Question

Write chemical reaction for the preparation of phenol from chlorobenzene.

Short answer

Phenol is prepared from chlorobenzene using NaOH at high temperature and pressure, followed by acidification.

Step-by-step solution

Understand the Ingredients

To prepare phenol from chlorobenzene, you need to know the starting material, which is chlorobenzene, and the desired product, which is phenol. Chlorobenzene has the molecular formula C6H5Cl, and phenol has the molecular formula C6H5OH.

Identify the Catalyst and Conditions

The preparation of phenol from chlorobenzene requires the use of aqueous sodium hydroxide (NaOH) as a reagent under high pressure and temperature (about 300°C and 200 atm). It is followed by acidification.

Write the Chemical Equation

Write down the chemical reaction: \[ \text{C}_6\text{H}_5\text{Cl} + 2 \text{NaOH} \xrightarrow{300°C, 200 \text{atm}} \text{C}_6\text{H}_5\text{ONa} + \text{H}_2\text{O} \]. This reaction forms sodium phenoxide by substituting the chlorine atom with the hydroxyl ion.

Convert Sodium Phenoxide to Phenol

To convert sodium phenoxide (C6H5ONa) to phenol (C6H5OH), the compound is acidified using a dilute acid, such as hydrochloric acid (HCl). The reaction is: \[ \text{C}_6\text{H}_5\text{ONa} + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{NaCl} \], which yields phenol and sodium chloride as a byproduct.

Key concepts

Chlorobenzene

Chlorobenzene is an organic compound with the formula \( ext{C}_6 ext{H}_5 ext{Cl} \). It is a colorless liquid that is insoluble in water but mixes well with various organic solvents. Chlorobenzene is an essential precursor for producing phenol in industrial chemistry.
When chlorobenzene is used to produce phenol, the chlorine atom in the compound needs to be replaced by a hydroxyl group \( (-OH) \). This transformation is achieved through a nucleophilic substitution reaction, where the chlorine atom is substituted by a hydroxyl group through a sequence of reactions.

• Chlorobenzene appears as a basic building block in organic chemistry.
• Its conversion to phenol involves chemical methods that leverage catalysis and extreme reaction conditions.

Sodium Hydroxide

Sodium hydroxide, often called lye or caustic soda, is a strong alkali with the chemical formula \( ext{NaOH} \). It plays a crucial role in the transformation of chlorobenzene to phenol by acting as the reagent and providing hydroxyl ions \( OH^- \) necessary for the reaction.
In the reaction between chlorobenzene and sodium hydroxide, freshly prepared aqueous \( ext{NaOH} \) is used. The reaction conditions enhance the rate of hydroxylation, which substitutes the chlorine atom with a hydroxyl group, forming sodium phenoxide.

• Hydroxyl ions from \( ext{NaOH} \) initiate the nucleophilic substitution of chlorine in chlorobenzene.
• The reaction takes place under high-temperature and high-pressure conditions to facilitate the conversion.

Reaction Conditions

The conversion of chlorobenzene to phenol via sodium phenoxide requires stringent reaction conditions. These include high temperature and pressure, which are essential to overcome the stability of the chlorobenzene molecule and to enhance the reaction's efficiency.
The typical reaction conditions are around 300°C and a pressure of 200 atmospheres. These extreme conditions help in breaking the \( ext{C-Cl} \) bond, which is otherwise very stable, and drive the reaction forward.

• High temperature contributes to sufficient energy for bond disruption and formation.
• High pressure assists in maintaining the desired phase and enhancing reaction rates.

Sodium Phenoxide

Sodium phenoxide \( ext{C}_6 ext{H}_5 ext{ONa} \) is an intermediate compound in the preparation of phenol from chlorobenzene. It forms when chlorobenzene reacts with sodium hydroxide under high temperature and pressure.
The resulting sodium phenoxide is not the end product; it requires further treatment to yield phenol. This transformation is achieved by acidifying sodium phenoxide with a dilute mineral acid such as hydrochloric acid (HCl). This additional reaction produces phenol and sodium chloride as a byproduct.

• Sodium phenoxide is a key intermediate, bridging the gap between chlorobenzene and phenol.
• Its conversion to phenol is relatively straightforward, involving simple acidification.