Reactions of benzene – Nitration

Nitration is a common reaction of benzene involving the substitution of a hydrogen atom in the benzene ring with a nitro group (NO2). The process typically requires a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) as a nitrating agent and catalyst, respectively. Below is a detailed note on the nitration reaction of benzene:

Nitration Reaction of Benzene

1.  Conditions for Nitration

Reagents: Concentrated nitric acid (HNO3) and sulfuric acid (H2SO4).

Catalyst: Sulfuric acid catalyzes by providing the nitronium ion (NO2⁺), the electrophile involved in the substitution.

Temperature: The reaction is carried out at a moderately elevated temperature, usually around 50-55°C.

2.  Reaction Mechanism

The nitration reaction involves the following steps:

Formation of Nitronium Ion (NO2⁺):

HNO3​+H2​SO4​​H2​NO3⁺​+HSO4⁻​

Generation of the Electrophile (NO2⁺):

H2​NO3⁺​​NO2⁺​+H2​O

Electrophilic Substitution:

C6​H6​+NO2^+​​C6​H5​NO2​+H⁺

3.  Formation of Nitrobenzene

The final product of the nitration of benzene is nitrobenzene (C6H5NO2).

The nitro group (NO₂) is introduced at the position originally occupied by a hydrogen atom on the benzene ring.

4.  Isomer Formation

Benzene is highly symmetrical, and all the hydrogen atoms are equivalent. Therefore, the nitration of benzene produces only one product – nitrobenzene.

Due to its symmetry, there are no positional isomers in the product.

5.  Regioselectivity

Benzene is highly regioselective in nitration, meaning that the substitution predominantly occurs at one specific position.

The electrophile (NO₂⁺) tends to attack the benzene ring at the ortho and para positions, with para being the major product.

6.  Reaction Outcome

The overall balanced chemical equation for the nitration of benzene is:

C6​H6​+HNO3​​C6​H5​NO2​+H2​O

Significance

– Nitrobenzene is a crucial intermediate in synthesizing various organic compounds, including dyes, pharmaceuticals, and agrochemicals.

– The regioselectivity and the absence of side products make the nitration of benzene a valuable tool in organic synthesis.

– This reaction is part of a broader class of electrophilic aromatic substitution reactions, which benzene and its derivatives frequently undergo.