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 compounds are named by prefixing the nitro with the root alkane name with the locant. Hence, the name of CH3
is amino methane and CH3
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
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 + AgIBy 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.
Nitro compounds undergo tautomerism in solution to azinitro form, which is acidic in nature. Hence, all the nitro compounds are weakly acidic in nature.
- 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.
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.
Similarly they react with chlorine to give trichloro product used to sterilize the soil.
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.
CH3NO2 $\to$ CCl3NO2
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
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 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.
On treating with Raney Nickle, nitro benzene is reduced to 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.
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.
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.
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.
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.
Uses of nitro compounds
- They are useful intermediates in the preparation of amino compounds and diazonium salts which are synthetic important materials to prepare almost all organic compounds.
- They are used to prepare the azo dyes and in soil sterility.
- Nitro benzene is used in shoe and floor polishes.
- It is also used in paint and solvents and to mask unpleasant odor in chemicals.
- It is used in the manufacture of Paracetamol (a popular analgesic)
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
- Aliphatic nitro compounds
- Aromatic nitro compounds
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.
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.
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.