Alcohols from Carbonyl Compounds: Reduction

Three major pathways exist: catalytic hydrogenation using transition metal catalysts like Raney Ni or Pd–C under mild to moderate heat and pressure; hydride reduction using sodium borohydride or lithium aluminum hydride as nucleophilic reagents; and borane reduction using borane solutions to selectively reduce electron-rich carbonyl groups. Each method offers different selectivity and reactivity profiles for various functional groups.

Lithium aluminum hydride is a stronger reducing agent due to greater polarity of the aluminum–hydrogen bond, making it more reactive and capable of reducing aldehydes, ketones, esters, acids, and acyl chlorides. Sodium borohydride is milder and selectively reduces aldehydes and ketones in the presence of esters, alkyl halides, and nitro groups. LAH requires dry aprotic solvents and reacts violently with water, while NaBH4 works in protic solvents like ethanol.

Aldehydes are reduced to primary alcohols, while ketones are reduced to secondary alcohols. In unsymmetrical ketones, the planar carbonyl group can undergo nucleophilic hydride attack from either face with equal probability, generating a chiral tetrahedral intermediate and producing a pair of enantiomeric products as a racemic mixture.

Catalytic hydrogenation adds hydrogen across the carbonyl double bond but also reduces any carbon–carbon multiple bonds present in the molecule, making it non-selective. Hydride reduction from sodium borohydride or lithium aluminum hydride selectively targets the carbonyl group through nucleophilic attack, leaving other functional groups intact depending on the reagent's reactivity profile.

Nucleophilic hydride attack forms alkoxide ions as the first step. The byproduct alkoxyborohydride or alkoxyaluminate then reduces three additional carbonyl molecules by successively transferring all hydrogen atoms. Since hydride is a poor leaving group, these hydride transfer steps are irreversible, driving the reaction to completion. The reaction mixture is then worked up with solvent or acid to protonate the alkoxide.

Borane reduction selectively reduces electron-rich carbonyl groups like carboxylic acids in the presence of other reducible functional groups such as esters and even ketones. The formation of a reactive triacylborate intermediate with a more electrophilic carbonyl group than the starting ester molecule drives the reduction reaction forward, enabling chemoselectivity unavailable with other hydride reagents.

Lithium aluminum hydride reacts violently with water and other protic solvents, liberating hydrogen gas and forming metal hydroxides or alkoxides. To prevent this dangerous decomposition and maintain the reagent's reducing power, LAH reductions are carried out in dry aprotic solvents like anhydrous diethyl ether or THF, ensuring safe and effective carbonyl reduction.