10.2 Preparation of Phenol

This section describes the laboratory and industrial methods for synthesizing phenol. While the laboratory method involves a two-step process from an aromatic primary amine (aniline), several industrial methods are used to produce phenol on a large scale.

Preparation from Phenylamine (Aniline)

The preparation of phenol from phenylamine involves two main stages: diazotization and hydrolysis.

Step 1: Formation of Benzene Diazonium Salt (Diazotization)

Phenylamine (aniline) is first converted into a diazonium salt.

Reactants:

• Phenylamine (${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{H}}_2$)
• Nitrous acid ($\mathrm{HN}{\mathrm{O}}_2$), generated in situ from sodium nitrite ($\mathrm{NaN}{\mathrm{O}}_2$) and hydrochloric acid ($\mathrm{HCl}$)

Conditions: The reaction must be carried out at a temperature below $10^{\circ }\mathrm{C}$ (usually $0-5^{\circ }\mathrm{C}$) to prevent decomposition of the unstable diazonium salt.

The amine group ($-N{H}_2$) is converted into a diazonium group ($-N_2^{+}$). The reaction is:

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{H}}_2+\mathrm{NaN}{\mathrm{O}}_2+2HCl\stackrel{<10^{\circ }\mathrm{C}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}\mathrm{C}{\mathrm{l}}^{-}+\mathrm{NaCl}+2{\mathrm{H}}_2\mathrm{O}$

Step 2: Hydrolysis of the Diazonium Salt

The benzene diazonium salt is converted to phenol by gentle warming with water.

Conditions: The aqueous solution is warmed to above $10^{\circ }\mathrm{C}$.

Reaction: The diazonium group is replaced by a hydroxyl group ($-OH$) from water. Nitrogen gas is evolved.

${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}\mathrm{C}{\mathrm{l}}^{-}+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Warm}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+{\mathrm{N}}_2(\mathrm{g})+\mathrm{HCl}$

Overall Reaction Scheme

The complete two-step synthesis from aniline to phenol is shown below.

---

Industrial Preparation of Phenol

In the FBISE curriculum, several industrial methods are essential for understanding the large-scale production of phenol.

1. From Chlorobenzene (Dow's Process)

Chlorobenzene is heated with $10\%$ $\mathrm{NaOH}$ at $360^{\circ }\mathrm{C}$ and $150$ atm pressure to form sodium phenoxide, which is then treated with $\mathrm{HCl}$ to release phenol.

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}+2NaOH\stackrel{360^{\circ }\mathrm{C},150atm}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$ ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+\mathrm{HCl}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+\mathrm{NaCl}$

2. From Sodium Benzene Sulfonate

Sodium benzene sulfonate is fused with solid $\mathrm{NaOH}$ at $320^{\circ }\mathrm{C}$ to yield sodium phenoxide.

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_3\mathrm{Na}+2NaOH\stackrel{320^{\circ }\mathrm{C}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$

3. From Cumene (Isopropylbenzene)

This is a modern commercial method. Cumene is oxidized by air to cumene hydroperoxide, which is then decomposed by dilute ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ into phenol and acetone.

---

Summary

Phenol is synthesized from phenylamine (aniline) in two steps:

1. Diazotization: Phenylamine reacts with $\mathrm{HN}{\mathrm{O}}_2$ (from $\mathrm{NaN}{\mathrm{O}}_2+\mathrm{HCl}$) below $10^{\circ }\mathrm{C}$ to form benzene diazonium chloride.
2. Hydrolysis: Benzene diazonium chloride is warmed above $10^{\circ }\mathrm{C}$ to form phenol, nitrogen gas, and HCl.

Commercially, phenol is prepared via Dow's Process (from chlorobenzene), sulfonation (from benzene sulfonic acid), or the Cumene process.

The diazonium salt serves as a key intermediate in organic synthesis. For further details on the reactivity of phenol, see .