LiAlH4

Meaning: LiAlH₄ (lithium aluminum hydride, LAH) is a source of nucleophilic hydride, H⁻, delivered from the AlH₄⁻ anion. The hydride attacks the electrophilic carbonyl carbon, forming an alkoxide, and a separate aqueous acid workup then protonates it to give the alcohol. Carbon gains C–H bonds, so the carbon is reduced. Because Al–H is more polar and weaker than B–H, LAH is a much stronger reducing agent than NaBH₄.

On the MCAT: Expect reaction-prediction questions with a product to choose. Aldehyde → primary alcohol. Ketone → secondary alcohol. Ester → two alcohols (a primary alcohol from the acyl part, plus the alcohol from the leaving group). Carboxylic acid → primary alcohol. Amide → amine (C=O becomes CH₂). Nitrile → primary amine. Passages often compare LAH with NaBH₄, which reduces only aldehydes and ketones, to test selectivity. A molecule with both a ketone and an ester shows the difference: NaBH₄ reduces only the ketone, while LAH reduces both.

Common confusion: Students think LAH can be used in water or alcohol like NaBH₄. It reacts violently with protic solvents, releasing H₂, so it is used in dry ether or THF, followed by a separate acid workup step. Another trap: LAH does not reduce isolated C=C bonds, so an alkene elsewhere in the molecule stays intact. Also remember that carboxylic acids and esters go all the way to alcohols, not to aldehydes.