Organic Compounds with Nitro Group

ORGANIC COMPOUNDS WITH NITRO GROUP

 

NITROALKANES

Nitro alkanes are derivatives of alkanes. They are isomeric to nitrites (esters) classified as primary, secondary and tertiary depending on the nature of carbon atom to which nitro group is linked.

 

Primary nitro alkane        Secondary Nitro alkane            Tertiary nitro alkane

—NO2 group is an ambident group. If it attacks through nitrogen. It is called nitro and if it attacks through oxygen atom, it is called nitrite. Hence nitrites and nitro compounds are isomers.
 

Physical properties of nitroalkane

Nitroalkanes are colourless liquids with pleasant smell while aromatic nitro compounds have characteristic odour. Nitro alkanes are sparingly soluble in water, are highly polar with strong dipole – dipole interactions due to which they have high boiling points. Most of nitroalkanes are quite stable and can be distilled without decomposition but alkyl nitrites are unstable and explode on heating. Nitro compounds like nitrobenzene, o – nitrophenol are steam volatile and can be purified by steam distillation.

Preparation of nitroalkane

(i) From alkyl halides:  

Alkyl halides react with silver nitrite in ethanolic solution to give nitro compounds. Alkyl nitrite is formed in minor quantity. This reaction is used to prepare  1nitro compounds primarily while 2oand 3o halides give major proportion of alkenes due to β – elimination. Contrary to this alkali nitrites give alkyl nitrites as major product. This is due to ionic nature of alkali nitrite.  

But if the reaction is carried out in solvents like DMF or DMSO, then even NaNO2 or KNO2 give good yield (about 60%) of nitro compound.  

Reactions:  

R—I + AgNO2 ——> RNO2 + Agl

C2H5l + AgNO2  ——> C2H5NO2 + Agl  

                                  Nitroethane

(ii) Nitration:  

Nitro derivatives of aromatic compounds like nitrobenzene are produced when benzene is allowed to react with nitrating mixture.(conc. HNO3/conc.H2SO4).            

 

 

Mechanism:  

Generation of nitronium ion                            

Attack of NO2 on benzene molecule              

             

Loss of proton:  

           

                                                 Nitrobenzene  

Direct nitration of alkane involves vapour phase nitration at high temperature.  

R — H + HONO2 ———> R — NO2 + H2O

                          675 K          low yield  

Problem faced in the method is that at such high temperature, a mixture of nitro alkanes is formed due to C – C cleavage.  

e.g. CH3CH2CH3 + HNO3 ——> CH3CH2CH2NO2 + CH3CH2NO2 + CH3NH2 + other products   

(iii) From amines:                 

3o nitroalkanes can be produced as follows:  

                    CH3                                                       CH3

                      |                                                            |

CH3 ———— C ———— NH2 ————> CH3 ———— C ———— NO2 + 2H2

                      |                                                            |

               3o butylamine                                        (83% yield)

 

chemical properties of nitroalkane

Reduction of nitro compounds

 

Alkylation of nitroalkanes

Alkylation of nitroalkanes is a chemical reaction in which an alkyl group (-CH2CH3, -CH(CH3)2, etc.) is added to a nitroalkane molecule (-NO2) in the presence of a strong base. The reaction is typically carried out using an alkyl halide (such as ethyl chloride) and a base (such as sodium ethoxide) in a solvent like ethanol.

The alkylation of nitroalkanes is an important method for the synthesis of a variety of organic compounds, including pharmaceuticals, pesticides, and dyes. The reaction proceeds by a nucleophilic substitution mechanism in which the nitro group acts as a nucleophile, attacking the electrophilic carbon atom of the alkyl halide. The alkyl group then replaces the halide group, forming a new carbon-carbon bond.

The alkylation of nitroalkanes is useful because the nitro group can be easily reduced to an amino group (-NH2) using reducing agents like tin and hydrochloric acid. This reaction provides a convenient way to introduce an alkyl group into an amino compound, which is important for the synthesis of many pharmaceuticals and other organic compounds.

However, the alkylation of nitroalkanes can also lead to the formation of unwanted by-products, such as elimination products, which can decrease the yield of the desired product and create environmental issues. Therefore, it is important to optimize the reaction conditions and control the reaction parameters carefully to maximize the yield and minimize the formation of by-products.

NITROARENES

Aromatic hydrocarbons react with concentrated nitric acid directly or in the presence of strong acid catalyst namely sulphuric acid to form nitro derivatives. Since hydrogen is replaced by monovalent nitro group, this reaction is called nitration.

The selection of the nitrating agent depends upon the reactivity of substrate. The most common amongst them is a mixture of concentrated nitric acid and concentrated sulphuric acid known as nitrating mixture. Concentrated sulphuric acid is used as it accelerates the nitration process by increasing the concentration of electrophilic nitronium ion.

 

Physical properties of nitroarenes

Nitroarenes are a class of organic compounds that contain a nitro group  attached to an aromatic ring. They have a range of physical properties that depend on the size and nature of the aromatic ring, the number and position of the nitro groups, and other factors such as the presence of substituents on the ring.

Some of the important physical properties of nitroarenes are:

1.     Solubility: Nitroarenes are generally less soluble in water but more soluble in organic solvents such as benzene, toluene, and ether. This is because the nitro group is polar and can form hydrogen bonds with water, but the aromatic ring is nonpolar and cannot form such bonds.

2.     Melting and boiling points: Nitroarenes have higher melting and boiling points compared to their parent hydrocarbons due to the presence of the polar nitro group. The melting and boiling points also depend on the size and nature of the aromatic ring and the number and position of the nitro groups.

3.     Density: Nitroarenes are generally denser than water and have a higher density compared to their parent hydrocarbons due to the presence of the nitro group.

4.     Color: Nitroarenes can have various colors, depending on the size and nature of the aromatic ring and the number and position of the nitro groups. For example, nitrobenzene is a pale yellow liquid, while 2,4,6-trinitrotoluene (TNT) is a yellow crystalline solid.

5.     Reactivity: Nitroarenes are reactive due to the presence of the nitro group, which is a strong electron-withdrawing group. They can undergo a variety of chemical reactions, including reduction, oxidation, and substitution.

 

Reduction of nitrobenzene

Nitrobenzene gives different products in different medium by using different reducing agent.

·         Reduction of nitrobenzene in acidic medium :

Nitrobenzene on reduction with Zn/HCl or Sn/ HCl gives aniline.

 

·         Catalytic reduction of nitrobenzene :

Nitrobenzene when reduced by hydrogen in presence of nickel or platinum as a catalyst gives aniline.

 

·         Reduction of nitrobenzene in neutral medium :

Nitrobenzene on reduction with Zn and aq. NH4Cl gives phenyl hydroxylamine.

 

·         Reduction of nitrobenzene with LiAlH4 :

Lithium aluminium hydride reduces nitrobenzene to azobenzene.

 

·         Reduction of nitrobenzene in alkaline (basic) medium :

 

·         Electrolytic reduction of nitrobenzene :

Nitrobenzene when reduced electrolytically, first gives phenyl hydroxylamine which immediately rearrenges to give p- aminophenol.

 

Nucleophilic aromatic substitution of substituted nitroarene