Chlorobenzene to Phenol (Dow’s Process)|Biswajit Das

The conversion of chlorobenzene to phenol using Dow’s process is a significant industrial method. This process is a classic example of nucleophilic aromatic substitution under harsh conditions. Here’s a detailed breakdown of the steps, conditions, and mechanism involved:

Overall Reaction

The overall reaction can be summarized as:

C6​H5​Cl+2NaOH→C6​H5​ONa+NaCl+H2​O

C6​H5​ONa+HCl→C6​H5​OH+NaCl

Step-by-Step Description of Dow’s Process

1. Preparation of Sodium Phenoxide:
   • Chlorobenzene is treated with a concentrated solution of sodium hydroxide (NaOH) at high temperature and pressure.
   • The typical conditions are approximately 350 °C and 3000 psi (200 atm).
   • A small amount of copper salt (CuCl2​) is often used as a catalyst to facilitate the reaction.
   • Under these conditions, the hydroxide ion (OH−) acts as a nucleophile and substitutes the chlorine atom on the benzene ring.
   • The initial product is sodium phenoxide (C6​H5​ONa), a salt of phenol.
2. Acidification of Sodium Phenoxide:
   • After the reaction in the first step is complete, the reaction mixture is cooled.
   • The resulting sodium phenoxide solution is then acidified with a strong acid, typically dilute hydrochloric acid (HCl).
   • The proton from the acid protonates the phenoxide ion, regenerating phenol.
   • The phenol, being less soluble in water, separates from the aqueous layer and can be isolated by methods like distillation.

Reaction Mechanism

Dow’s process does not follow a typical SN​Ar (nucleophilic aromatic substitution) pathway, as a strong electron-withdrawing group is not present to stabilize the intermediate. Instead, it proceeds through an elimination-addition mechanism involving a highly reactive intermediate called benzyne.

1. Formation of Benzyne:
   • The strong base, sodium hydroxide, removes a proton from the benzene ring, specifically from the carbon adjacent to the chlorine atom.
   • This is an E2-like elimination. The electron pair from the C-H bond moves to form a triple bond with the carbon bearing the chlorine atom, and the chlorine atom leaves as a chloride ion.
   • This forms the highly strained and unstable benzyne intermediate (C6​H4​).
2. Nucleophilic Addition to Benzyne:
   • The hydroxide ion (OH−) then attacks the triple bond of the benzyne intermediate.
   • The attack can occur at either of the two carbons of the triple bond, as they are equivalent.
   • This addition leads to the formation of a carbanion intermediate.
3. Protonation:
   • The carbanion intermediate quickly abstracts a proton from the solvent (water) to form phenol.

Summary Table

Significance and Modern Alternatives

• Dow’s process was one of the first commercially viable methods for producing phenol on an industrial scale.
• The use of such extreme conditions (high temperature and pressure) highlights the difficulty of achieving nucleophilic substitution on an unactivated benzene ring.
• While historically important, modern industrial production of phenol largely uses the cumene process, which is more economically and environmentally friendly. The cumene process involves the oxidation of cumene (isopropylbenzene) to cumene hydroperoxide, which is then cleaved to phenol and acetone.