Reduction of Carboxylic Acids

Carboxylic acids are reduced to primary alcohols by lithium tetrahydridoaluminate(III), LiAlH₄.Conditions: LiAlH₄ in dry ether at room temperature, followed by the careful addition of dilute acid. The ether must be dry because LiAlH₄ reacts violently with water.The COOH group is reduced to CH₂OH. The reaction passes through an aldehyde, but the aldehyde is reduced more easily than the acid, so it cannot be isolated and the product is the primary alcohol. Using [H] for the reducing agent:RCOOH + 4[H] → RCH₂OH + H₂OFour [H] are needed: two to reduce the acid to the aldehyde (forming water) and two more to reduce the aldehyde to the alcohol.Carboxylic acids are harder to reduce than aldehydes and ketones. Sodium tetrahydridoborate(III), NaBH₄, reduces aldehydes and ketones but does not reduce carboxylic acids, so the more powerful LiAlH₄ is needed.The number of carbon atoms does not change: propanoic acid gives propan-1-ol, and benzoic acid gives phenylmethanol, C₆H₅CH₂OH.Worked example: ethanoic acid is reduced with LiAlH₄:
CH₃COOH + 4[H] → CH₃CH₂OH + H₂O
6.00 g of ethanoic acid (M_r = 60.0) is 0.100 mol. The ratio is 1 : 1, so the maximum amount of ethanol is 0.100 mol, which is 0.100 × 46.0 = 4.60 g.