15.2 Aromatic Nitrocompounds

BySubarnam
15.2 Nitrobenzene
🎯 Learning outcomes

After completion of the chapter, students should be able to:

  • Show preparation of nitrobenzene from benzene.
  • State physical properties of nitrobenzene.
  • Explain chemical properties of nitrobenzene, i.e. reduction in different media and electrophilic substitution reactions (nitration, sulphonation & bromination).
  • State important uses of nitro compounds.

Principle

Nitrobenzene is prepared in the laboratory by direct nitration of benzene with nitrating mixture, i.e. a mixture of concentrated sulphuric acid and conc. HNO3 at a temperature of about 60°C.

Benzene + conc. HNO3 conc. H2SO4→ NO2 Nitrobenzene + H2O
  1. Nitrobenzene is a pale-yellow oily liquid with a bitter almond smell.
  2. It solidifies at 5.7°C and boils at 210°C and its specific gravity is 1.2.
  3. It is insoluble in water but soluble in organic solvents like ether, benzene, chloroform, etc.

Reduction in different media

  1. In acidic medium (Fe/HCl, Zn/HCl, or Sn/HCl)
    NO2 Nitrobenzene + 6[H] Zn/HCl→ NH2 Aniline + 2H2O
  2. In catalytic medium (H2 with Pd, Pt, or Ni)
    NO2 Nitrobenzene + 3H2 Ni→ NH2 Aniline + 2H2O
  3. In alkaline medium
    2 NO2 Nitrobenzene + 8[H] Zn/NaOH, alcoholic→ NN Azobenzene + 4H2O
    2 NO2 Nitrobenzene + 10[H] Zn/NaOH, aqueous→ NNHH Hydrazobenzene + 4H2O
    2 NO2 Nitrobenzene + 6[H] NaOH/Na2AsO3→ NNO Azoxybenzene + 3H2O
  4. With lithium aluminium hydride
    2 NO2 Nitrobenzene + 8[H] LiAlH4→ NN Azobenzene + H2O
  5. In neutral medium
    NO2 Nitrobenzene + 4[H] Zn/NH4Cl→ NHOH Phenyl hydroxyl amine + H2O
  6. Electrolytic medium
    NO2 Nitrobenzene + 4[H] conc. H2SO4, electricity→ HONH2 4-aminophenol (p-aminophenol) + H2O

The nitro group is a deactivating group in electrophilic aromatic substitution reactions; hence, these reactions are slower than in benzene.

The nitro group is meta-directing in electrophilic aromatic substitution reactions; hence, meta-substituted products are obtained. For example,

NO2 Nitrobenzene + HNO3 (conc.) Conc. H2SO4, 90°C→ NO2NO2 m-Dinitrobenzene + H2O
NO2 Nitrobenzene + 3HNO3 (fuming) Conc. H2SO4, 100°C→ NO2NO2O2N 1,3,5-Trinitrobenzene + 3H2O

Nitrobenzene undergoes sulphonation with concentrated fuming sulphuric acid when heated. It also undergoes bromination with bromine in the presence of FeBr3 as a catalyst when heated.

Practice writing these reactions – check from the book if you have written them correctly.

Nitrobenzene does not undergo Friedel–Crafts reactions.

  1. Nitromethane can be used to produce chloropicrin, which is used as a broad-spectrum antimicrobial, fungicide, herbicide, insecticide and a component of tear gas.
  2. These can be used as solvents.
  3. A few nitroalkanes are used as explosives.
  4. Nitrobenzene is used to prepare aniline and other dye stuff.
  5. It is also used as a solvent in shoe polishes.

📥 Download PDF 3.3 Aromatic Nitro Compounds- Notes

📥 Download PDF 3.3 Aromatic Nitro Compounds- Important NEB questions
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