Nitroalkane and Nitrobenzene

Organic compounds having -NO₂ functional group is called nitro compounds (nitroalkane and nitrobenzene). When one of the hydrogen atoms of an alkane is replaced by nitro group (-NO₂), the compound is nitroalkane.

Nomenclature:

Common as well as IUPAC names of nitroalkanes are same.

CH₃NO₂ – Nitromethane
CH₃CH₂NO₂ – Nitroethane
CH₃CH₂CH₂NO₂ – 1-Nitropropane

General methods of preparations of nitroalkane:

1. From haloalkane: Haloalkane is treated with alcoholic AgNO2 to give nitroalkane.

2. From alkane: Alkane is heated with fuming nitric acid at 450°C to give nitroalkane.

Physical Properties:

1. They are colorless liquid with a pleasant odour.
2. They are sparingly soluble in water.

Chemical Properties:

A. Reduction

1. Reduction in acidic medium: Nitroalkanes are reduced in acidic medium using Sn/HCl, Fe/HCl, or Zn/HCl to give primary amine.

2. Reduction in neutral medium: Nitroalkanes are reduced in a neutral medium using Zn/H₂O or Zn/NH4Cl to give hydroxylamine.

3. Catalytic reduction: Nitroalkanes are reduced in a catalytic medium using Pt, Pd, or Ni as a catalyst to give primary amine.

4. Reduction by Lithium aluminium hydride (LiAlH₄): Nitroalkanes are reduced by LiAlH4 to give primary amine.

Uses:

• To prepare amine.
• To prepare dyes, drugs, etc.

Nitrobenzene

Laboratory preparation of nitrobenzene:

Principle:
Nitrobenzene is prepared in the laboratory by heating benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid at 60°C.

Procedure:

A mixture of 60 ml of concentrated nitric acid and 60 ml of concentrated sulphuric acid is taken in a round bottom flask. This mixture is known as nitrating mixture. 60 ml of benzene is added to the mixture a little at a time with continuous shaking and cooling after each addition. When benzene is added completely, the mixture is heated in the water bath at 60 °C for an hour until a pale yellow oily layer of nitrobenzene appears having bitter almond odour. The layer of nitrobenzene is separated by a separating funnel.

Purification:

The impure nitrobenzene is contaminated with acidic impurities. Then it is washed with dilute Na₂CO₃ to remove acidic impurities and then with water several times. Now, it is dried with anhydrous CaCl₂ and finally redistilled about 210-215 °C using an air condenser to get pure nitrobenzene.

Physical Properties:

1. It is a pale yellow oily liquid having a bitter almond odour.
2. It is insoluble in water but soluble in organic solvents.
3. Its boiling point is 210 °C.
4. It is highly toxic.

Chemical Properties:

A. Reduction in different media

1. Reduction in acidic medium: Nitrobenzene is reduced to aniline in acidic solution using Sn/HCl, Fe/HCl, or Fe/HCl.

2. Reduction in neutral medium: Nitrobenzene is reduced to phenyl hydroxyl amine in a neutral solution using Zn/H₂O or Zn/NH₄Cl.

3. Reduction in alkaline medium: Nitrobenzene undergoes a bimolecular reduction in alkaline medium using different reagents to give different products.

4. Catalytic reduction: Nitrobenzene is reduced to aniline in presence of Ni, Pt, or Pd as a catalyst.

5. Electrolytic medium: Nitrobenzene is reduced into p-Aminophenol by electrolysis in a strongly acidic medium.

6. Reduction by LiAlH₄: Nitrobenzene is reduced into azobenzene with lithium aluminium hydride.

B. Electrophilic substitution reaction

From the above resonating structure, it is clear that the -NO₂ group decreases the electron density at ortho and para position. Consequently, the incoming electrophile prefers to attack at the meta position. So, -NO₂ group is meta directing group.

1. Chlorination:

2. Nitration:

3. Sulphonation:

Uses of nitrobenzene:

• To prepare aniline.
• For making shoe polishes.
• To prepare dyes, drugs, explosives, etc.
• As solvent.