Sulphonation of Benzene

Sulphonation of benzene is an electrophilic substitution reaction in which a hydrogen atom of the benzene ring is replaced by a sulphonic acid group (–SO₃H) to form benzene sulphonic acid. This reaction occurs in the presence of concentrated sulphuric acid or oleum. Like other reactions of benzene, sulphonation proceeds via substitution in order to preserve the aromatic stability of the ring.

Reagents and Conditions

Sulphonation of benzene is carried out using concentrated H₂SO_4, which provides the reacting species needed for the reaction or oleum (H₂SO₄ + SO₃) is stronger because it contains free SO₃, so the reaction happens faster.

These reagents are used because:

• Benzene is very stable (aromatic), so it does not react easily.
• It needs a strong electrophile to react.
• These reagents help in forming SO₃, which is strong enough to attack benzene.

Chemical Reaction

Sulphonation of benzene involves the reaction of benzene with concentrated sulphuric acid or oleum to form benzene sulphonic acid.

• In this reaction, one hydrogen atom of benzene is replaced by a sulphonic acid group (–SO₃H).
• The product formed is benzene sulphonic acid (C₆H₅SO₃H).
• This is an example of an electrophilic substitution reaction, because a substituent replaces hydrogen without disturbing the aromatic ring.

Electrophile Formation

In sulphonation of benzene, the electrophile is sulphur trioxide (SO₃). When concentrated sulphuric acid (H₂SO₄) or oleum is used, it produces SO₃ (sulphur trioxide).

• SO₃ is electron-deficient (especially the sulphur atom).
• It has a strong tendency to accept electrons.
• Hence, it behaves as an electrophile.

Mechanism of Sulphonation of Benzene

Sulphonation of benzene follows an Electrophilic Aromatic Substitution (EAS) mechanism. It occurs in three main steps:

Step 1: Formation of Electrophile

• Concentrated sulphuric acid produces SO₃ (sulphur trioxide).
• SO₃ acts as the electrophile.

Step 2: Formation of Carbocation (Arenium Ion)

• The π-electrons of benzene attack the electrophile (SO₃).
• A sigma complex (arenium ion) is formed.
• In this step one bond is formed between benzene and SO₃ and Aromaticity is temporarily lost.
• This is the slow (rate-determining step).

Step 3: Deprotonation

• A hydrogen ion (H⁺) is removed from the ring.
• Aromaticity is restored.
• Benzene sulphonate ion (C₆H₅SO₃⁻) is formed.

Step 4: Protonation

• The benzene sulphonate ion accepts a proton (H⁺) from H₃O⁺ to form benzene sulphonic acid.
• Final product formed: benzene sulphonic acid (C₆H₅SO₃H).

Related Articles:

• Friedel Crafts Reaction
• Classification of Hydrocarbons