What NEET Asks
- Direct reagent-product identification questions are common.
- Focus on conditions leading to complete vs. partial/selective reduction.
- Questions involving compounds with multiple functional groups.
Key Points
- Nitro compounds (R-NO2) are typically reduced to primary amines (R-NH2).
- Acidic medium: Sn/HCl, Fe/HCl (or Zn/HCl) are common reagents. They supply nascent hydrogen for reduction.
- Catalytic hydrogenation: H2 with Ni, Pd, or Pt catalyst is effective and mild. It's often preferred for unsaturated compounds (e.g., nitroalkenes) to avoid reducing C=C bonds.
- Strong reducing agents: LiAlH4 reduces nitro groups to amines, but is non-selective and will reduce other reducible groups like esters, carboxylic acids, ketones, aldehydes.
- Neutral medium: Zn dust/NH4Cl reduces nitro compounds to N-alkyl/aryl hydroxylamines (e.g., nitrobenzene to N-phenylhydroxylamine).
- Selective reduction: In dinitro compounds (e.g., m-dinitrobenzene), mild reducing agents like (NH4)2S or Na2S can selectively reduce only one nitro group.
Must-Know Formula / Reaction
R-NO2 + 6[H] --(Reducing agent)--> R-NH2 + 2H2O
- R-NO2: Any aliphatic or aromatic nitro compound.
- [H]: Nascent hydrogen, provided by reducing agents like Sn/HCl, Fe/HCl.
- Reducing agent: Can also be H2/catalyst (Pd, Pt, Ni) or LiAlH4.
- R-NH2: The corresponding primary amine.
Common Mistakes
- Students often confuse reagents for complete reduction (Sn/HCl) with those for partial reduction (Zn/NH4Cl).
- Don't forget that LiAlH4 is a strong, non-selective agent; consider other functional groups in the molecule.
- Incorrectly predicting products for selective reduction of dinitro compounds using mild agents like (NH4)2S.
Rapid Revision
- Sn/HCl, Fe/HCl, H2/Pd/Pt/Ni all convert R-NO2 to R-NH2.
- Zn/NH4Cl converts R-NO2 to R-NHOH (N-alkyl/aryl hydroxylamine).
- (NH4)2S reduces one NO2 group in dinitro compounds.
- LiAlH4 is strong, reduces nitro but also many other groups.