The most common sources of the hydride nucleophile are lithium aluminium hydride (LiAlH₄) and sodium borohydride (NaBH₄). Note! The hydride anion is not present during this reaction; rather, these reagents serve as a source of hydride due to the presence of a polar metal-hydrogen bond. Because aluminium is less electronegative than boron, the Al-H bond in LiAlH₄ is more polar, thereby, making LiAlH₄ a stronger reducing agent.

Addition of a hydride anion (H:^–) to an aldehyde or ketone gives an alkoxide anion, which on protonation yields the corresponding alcohol. Aldehydes produce 1º-alcohols and ketones produce 2º-alcohols.

In metal hydrides reductions the resulting alkoxide salts are insoluble and need to be hydrolyzed (with care) before the alcohol product can be isolated. In the sodium borohydride reduction the methanol solvent system achieves this hydrolysis automatically. In the lithium aluminium hydride reduction water is usually added in a second step. The lithium, sodium, boron and aluminium end up as soluble inorganic salts at the end of either reaction.  Note! LiAlH₄ and NaBH₄ are both capable of reducing aldehydes and ketones to the corresponding alcohol.

Examples

Mechanism

This mechanism is for a LiAlH₄ reduction.  The mechanism for a NaBH₄ reduction is the same except methanol is the proton source used in the second step.

1) Nucleophilic attack by the hydride anion

2) The alkoxide is protonated

Going from Reactants to Products Simplified