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Nitro Group

Organic compounds containing nitrogen can be broadly classified into two groups.

  1. Compounds containing nitro functional group -NO2
  2. Compounds containing amine functional group -NH2 and their derivatives.
In this nitro compounds are primary source of organic compounds with functional group -NO2 to start the synthesis of many other important compounds

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Factor by Grouping
 

Nomenclature of Nitro Compounds

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Organic compounds containing nitro -NO2 functional group are called as nitro compounds. They are obtained by replacing one or more hydrogen in the hydrocarbon with nitro group. Here, the nitrogen is directly attached to the carbon atom in the organic chain.

The general formula of nitro compounds is

Nitro

Nitro compounds are named by prefixing the nitro with the root alkane name with the locant. Hence, the name of CH3NO2 is amino methane and CH3-CH(NO2)-CH3 is 2-nitropropane.

Preparation Method of Nitro Compounds

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Nitro compounds are prepared by any one of the following methods.

1. By direct nitration of alkanes


Alkanes are treated with nitrating mixture (fuming nitric acid) with concentrated sulfuric acid to give nitro compounds. This method will not give good yield in case of aliphatic nitro compounds, but will give best yield for aromatic nitro compounds like nitro benzene.

Ethane on nitration gives nitro ethane.

CH3-CH3 + HNO3/H2SO4 CH3-CH2-NO2

Benzene on nitration gives nitro benzene

Nitro Benzene

2. By treating iodoalkanes with silver nitrite


Iodo alkanes on reacting with silver nitrite give nitro compounds. This method will not give good yield for nitro benzene. But all the aliphatic nitro compounds can be prepared by this method. 

For example iodoethane on treating with silver nitrite gives nitro ethane.

CH3-CH2-I + AgNO2 CH3-CH2-NO2 + AgI

By this method tertiary nitro compounds cannot be prepared as tertiary iodoalkanes undergo elimination reaction rather than substitution reaction.

3. By treating amines with alkaline KMnO4


Tertiary amines on treating with alkaline KMnO4 gives tertiary nitro compounds. This method works only with tertiary amines.

Tertiary Nitro

Physical Properties of Nitro Compounds

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  • Aliphatic nitro compounds are mostly gaseous in nature and higher members are liquids.
  • Aromatic nitro compounds are yellow color liquids which intensified to brown color by time.
  • They are polar due to the presence of nitro group.

Chemical Properties of Nitro Compounds

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Nitro compounds undergo tautomerism in solution to azinitro form, which is acidic in nature. Hence, all the nitro compounds are weakly acidic in nature. They dissolve in bases to give salts and can be regenerated by adding a stronger acid like HCl.

Similarly they can be reduced to amino compounds in acidic or basic medium. They condense with acetaldehyde to give condensation product.
Azinitro

Similarly they react with chlorine to give trichloro product used to sterilize the soil.

CH3NO2 $\to$ CCl3NO2
Nitro benzene is electron withdrawing group by both inductive effect and resonance effect. Hence it deactivates the benzene ring and it is meta directing group. On nitrating with fuming nitric acid and sulfuric acid it gives 1,3 dinitro benzene.

1-3-Dinitro Benzene
Similarly it is electron withdrawing group and stabilize the carbocation formed. Hence it will undergo nucleophilic substitution reaction easily. Aliphatic nitro compounds are boiled with dilute HCl to give carboxylic acid and hydroxyl amine hydrochloride.

For example Methyl amine gives acetic acid and hydroxyl amine hydrochloride.

CH3NO2 + HCl + H2O $\to$ CH3COOH + NH2OH.HCl

Reduction of Nitro Compounds

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Majority of the reaction of organic nitro compounds can be studied under their reduction. Reduction of nitro compounds can be studies separately for aliphatic nitro compounds and aromatic nitro compounds.

1. Reduction of aliphatic nitro compounds


Aliphatic nitro compounds can be reduced to alkanes by catalytic hydrogenation over platinum at high temperature.

Aliphatic Nitro

Aliphatic nitro compounds can be reduced to amines by treating with Sn/HCl. This is the effect way to convert nitro compounds to amines.

CH3NO2 $\to$ CH3NH2

Aliphatic nitro compound can be reduced to hydroxyl amines by treating with diborane.

CH3NO2 $\to$ CH3NHOH

Aliphatic nitro compounds are reduced to oximes using stannous chloride.

CH3NO2 $\to$ CH3-N = OH

2. Reduction of aromatic nitro compounds


Nitro benzene can be reduced to aniline by Sn/HCl.

Aniline

On treating with Raney Nickle, nitro benzene is reduced to phenyl hydroxylamine.

Phenyl Hydroxylamine

On treating with lithium aluminium hydride, nitro benzene is reduced to diazobenzene. There are many products formed due to condensation of the reduction products formed. Hence the reduction of nitro benzene is more complex with more products.

Isomerism in Nitro Compounds

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The isomerism exhibited by organic nitro compounds can be classified into the following types.

1. Chain isomerism


Compounds having same molecular formula and difference in the nature of carbon chain is called as chain isomers. For example 1-nitro butane and 2-methyl 1-nitro propane.

CH3-CH2-CH2-CH2-NO2 CH3-CH(CH3)-CH2-NO2

2. Position isomerism


Compounds having same molecular formula and difference in the position of attachment of functional group is called as position isomers. For example 1-nitro propane and 2-nitro propane.

CH3-CH2-CH2-NO2 CH3-CH(NO2)-CH3

3. Functional group isomers


Nitro compounds are functional group isomers for nitrite compounds in which the oxygen is bonded with the carbon chain. The following diagram explains the difference between nitro methane and methyl nitrite.
Group Isomers

4. Tautamerism


In solution the nitro form is in equilibrium with azinitro form. The azinitro form is acidic in nature and hence nitro methane is soluble in alkalis.

Tautamerism

Uses of nitro compounds

  1. They are useful intermediates in the preparation of amino compounds and diazonium salts which are synthetic important materials to prepare almost all organic compounds.
  2. They are used to prepare the azo dyes and in soil sterility.
  3. Nitro benzene is used in shoe and floor polishes.
  4. It is also used in paint and solvents and to mask unpleasant odor in chemicals.
  5. It is used in the manufacture of Paracetamol (a popular analgesic)

Nitro Functional Group

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The NO2 group attached with organic chain is called as nitro functional group. All the compounds containing the nitro functional group are called as organic nitro compounds.

They are classified as
  1. Aliphatic nitro compounds
  2. Aromatic nitro compounds

Nitro Groups

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The NO2 group is called as nitro group. It is electron withdrawing group due to its -Inductive effect as well as -Resonance effect. The structure of NO2 group is given below.
Nitro Group

Aliphatic Nitro Compounds

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If the nitro functional group is attached with aliphatic carbon chain it is called as aliphatic nitro compound. They are weakly acidic due to their azinitro form and they are soluble in alkalies and can undergo condensation reaction with aldehydes.

For example nitro methane is an example for aliphatic nitro compound.

CH3-NO2 - Nitro methane.

Aromatic Nitro Compounds

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If the nitro functional group is attached with benzene ring it is called as aromatic nitro compound. For example nitro benzene is aromatic nitro compound. All aromatic nitro compounds are yellow colored liquids which may change to dark brown color due to oxidation.
Aromatic Nitro
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