Question

In Dow’s process for manufacture of phenol, PhCl is fused with NaOH at elevated temperature under pressure.
$$PhCl\underset{623K,300atm}{\overset{NaOH}{\to }}[Intermediate]\stackrel{H_{2}O}{\to }Phenol(A)+(B+C)sideproduct$$
Which of the following statements are correct?
(This question has multiple correct options)
(A) Phenol is formed via the formation of benzyne intermediate
(B) p – phenyl phenol is also formed as a by product
(C) Diphenyl ether is also formed as a by – product
(D) Biphenylene is also formed as a by – product

Answer 1

Hint: Dow’s process is hydrolysis of chlorobenzene. It is a nucleophilic substitution reaction. It is formed by an intermediate of benzene. Nucleophilic substitution is a type of reaction in which an electron rich compound replaces a leaving group.

Complete answer:
- We refer to Dow's Process to the hydrolysis of chlorobenzene for the preparation of phenol.
-We can easily convert benzene chlorobenzene by electrophilic aromatic substitution.
- Dow’s process is preparation of phenol from haloarenes, it is an industrial preparation of phenol.
- Chlorobenzenes with 10% NaOH solution is heated at 623K under 300 atm pressure.
- The major product of Dow’s process is phenol but other by- products are also formed as a result of reaction between the species formed during the reaction.

- Now let’s understand the reaction mechanism:
-In the first step, the base (NaOH) removes one hydrogen from the benzene ring and forms a carbanion which loses $$C{l}^{-}$$ ion to form benzyne intermediate.

-In the second step, an attack of nucleophile takes place. The benzyne intermediate formed gets attacked by nucleophile $$O{H}^{-}$$to form carbanion which subsequently gets protonated to form phenol.

- Now for side products, let’s see the conversion of reactants into reacting species.

- Due to the presence of phenoxide ions diphenyl ether is also a by – product.
- Another by – product we see is para- phenyl phenol which is formed by benzyne and phenoxide ions.
 

Therefore, the correct statements are options A. Phenol is formed via the formation of benzyne intermediate, B. p – phenyl phenol is also formed as a by-product, C. Diphenyl ether is also formed as a by – product.

Note:
-After the formation of benzyne, $$O{H}^{-}$$ nucleophilic attack takes place.
-The reaction mixture also contains some phenoxide ions. So, Diphenyl ether is a possible side product.
- The major product of Dow’s process is phenol but other by- products are also formed as a result of reaction between the species formed during the reaction.