Reduction of Carboxylic Acids, Aldehydes & Ketones (DP IB Chemistry): Revision Note

Reduction of carboxylic acids, aldehydes & ketones

Carbonyl compounds

• Alcohols can be oxidised to carbonyl compounds in the presence of a suitable oxidising agent

  • Primary alcohol → aldehyde → carboxylic acid

  • Secondary alcohol → ketone

  • Tertiary alcohol - no reaction

• These reactions can be reversed in the presence of a suitable reducing agent

  • Carboxylic acid → aldehyde → primary alcohol

  • Ketone → secondary alcohol

• The two most common reducing agents for carbonyl compounds are:

  • Lithium aluminium hydride, LiAlH₄

    • Used in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid

    • Strong enough to reduce carboxylic acids as well as aldehydes and ketones

  • Sodium borohydride, NaBH₄

    • Used in aqueous or alcoholic solutions

    • Less reactive and safer to handle, but it can only reduce aldehydes and ketones, not carboxylic acids

  • Both of these reagents produce the nucleophilic hydride ion, H^– 

Reduction reactions

• Equations for reduction reactions can be written using [H] to represent the reducing agent

Carboxylic acids

• Carboxylic acids are reduced to primary alcohols

  • This requires heating with LiAlH₄ under reflux in dry ether followed by dilute acid

  • Remember that NaBH₄ cannot reduce carboxylic acids

Reduction of a carboxylic acid

Aldehydes

• Aldehydes are reduced to primary alcohols

  • This can be with LiAlH₄ or NaBH₄ 

Reduction of an aldehyde

Ketones

• Ketones are reduced to secondary alcohols

  • This can be with LiAlH₄ or NaBH₄ 

Reduction of a ketone