Preparation of Phenol from Benzene Sulphonic Acid

Phenol can be prepared from benzene sulphonic acid through a series of chemical reactions. The main steps involved in this process are as follows:

1. Sulphonation of Benzene:
The first step is the sulphonation of benzene to form benzene sulphonic acid. This reaction is carried out by treating benzene with concentrated sulphuric acid (H2SO4) at a temperature of around 50-60°C. The reaction is as follows:


The electrophilic substitution of benzene occurs in the presence of a strong acid catalyst (H2SO4). The sulphonic acid group (-SO3H) is introduced into the benzene ring, resulting in the formation of benzene sulphonic acid.

2. Conversion of Benzene Sulphonic Acid to Sodium Phenoxide:
The next step involves the conversion of benzene sulphonic acid to sodium phenoxide. This is achieved by treating benzene sulphonic acid with sodium hydroxide (NaOH) or sodium carbonate (Na2CO3) at a high temperature of about 300-350°C. The reaction is as follows:


The sulphonic acid group is replaced by a hydroxyl group (-OH), resulting in the formation of sodium phenoxide.

3. Acidification of Sodium Phenoxide to Obtain Phenol:
The final step involves the acidification of sodium phenoxide to obtain phenol. This is done by treating the sodium phenoxide solution with dilute hydrochloric acid (HCl). The reaction is as follows:


The hydroxide group (-OH) of sodium phenoxide is protonated by the hydrogen ion (H+) from the hydrochloric acid, resulting in the formation of phenol. Sodium chloride (NaCl) is formed as a byproduct in this reaction.

The obtained phenol can be further purified by processes like distillation or recrystallization.

In summary, phenol can be prepared from benzene sulphonic acid by sulphonation of benzene to form benzene sulphonic acid, conversion of benzene sulphonic acid to sodium phenoxide, and acidification of sodium phenoxide to obtain phenol. Each step involves specific reactants and conditions, leading to the formation of phenol as the final product.