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Nitro Group

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Nitro compounds are historically important for precursors of azo dyes and explosives. The remarkable synthetic importance of nitro compounds has ensured long standing studies of their utilization in organic synthesis. The versatility of Nitro compounds in organic synthesis is largely due to their easy availability and transformation into a variety of diverse functionalities.

The organic compounds where Nitro group (-NO2) is directly attached to the carbon of hydrocarbon chain are known as aliphatic nitro compounds. Nitro compounds are often explosive, especially when the compound contains more than one Nitro group and is impure. The Nitro group is one of the most common explosophores used globally.

Nitro compounds are named by adding prefix "nitro" before the name of the parent hydrocarbon. Thus, aliphatic nitro compounds are called nitroalkanes and aromatic ones are called nitroarenes. Arabic numerals are used to indicate the position of Nitro group and other substitutes if any.

The nomenclature of both aliphatic and aromatic nitro compounds are given below.

Nitro compounds
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Preparation

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Methods of preparation of nitroalkanes are given below.

1. Oxidation of amines


An amino group attached to a tertiary carbon atom is oxidized with potassium permanganate to give Nitro alkane.

Oxidation of Amines

2. Action of silver nitrite on alkyl halide


Alkyl halides undergo nucleophilic substitution with silver nitrite to yield nitroalkanes.

Preparation of Nitroalkanes

3. Hydrolysis of $\alpha$-nitroalkenes


An $\alpha$-nitroalkenes on hydrolysis in the presence of an acid or alkali produces nitroalkane.

Hydrolysis of Alpha Nitroalkene

Properties

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Some of the physical properties of Nitro compounds are listed below.

  1. Lower Nitro alkanes are colorless liquids with pleasant smell. Aromatic Nitro compounds like nitrobenzene have a pale yellow color.
  2. Nitrobenzene is also called mirbane.
  3. Nitrobenzene particularly has a smell of bitter almonds.
  4. Aromatic Nitro compounds are not soluble in water at all. They are soluble in organic solvents.
  5. Nitro compounds have abnormally high boiling points because of their highly polar nature.

Chemical properties of nitro compounds


1. Hydrolysis

Primary, secondary and tertiary nitroalkanes give different products.

Hydrolysis of Nitroalkanes

2. Action with nitrous acid

Primary, secondary, tertiary Nitro alkanes react differently.

Properties of Nitroalkanes

3. Ring substitution in nitroarenes

Nitro group deactivates the ring and hence makes it less reactive towards electrophilic ring substitution.

Ring Substitution in Nitroarenes

Functional Group

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Aliphatic Nitro compounds


Nitro compounds are characterized by the presence of Nitro group (-NO2) in their molecules.The aliphatic Nitro compound in which the Nitro group is attached to the alkyl group.
Aliphatic nitro compound
R-NO
2

The aliphatic Nitro compounds may be further classified into primary, secondary or tertiary carbon atom respectively.

Aliphatic Nitro Compounds

Aromatic Nitro compounds


The nitrogen containing organic compounds where Nitro group (-NO2) is directly attached to benzene ring are known as aromatic Nitro compounds. The compounds where Nitro group is attached to side chain of aromatic ring behave similar to aliphatic Nitro compounds.

Aromatic Nitro Compounds

Reduction

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Reduction of a Nitro group to an amino group is one of the most important reactions in aromatic chemistry and there are many methods available to achieve this transformation.

The various reduction stages of Nitro group (-NO2) are given below.

Nitro Group Reduction

The final product however depends upon the nature of reducing agent as well as the pH of the medium.

1. Catalytic reduction


This involves the use of dihydrogen gas as reducing agent in the presence of catalysts such as raney Ni, Pt or Pd.

Reduction of Nitro Compound


2. Reduction in acidic medium


This involves the use of combination of active metal such as Zn/HCl, Fe/HCl or Sn/HCl as reducing agents.

Reduction in Acidic Medium

3. Reduction with LiAlH4


Aliphatic Nitro compounds are reduced to primary amines with LiAlH4. Aromatic Nitro compounds on reduction with LiAlH4 give azo compounds. 

Reduction of Nitro Group