Electrophilic Substitution Of Benzene

Resonance involved in the benzene ring makes the delocalized electron span effectively over the carbon atoms in the benzene ring. It partially stabilizes the arenium ion too. Partial stability of arenium ion makes benzene highly prone to electrophilic substitution reactions.

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What is Electrophilic Substitution of Benzene?

Electrophilic substitution of benzene is the one where an electrophile substitutes the hydrogen atom of benzene. As the aromaticity of benzene is not disturbed in the reaction, these reactions are highly spontaneous in nature. Basic examples of electrophilic substitution reaction of benzene are nitration, sulfonation, halogenation, Friedel Craft’s alkylation and acylation, etc.


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The Mechanism for Electrophilic Substitution of Benzene

An electrophilic substitution reaction generally involves three steps:

1. Generation of electrophile: In the presence of Lewis acid, generation of electrophile takes place. As the Lewis acid accepts the electron pair from the attacking reagent.

Generation of electrophile

2. Formation of arenium ion: The electrophile generated attacks on the benzene ring to form a positively charged cyclohexadienyl cation better called an arenium ion containing one sp3 hybridized carbon atom. The positive charge is effectively distributed over three carbon atoms by resonance which makes it partially stable.

Formation of arenium ion

As the delocalization of electrons stops at an sp3 hybridized carbon atom, the arenium ion is not aromatic in nature.

3. Removal of positive charge from the carbocation intermediate: The arenium ion finally loses its proton from sp3 hybridized carbon to a Lewis base restoring the aromaticity.

Removal of positive charge from the carbocation intermediate

Few examples of electrophilic aromatic substitution

1. Nitration of Benzene

Benzene reacts with nitric acid at 323-333k in presence of sulphuric acid to form nitrobenzene. This reaction is known as nitration of Benzene.

Nitration of Benzene

2. Sulphonation of Benzene

Sulphonation of benzene is a process of heating benzene with fuming sulphuric acid (H2SO4 +SO3) to produce benzenesulphonic acid. The reaction is reversible in nature.

Sulphonation of Benzene

3. Halogenation of Benzene

Benzene reacts with halogens in the presence of Lewis acid like FeCl3, FeBr3 to form aryl halides. This reaction is termed halogenation of benzene.

Halogenation of Benzene

Frequently Asked Questions – FAQs


Why does benzene undergo electrophilic substitution?

Benzene is a planar molecule which has delocalized electrons above and below the plane of the ring. Being electron-rich, it is highly attractive to electron-deficient species i.e., electrophiles.

What are the three steps involved in the electrophilic substitution of benzene?

Step 1: Generation of electrophile.
Step 2: Formation of the carbocation.
Step 3: Removal of the proton.

What is an electrophilic substitution reaction? Give example

In electrophilic aromatic substitution reactions, an atom attached to an aromatic ring is replaced with an electrophile.
Examples of such reactions include aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions

What are the products of nitration, chlorination and Friedel Craft alkylation of benzene ?

Nitrobenzene, chlorobenzene and methyl benzene respectively are the products of nitration, chlorination and Friedel Craft alkylation of benzene.

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