Reduction of Carbonyls (Oxford AQA International A Level Chemistry)

Reduction of Carbonyls

• Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols

• Possibly the most common reducing agent for this is sodium tetrahydridoborate, NaBH₄

  • You may also see this named as sodium borohydride in some sources

• In an aqueous solution, NaBH₄ generates the hydride ion nucleophile, :H^-

• The hydride ion will reduce a carbonyl group in an aldehyde or a ketone, but is not strong enough to reduce a C=C double bond

  • This is because it is attracted to the C in the C=O bond, but is repelled by the high electron density of the C=C bond

• When this reaction takes place, it is an example of a nucleophilic addition reaction

Aldehyde to a primary alcohol

Ketone to a secondary alcohol

Nucleophilic addition mechanism

• The nucleophilic addition mechanism involves two steps

• Step 1:

  • Nucleophilic attack: The lone pair of electrons from the nucleophile attacks the partially positive carbon atom on the carbonyl group

• Step 2:

  • New O-H bond forms: The lone pair of electrons from the negative oxygen atom attracts and forms a bond with H⁺ ion from the solvent

General mechanism for nucleophilic addition