Carboxylic acid reduction
Abstract
The reduction of carboxylic acids into the corresponding primary alcohols requires a powerful hydride (typically, LiAlH4), owing to the reduced electrophilicity of the carbonyl carbon caused by resonance donation from the adjacent oxygen atom.
Keywords
Carboxylic acid, Reduction, Primary alcohol
Category
Carboxylic acid
Mechanism
Here, LiAlH4 acts as a hydride donor but also as a base. First, the carboxylic acid is deprotonated by LiAlH4, giving the corresponding carboxylate ion. Then, a hydride attacks the electrophilic carbonyl carbon, breaking the C=O bond and transferring the electron pair to the oxygen atom, which becomes an alkoxide coordinated to the metal center. The resulting tetrahedral intermediate then collapses with departure of aluminium-coordinated oxygen atom, generating an aldehyde intermediate. This aldehyde is immediately reduced by a second hydride delivery, breaking the C=O bond and giving a second aluminium alkoxide intermediate. Finally, the metal alkoxide is hydrolyzed during aqueous workup, releasing the primary alcohol and aluminium hydroxide as by-product.
