Nitroarenes

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