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 hydride reductions, the resulting alkoxide salts are insoluble and need to be protonated (with care) before the alcohol product can be isolated. In the sodium borohydride reduction the ethanol solvent system achieves this hydrolysis automatically. In the lithium aluminum hydride reduction water is usually added in a second step. The lithium, sodium, boron and aluminum 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.

Mechanism

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

1) Nucleophilic attack by the hydride anion

2) The alkoxide is protonated