15.2 Aromatic Nitrocompounds
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.
- Nitrobenzene is a pale-yellow oily liquid with a bitter almond smell.
- It solidifies at 5.7°C and boils at 210°C and its specific gravity is 1.2.
- It is insoluble in water but soluble in organic solvents like ether, benzene, chloroform, etc.
Reduction in different media
- In acidic medium (Fe/HCl, Zn/HCl, or Sn/HCl)
Nitrobenzene + 6[H] Zn/HCl→ Aniline + 2H2O
- In catalytic medium (H2 with Pd, Pt, or Ni)
Nitrobenzene + 3H2 Ni→ Aniline + 2H2O
- In alkaline medium
2Nitrobenzene + 8[H] Zn/NaOH, alcoholic→ Azobenzene + 4H2O2Nitrobenzene + 10[H] Zn/NaOH, aqueous→ Hydrazobenzene + 4H2O2Nitrobenzene + 6[H] NaOH/Na2AsO3→ Azoxybenzene + 3H2O
- With lithium aluminium hydride
2Nitrobenzene + 8[H] LiAlH4→ Azobenzene + H2O
- In neutral medium
Nitrobenzene + 4[H] Zn/NH4Cl→ Phenyl hydroxyl amine + H2O
- Electrolytic medium
Nitrobenzene + 4[H] conc. H2SO4, electricity→ 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,
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.
- Nitromethane can be used to produce chloropicrin, which is used as a broad-spectrum antimicrobial, fungicide, herbicide, insecticide and a component of tear gas.
- These can be used as solvents.
- A few nitroalkanes are used as explosives.
- Nitrobenzene is used to prepare aniline and other dye stuff.
- It is also used as a solvent in shoe polishes.