Reduction of Carbonyls (AQA A-Level Chemistry): Revision Notes

7.2.2 Reduction of Carbonyls

Reduction of Carbonyl Compounds

The reduction of carbonyl compounds (aldehydes and ketones) is a key reaction in organic chemistry. This reaction involves the conversion of carbonyl groups ($C=O$) into alcohols, which occurs through nucleophilic addition using a reducing agent. Below is a structured breakdown of this topic:

Overview of Reduction Reactions in Carbonyl Compounds

• Aldehydes can be reduced to form primary alcohols.
• Ketones can be reduced to form secondary alcohols.
• The reducing agent commonly used for this purpose is sodium borohydride ($NaBH₄$), also known as sodium tetrahydridoborate.

Sodium Borohydride ($NaBH₄$) as a Reducing Agent

• $NaBH₄$ is a selective reducing agent that provides hydride ions ($H⁻$).
• In aqueous solution, $NaBH₄$ releases $H⁻$ions which act as nucleophiles, attacking the electron-deficient carbon in the carbonyl group.
• $NaBH₄$ is preferred in laboratory settings because it selectively reduces the carbonyl group without affecting other double bonds, such as $C=C$, due to their high electron density.

Mechanism of the Reduction Reaction: Nucleophilic Addition

• These reduction reactions of carbonyl compounds are classic examples of nucleophilic addition:
  1. The hydride ion ($H⁻$) from $NaBH₄$ acts as a nucleophile and is attracted to the partially positive carbon in the $C=O$ group.
  2. The hydride ion donates a pair of electrons to the carbon, which reduces the carbonyl to a single bond.
  3. A proton ($H⁺$) from water or another solvent then protonates the negatively charged oxygen, forming an alcohol.