reducing agent
Summary
A reducing agent donates electrons (or hydride) to another species and is itself oxidized. In organic chemistry, common ones are NaBH₄, LiAlH₄ and H₂ with a metal catalyst. The key test point: LiAlH₄ is stronger and reduces esters and carboxylic acids, while NaBH₄ typically does not.
Detail
Meaning: A reducing agent (reductant) reduces another species by giving it electrons, so the agent is the one that is oxidized. In organic chemistry, reduction of a substrate usually means more C–H bonds or fewer C–O bonds at a carbon. Hydride donors such as NaBH₄ and LiAlH₄ deliver H⁻ to the carbonyl carbon, giving an alkoxide that is protonated to an alcohol. Catalytic hydrogenation (H₂ with Pd, Pt or Ni) adds H₂ across C=C bonds.
On the MCAT: A passage gives a starting material and a reagent, and you predict the product. Aldehydes give primary alcohols and ketones give secondary alcohols with either hydride reagent. LiAlH₄ also reduces carboxylic acids and esters to primary alcohols, and amides to amines. NaBH₄ is milder and typically leaves esters and acids alone, so in a molecule with both a ketone and an ester it reduces the ketone selectively. LiAlH₄ reacts violently with water, so it is used in dry ether followed by an aqueous workup. Oxidizing agents such as PCC do the opposite job.
Common confusion: Students call the reducing agent the species that is reduced. It is the reverse: the reducing agent is oxidized, and the substrate is the one reduced. Check the substrate: if it gains H or loses O, the reagent acting on it is the reducing agent.
Sources
- Organic Chemistry
- Organic Chemistry: Structure and Function
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.