Reducing Agents in Organic Chemistry: JEE Main

Reduction questions in JEE Main Organic Chemistry require you to match the reagent to the product — and each common reducing agent has a distinct selectivity profile. Using LiAlH₄ where NaBH₄ is correct, or confusing catalytic hydrogenation with chemical reduction, costs marks. This guide maps every major reducing agent to its targets and gives the selectivity rules you need to answer any JEE reduction question correctly.

Lithium Aluminium Hydride (LiAlH₄): The Powerful Reducer

LiAlH₄ (LAH) is a powerful, non-selective hydride source in anhydrous ether. It reduces almost every polar multiple bond containing a heteroatom: aldehydes → primary alcohols; ketones → secondary alcohols; carboxylic acids → primary alcohols; esters → two primary alcohols (one from the acid part, one from the alcohol part); acid chlorides → primary alcohols; amides → amines; nitriles → primary amines. Critically, LiAlH₄ does NOT reduce isolated C=C alkene bonds (no reaction with ordinary alkenes or arenes under typical conditions). It does react with water violently — strictly anhydrous conditions.

LiAlH₄ reduces nitro groups (-NO₂) to amines (-NH₂). It cannot distinguish between a ketone and an ester in the same molecule — both are reduced. For selective reduction of one group, use NaBH₄. JEE uses LAH reduction of amides and nitriles frequently in "give product" questions. Take a free organic chemistry mock. For the nitrogen compound context see our nitrogen compounds guide.

Sodium Borohydride (NaBH₄): The Selective Reducer

NaBH₄ is a milder hydride source. It selectively reduces aldehydes and ketones to alcohols without reducing esters, carboxylic acids, amides, or isolated C=C bonds. This selectivity makes it the reagent of choice when a molecule contains both a ketone and an ester and only the ketone must be reduced. NaBH₄ can be used in protic solvents (water, alcohol) unlike LiAlH₄. It does not reduce nitro groups, nitriles, or carboxylic acids under standard conditions.

Selectivity comparison: LiAlH₄ > NaBH₄ in reducing power. DIBAL-H (diisobutylaluminium hydride) reduces esters to aldehydes at −78°C (partial reduction — the aldehyde is trapped before further reduction). This DIBAL-H controlled partial reduction is a JEE Advanced topic but appears in JEE Main multi-correct questions. For carbonyl chemistry see our aldehydes and ketones guide.

Catalytic Hydrogenation (H₂/Pd, Ni, Pt)

Catalytic hydrogenation reduces C=C and C≡C bonds (alkenes and alkynes to alkanes), C=O in aldehydes and ketones to alcohols (under high H₂ pressure), nitro groups to amines (H₂/Pd or Fe/HCl work too), and aromatic rings under forcing conditions. Selectivity: H₂/Pd with Lindlar's catalyst (poisoned Pd/CaCO₃/quinoline) reduces alkynes specifically to cis-alkenes (syn addition of H₂ to the same face). Na/liquid NH₃ (Birch reduction) reduces alkynes to trans-alkenes (anti addition via radical anion mechanism). JEE loves the cis vs trans alkene from alkyne reduction — Lindlar → cis; Na/NH₃ → trans.

Other Important Reduction Reagents

Clemmensen reduction: Zn-Hg amalgam + conc. HCl. Converts C=O (aldehyde or ketone) to CH₂ (methylene). Used in acidic conditions — not suitable for acid-sensitive molecules. Wolff-Kishner reduction: NH₂NH₂ + KOH (heat). Same net transformation (C=O → CH₂) but in basic conditions — not suitable for base-sensitive molecules. Together: if the substrate is acid-sensitive, use Wolff-Kishner; if base-sensitive, use Clemmensen. Meerwein-Pondorf-Verley (MPV): aluminium isopropoxide reduces ketones selectively in the presence of esters. Borane (BH₃): reduces carboxylic acids to primary alcohols (contrast with LiAlH₄ which also does this but more aggressively). For the full named reactions see our named reactions guide and for oxidation agents see our organic reaction mechanisms guide.