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Physical And Chemical Properties Of Nitro Compounds

Physical Properties of nitro compounds

  • Physical state and smell: Nitroalkanes are colourless pleasant smelling liquid. Except for a few compounds including nitrobenzene which are pale yellow, oily liquids, most of the nitroarenes are yellow crystalline solids.
  • Solubility: Nitroalkanes are sparingly soluble in water and are soluble in organic solvents. They are soluble in organic solvents like alcohol, benzene, ether etc.
  • Boiling point: Since the nitro group is a polar group of the electronegativity difference between nitrogen and oxygen atoms, the nitro compounds have specific dipole moment values which vary from 3 to 4 Debye. The boiling point of benzene is 353 K while nitrobenzene boils at 483 K because of dipole-dipole interaction.
  • Stability: Nitroalkanes are quite stable and can be distilled under normal atmospheric pressure. 
  • Chemical properties of nitro-compound 
  • Acidity: Due to tautomerism, primary and secondary alkane are weakly acids. 
  • Halogenation: Nitro alkane containing alpha hydrogen undergoes halogenation in the presence of a base to give a halogen-substituted product. It forms nitrochloroform or chloropicrin. It is used as an insecticide and war gas.
  • Reaction with Grignard reagent

The nitroalkanes react with a Grignard reagent to form alkane.

  • Hydrolysis of Nitroalkanes: The hydrolysis of nitroalkanes takes place in acidic medium.
  • Primary nitro compounds are hydrolysed by boiling either with hydrochloric acid or with 85 % sulphuric acid. The product formed is carboxylic acid and hydroxylamine.
  • The secondary nitroalkanes are hydrolysed into ketone and nitrous oxide by boiling with hydrochloric acid.
  • The tertiary nitroalkanes are not readily hydrolysed.
  • Nitroarenes do not undergo hydrolysis.

  • Reaction with Nitrous acid(HNO2): This method is also applied for distinguishing primary, secondary and tertiary nitroalkanes.

  • Reduction of Nitro compound

The reduction proceeds through the following stages:

Based on the medium, there are many reduction products of an aromatic and aliphatic nitro compound. They are discussed below:

  1. Reduction in acidic medium

  Nitroalkane is converted to amines whereas nitrobenzene converts into aniline on reduction. The reducing agents in acidic medium are : Sn/HCl, Zn/HCl,Fe/HCl, Sncl2/HCl etc.

  1. Reduction in neutral medium

Both aliphatic and aromatic nitro compounds are converted into hydroxylamines in neutral medium. The reducing agent in the neutral medium is Zn dust, Zn/H2O, Al-Hg couple/H2O.

Note: Hydroxylamine on warming with Tollens reagent (ammoniacal silver nitrate) are easily oxidised back to the corresponding nitroso compound and thus reduce the tollen’s reagent. This reaction is used for test of nitro compounds and also known as Baker-Muliken’s test.

  1. Catalytic reduction:

On catalytic reduction, both nitroalkane and nitrobenzene give 1-degree amines. The reducing agent for catalytic reduction is H2/Ni, H2/pt etc.

  1. Reduction with LiAlH4:

The aliphatic compound produces amine, whereas the aromatic compound does not form amines.

  1. Electrophilic reduction: Final product depends upon the pH of the reaction medium.

  1. Reduction in Basic medium

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