Reduction with LiAlH4

Aldehydes and ketones are reduced to alcohols by lithium tetrahydridoaluminate(III), LiAlH₄ (lithium aluminium hydride). It is a powerful reducing agent and a source of hydride ions, H⁻.Conditions: LiAlH₄ in dry ether (ethoxyethane) at room temperature, followed by the careful addition of dilute acid to release the alcohol. The ether must be dry because LiAlH₄ reacts violently with water, producing hydrogen.In equations the reducing agent is written as [H].Aldehydes are reduced to primary alcohols:
RCHO + 2[H] → RCH₂OH
Ketones are reduced to secondary alcohols:
RCOR' + 2[H] → RCH(OH)R'This is the reverse of the oxidation of alcohols to carbonyl compounds. The carbon skeleton does not change: only the C=O group becomes CH(OH).For example, ethanal gives ethanol, and propanone gives propan-2-ol:
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃Worked example: 3.60 g of butanone, CH₃COCH₂CH₃ (M_r = 72.0), is reduced with LiAlH₄. Amount of butanone = 3.60 ÷ 72.0 = 0.0500 mol. The product is the secondary alcohol butan-2-ol (M_r = 74.0), formed in a 1 : 1 ratio, so the maximum mass is 0.0500 × 74.0 = 3.70 g.