13.15 : Carboxylic Acids to Primary Alcohols: Hydride Reduction

Carboxylic acids, upon treatment with LiAlH₄, are reduced to yield primary alcohols.

Here, the hydride reagent acts as a nucleophile as well as a strong base. It deprotonates the carboxylic acid to form a carboxylate salt, AlH₃, and hydrogen gas.

Next, the AlH₃ produced transfers a hydride to the carbonyl carbon of the carboxylate ion. This results in a tetrahedral intermediate featuring an O–Al bond, which upon subsequent elimination, generates an aldehyde.

The aldehyde formed is unisolable as it is more reactive than the carboxylate anion. Instead, it is immediately reduced by LiAlH₄ to an alkoxide.

Lastly, the alkoxide is protonated via hydrolysis to yield the primary alcohol.

Alternatively, carboxylic acids can also be reduced to primary alcohol using borane in THF. Borane can selectively reduce carboxylic acid in the presence of other reducible functional groups such as a ketone or a nitro group.