduce the carbonyl group in the presence of other multiple bonds.

Like catalytic hydrogenation, the hydride reagents reduce aldehydes to primary alcohol and the ketones to secondary alcohol. Some commonly used hydride reagents are lithium aluminum hydride (LiAlH₄) and sodium borohydride (NaBH₄), which generates the hydride ion that acts as a strong nucleophile that attacks the electrophilic carbonyl carbon. Since hydride ions can attack from either face of the carbonyl group, the unsymmetrical ketones can form chiral products. Although the hydride ions are strong nucleophiles, they are not strong enough to attack the non-electrophilic C=C bonds. Hence, the carbonyl group can be selectively reduced while keeping the C=C bond intact.

Among LiAlH₄ and NaBH₄, the LiAlH₄ is a stronger reducing agent since the Al-H bond is much stronger than the B-H bonds. Hence, LiAlH₄ can reduce almost all carbonyl compounds, whereas NaBH₄ can selectively reduce only aldehydes and ketones. Due to its violent reactive nature with water, the LiAlH₄ reactions are carried out in non-aqueous solvents.