Carboxylic Acids to Primary Alcohols: Hydride Reduction

LiAlH4 acts as both a nucleophile and strong base in carboxylic acid reduction. It deprotonates the carboxylic acid to form a carboxylate salt, AlH3, and hydrogen gas. The AlH3 then transfers a hydride to the carbonyl carbon, initiating the reduction sequence that ultimately yields a primary alcohol after hydrolysis.

The aldehyde intermediate is unisolable because it is more reactive than the carboxylate anion from which it forms. Instead of accumulating, the aldehyde is immediately reduced by excess LiAlH4 to an alkoxide, which is then protonated during hydrolysis to yield the final primary alcohol product.

During reduction, AlH3 transfers a hydride to the carbonyl carbon of the carboxylate ion, creating a tetrahedral intermediate with an O–Al bond. Upon subsequent elimination, this intermediate generates an aldehyde. This intermediate is a key structural feature that bridges the carboxylate and aldehyde stages of the reduction pathway.

Borane in tetrahydrofuran can selectively reduce carboxylic acids to primary alcohols in the presence of other reducible functional groups such as ketones or nitro groups. In contrast, LiAlH4 is a non-selective strong reducing agent. This selectivity makes borane advantageous when multiple reducible groups are present in the same molecule.

Weaker reducing agents like lithium tri-tert-butoxyaluminum hydride or diisobutylaluminum hydride cannot reduce carboxylic acids to primary alcohols because they lack sufficient reducing power. Only strong reducing agents such as LiAlH4 or borane possess the necessary reactivity to cleave the C=O bond and reduce the carboxylic acid functional group to a primary alcohol.

The final step is hydrolysis of the alkoxide intermediate. After the aldehyde is reduced by LiAlH4 to form an alkoxide, aqueous workup protonates the alkoxide, yielding the primary alcohol product. This hydrolysis step is essential for isolating the desired alcohol from the reaction mixture.

Borane in THF selectively reduces carboxylic acids while tolerating other reducible functional groups present in the molecule. Ketones and nitro groups are examples of functional groups that remain unreacted when borane is used, making this reagent ideal for selective reduction in complex molecules containing multiple reactive sites.