# 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 ($\mathrm{C_6H_5NH_2}$) - Nitrous acid ($\mathrm{HNO_2}$), generated *in situ* from sodium nitrite ($\mathrm{NaNO_2}$) 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 ($-NH_2$) is converted into a diazonium group ($-N_2^+$). The reaction is: $$ \mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{<10^{\circ}C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O} $$ ### 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. $$ \mathrm{C_6H_5N_2^+Cl^- + H_2O \xrightarrow{Warm} C_6H_5OH + N_2(g) + HCl} $$ ### 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. $$ \mathrm{C_6H_5Cl + 2NaOH \xrightarrow{360^{\circ}C, 150 atm} C_6H_5ONa + NaCl + H_2O} $$ $$ \mathrm{C_6H_5ONa + HCl \rightarrow C_6H_5OH + NaCl} $$ ### 2. From Sodium Benzene Sulfonate Sodium benzene sulfonate is fused with solid $\mathrm{NaOH}$ at $320^{\circ}\mathrm{C}$ to yield sodium phenoxide. $$ \mathrm{C_6H_5SO_3Na + 2NaOH \xrightarrow{320^{\circ}C} C_6H_5ONa + Na_2SO_3 + H_2O} $$ ### 3. From Cumene (Isopropylbenzene) This is a modern commercial method. Cumene is oxidized by air to cumene hydroperoxide, which is then decomposed by dilute $\mathrm{H_2SO_4}$ into phenol and acetone. --- ## Summary Phenol is synthesized from phenylamine (aniline) in two steps: 1. **Diazotization:** Phenylamine reacts with $\mathrm{HNO_2}$ (from $\mathrm{NaNO_2 + HCl}$) 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 .