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.

nitroalkane general formula

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.
nitroalkane form haloalkane

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

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.
reduction of nitroalkane in acidic medium

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

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

  1. Reduction by Lithium aluminium hydride (LiAlH4): Nitroalkanes are reduced by LiAlH4 to give primary amine.
reduction of nitroalkane with LiAlH4

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.

nitrobenzene lab preparation

Fig: Lab preparation of Nitrobenzene

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.

distillation of nitrobenzene
Fig: Distillation of 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.
reduction of nitrobenzene in acidic medium

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

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

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

  1. Electrolytic medium: Nitrobenzene is reduced into p-Aminophenol by electrolysis in a strongly acidic medium.
reduction of nitrobenzene in electrolytic medium

  1. Reduction by LiAlH4: Nitrobenzene is reduced into azobenzene with lithium aluminium hydride.
reduction of nitrobenzene with LiAlH4

B. Electrophilic substitution reaction
resonance structure of nitrobenzene

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:
chlorination of nitrobenzene

  1. Nitration:
nitration of nitrobenzene

  1. Sulphonation:
sulphonation of nitrobenzene

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

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