Reduction of Carboxylic Acids, Aldehydes & Ketones (SL IB Chemistry)

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, in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid
    • This is the stronger of these reducing agents and can reduce carboxylic acids
  • Sodium borohydride, NaBH₄, in aqueous or alcoholic solutions
    • This is the less hazardous of these reducing agents but it cannot reduce 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

Carboxylic acids are reduced to primary alcohols with LiAlH₄ 

Aldehydes

• Aldehydes are reduced to primary alcohols
  • This can be with LiAlH₄ or NaBH₄ 

Reduction of an aldehyde

Aldehydes are reduced to primary alcohols with LiAlH₄ or NaBH₄ 

Ketones

• Ketones are reduced to secondary alcohols
  • This can be with LiAlH₄ or NaBH₄ 

Reduction of a ketone

Ketones are reduced to secondary alcohols with LiAlH₄ or NaBH₄