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Preparation of Benzene | chemca

Hydrocarbons

Preparation of Benzene

Synthesis from Ethyne, Phenol, Benzoic Acid, and Sulphonic Acid.

By chemca Team • Updated Jan 2026

Benzene ($C_6H_6$) is the parent aromatic hydrocarbon. It is commercially isolated from coal tar, but can be prepared in the laboratory using cyclic polymerization, decarboxylation, or reduction methods.

1. Cyclic Polymerization of Ethyne

From Alkynes

When Ethyne (Acetylene) is passed through a Red Hot Iron Tube at 873 K, it undergoes cyclic polymerization to form Benzene.

$$ 3HC \equiv CH \xrightarrow{\text{Red Hot Fe Tube, 873K}} \underset{\text{Benzene}}{C_6H_6} $$

This reaction effectively converts an aliphatic compound into an aromatic compound.

2. Decarboxylation of Aromatic Acids

From Sodium Benzoate

When the sodium salt of benzoic acid (Sodium Benzoate) is heated with Soda-lime ($NaOH + CaO$), it loses a molecule of carbon dioxide to form Benzene.

$$ \underset{\text{Sodium Benzoate}}{C_6H_5COONa} + NaOH \xrightarrow{CaO, \Delta} \underset{\text{Benzene}}{C_6H_6} + Na_2CO_3 $$

Role of CaO: Calcium Oxide (Quick lime) is used to keep the sodium hydroxide dry (it is hygroscopic) and to permit the reaction at a higher temperature without fusing the glass container.

3. Reduction of Phenol

From Phenol

Benzene is prepared by passing phenol vapors over heated Zinc dust. Zinc acts as a reducing agent, removing oxygen from phenol.

$$ \underset{\text{Phenol}}{C_6H_5OH} + Zn \xrightarrow{\Delta} \underset{\text{Benzene}}{C_6H_6} + ZnO $$

4. Hydrolysis of Sulphonic Acid

From Benzene Sulphonic Acid

Benzene sulphonic acid yields benzene upon hydrolysis with superheated steam or by heating with dilute $HCl$ under pressure.

$$ C_6H_5SO_3H + H_2O \text{ (steam)} \xrightarrow{\Delta} C_6H_6 + H_2SO_4 $$

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