Preparing Phenol — Laboratory Methods

Phenol (carbolic acid) can be made from several aromatic starting materials.

(1) From haloarenes (Dow process): chlorobenzene is heated with aqueous NaOH at high temperature (~623 K) and pressure (~300 atm) to give sodium phenoxide, then acidified: C6H5Cl→NaOH, 623 K, 300 atmC6H5ONa→H+C6H5OH\text{C}_6\text{H}_5\text{Cl} \xrightarrow{\text{NaOH, 623 K, 300 atm}} \text{C}_6\text{H}_5\text{ONa} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH}

(2) From benzenesulphonic acid: benzene is sulphonated, and the sodium benzenesulphonate is fused with NaOH, then acidified: C6H6→H2SO4C6H5SO3H→NaOH (fuse)C6H5OH\text{C}_6\text{H}_6 \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{SO}_3\text{H} \xrightarrow{\text{NaOH (fuse)}} \text{C}_6\text{H}_5\text{OH}

(3) From diazonium salts: a benzenediazonium salt is warmed with water (or dilute acid): C6H5N2+Cl−+H2O→warmC6H5OH+N2+HCl\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{H}_2\text{O} \xrightarrow{\text{warm}} \text{C}_6\text{H}_5\text{OH} + \text{N}_2 + \text{HCl}

Preparing Phenol — The Cumene Process (Industrial)

The most important industrial method is the cumene process.

Step 1: Cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide.

Step 2: The cumene hydroperoxide is treated with dilute acid, which cleaves it into phenol and acetone (propanone): Cumene→air (O2)cumene hydroperoxide→H+phenol+acetone\text{Cumene} \xrightarrow{\text{air (O}_2)} \text{cumene hydroperoxide} \xrightarrow{\text{H}^+} \text{phenol} + \text{acetone}

Routes to phenol from cumene, haloarene and diazonium salt

This is economical because it produces two useful products (phenol and acetone) in one process.

Key Point: Four routes to phenol — chlorobenzene (Dow, harsh conditions), benzenesulphonic acid (NaOH fusion), diazonium salt (warm water), and the industrial cumene process (gives phenol + acetone). The cumene process is the one to remember for "industrial preparation."

[NEET Important] "Phenol from cumene" gives phenol AND acetone — this co-production is a favourite exam point. Cumene is isopropylbenzene; it is air-oxidised to cumene hydroperoxide, then acid-cleaved.

Why These Methods Work

Each method replaces a leaving group on the ring with -OH:

  • In the Dow process, the strong nucleophile OH−^- displaces chloride — but only under drastic conditions, because aryl halides are unreactive (Chapter 6).
  • In the benzenesulphonic acid route, the -SO3_3Na group is replaced by -OH on fusion with alkali.
  • In the diazonium route, the very good leaving group N2_2 departs as the diazonium salt reacts with water.
  • In the cumene process, a rearrangement of the hydroperoxide under acid gives phenol and acetone.

Worked comparison: the laboratory routes (diazonium, benzenesulphonic acid) are convenient on a small scale, while the cumene process dominates industry because it is cheap and co-produces acetone.

[JEE Tip] When asked "how is phenol prepared from benzene using only conc. H2_2SO4_4 and NaOH," the answer is the benzenesulphonic acid route: benzene → benzenesulphonic acid (conc. H2_2SO4_4) → sodium salt → fuse with NaOH → sodium phenoxide → acidify → phenol.

Solved Examples

Example 1: Phenol from chlorobenzene

Write the conditions for preparing phenol from chlorobenzene.

Solution: Heat chlorobenzene with aqueous NaOH at ~623 K and ~300 atm (the Dow process) to give sodium phenoxide, then acidify to obtain phenol.

Example 2: Cumene process products

What two products are obtained when cumene hydroperoxide is treated with dilute acid?

Solution: Phenol and acetone (propanone). This is the basis of the industrial cumene process.

Example 3: Phenol from a diazonium salt

What happens when benzenediazonium chloride is warmed with water?

Solution: It is hydrolysed to phenol, releasing nitrogen gas: C6H5N2+Cl−+H2O→C6H5OH+N2+HCl\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{N}_2 + \text{HCl}

Example 4: Phenol from benzene (acid + alkali only)

How is phenol prepared from benzene using conc. H2_2SO4_4 and NaOH?

Solution: Benzene + conc. H2_2SO4_4 → benzenesulphonic acid → neutralise to sodium benzenesulphonate → fuse with NaOH → sodium phenoxide → acidify → phenol.

Example 5: Industrial preference

Why is the cumene process the preferred industrial method for making phenol?

Solution: It is economical because it co-produces a second valuable product, acetone, alongside phenol from cheap cumene and air. The acetone offsets the cost, making phenol cheaper to manufacture this way.

Example 6: Identify the starting material

Cumene is the starting material in the industrial preparation of phenol. What is cumene's other name?

Solution: Cumene is isopropylbenzene (C6_6H5_5-CH(CH3_3)2_2).

Example 7: Role of NaOH fusion

In the benzenesulphonic acid route, why is sodium benzenesulphonate fused with NaOH?

Solution: The molten (fused) NaOH provides the strong nucleophile OH−^- at high temperature to displace the -SO3_3Na group, converting it to -O−Na+^-\text{Na}^+ (sodium phenoxide), which is then acidified to phenol.

Example 8: Drastic conditions for the Dow process

Why does the conversion of chlorobenzene to phenol require such drastic conditions?

Solution: Chlorobenzene is an aryl halide; its C-Cl bond has partial double-bond character (resonance) and is hard to break, so nucleophilic substitution by OH−^- needs very high temperature and pressure.

Example 9: Product of cumene oxidation (step 1)

What is formed when cumene is oxidised by air?

Solution: Cumene hydroperoxide — the air oxidises the benzylic C-H of the isopropyl group to a -OOH (hydroperoxide). It is then cleaved by acid to phenol and acetone.

Example 10: Choosing a method

Which method would you use to prepare a pure sample of phenol in the laboratory from aniline?

Solution: Diazotise aniline (NaNO2_2/HCl, 0-5 °C) to the diazonium salt, then warm with water to hydrolyse it to phenol (releasing N2_2).