Preparation of Phenols

The Four Routes at a Glance

A phenol carries a hydroxyl group bonded directly to the sp² carbon of an aromatic ring. That single structural fact controls how phenols are made. The aromatic C–O bond cannot be installed by the easy substitution and hydration reactions used for ordinary alcohols; aromatic carbon resists nucleophilic attack. The four NCERT routes therefore each rely on a special trick to force a hydroxyl onto the ring — either brute-force conditions, a good leaving group, or a rearrangement.

Three of these are essentially laboratory or older commercial routes, while one — the cumene hydroperoxide route — dominates real-world production. NCERT states plainly that most of the worldwide production of phenol is from cumene. The map below orients all four around their common product, sodium phenoxide or phenol itself.

Notice a recurring pattern across three of the four routes: the immediate product is not phenol but sodium phenoxide, the conjugate base. Because phenol is a weak acid, it is liberated only in the final acidification step. Keeping this in mind helps you write balanced equations correctly and prevents the common slip of omitting the acidification stage in the haloarene and sulphonic-acid routes. The cumene process is the exception — it delivers neutral phenol directly, alongside acetone.

From Haloarenes (Dow Process)

Chlorobenzene is fused with sodium hydroxide at 623 K and 320 atmospheres pressure. Sodium phenoxide is produced, and acidification of the phenoxide gives phenol. This is the industrial Dow process, and it is also one of the methods you met in the haloarenes unit.

$\ce{C6H5Cl + 2NaOH ->[\text{623 K}][\text{320 atm}] C6H5ONa + NaCl + H2O}$

$\ce{C6H5ONa + HCl -> C6H5OH + NaCl}$

The extreme temperature and pressure are the whole point of this reaction. The C–Cl bond in chlorobenzene is far less reactive than the C–X bond in an alkyl halide because the chlorine lone pair conjugates into the ring, giving the bond partial double-bond character, and because the carbon is sp² hybridised. Only forcing conditions allow hydroxide to displace chloride.

From Benzenesulphonic Acid

Benzene is first sulphonated with oleum to give benzenesulphonic acid. The sulphonic acid group is then replaced by hydroxyl through alkali fusion: heating with molten sodium hydroxide gives sodium phenoxide, which on acidification yields phenol. NIOS notes that this alkali-fusion route was the first commercial synthesis of phenol, developed in Germany in 1890.

$\ce{C6H6 ->[\text{oleum}] C6H5SO3H}$

$\ce{C6H5SO3H + 2NaOH ->[\Delta][\text{fuse}] C6H5ONa + Na2SO3 + H2O}$

$\ce{C6H5ONa + H^+ -> C6H5OH}$

This is exactly the synthesis NCERT exercise 7.12 asks you to write when given only benzene, concentrated H₂SO₄ and NaOH: sulphonate, fuse with alkali, then acidify.

From Diazonium Salts

This is the gentlest route and the one NIOS calls the most general laboratory method. An aromatic primary amine — aniline — is treated with nitrous acid, generated in situ from sodium nitrite and hydrochloric acid, at 273–278 K. This diazotisation gives a benzenediazonium salt. Warming the salt with water, or treating it with a dilute acid, hydrolyses it to phenol with loss of nitrogen gas.

$\ce{C6H5NH2 ->[\text{NaNO2 + HCl}][\text{273-278 K}] C6H5N2^+Cl^-}$

$\ce{C6H5N2^+Cl^- + H2O ->[\text{warm}] C6H5OH + N2 ^ + HCl}$

The driving force is the exceptional leaving ability of the diazonium group: nitrogen gas is an outstanding leaving group, so the substitution proceeds under mild warming rather than the brutal conditions the haloarene route demands.

From Cumene (Industrial Route)

This is the method that matters commercially. NCERT states that phenol is manufactured from the hydrocarbon cumene, and that most of the worldwide production of phenol is from cumene. Cumene — isopropylbenzene — is oxidised by air to cumene hydroperoxide, which is then treated with dilute acid to give phenol and acetone together.

$\ce{C6H6 + CH3CH=CH2 ->[\text{H}_3\text{PO}_4] C6H5CH(CH3)2}$

$\ce{C6H5CH(CH3)2 + O2 ->[\text{air}] C6H5C(CH3)2OOH}$

$\ce{C6H5C(CH3)2OOH ->[\text{dil. acid}] C6H5OH + (CH3)2CO}$

The economic genius of the route is the by-product. Acetone is itself a high-value industrial solvent, so a single plant produces two saleable chemicals from cheap benzene and propene. That co-production is precisely why cumene displaced both the Dow and alkali-fusion processes. The starting hydrocarbon is also cheap to assemble: cumene is itself made by Friedel–Crafts alkylation of benzene with propene over phosphoric acid, so the entire chain begins from two of the most abundant petrochemical feedstocks. For the exam, hold on to the headline NCERT statement — phenol is manufactured from cumene, and acetone is obtained in large quantities as a by-product of the same reaction.

The Cumene Process Step by Step

For NEET you are expected to reproduce the equations and the structure of the hydroperoxide; the acid-catalysed step is best understood as a hydrolytic rearrangement, the term NIOS uses. The schematic below tracks the three transformations and the carbon skeleton at each stage.

In words: air oxidation inserts an –O–O–H unit onto the benzylic carbon of the isopropyl group, giving the tertiary hydroperoxide. Treatment with dilute acid protonates the –OOH, triggering migration of the phenyl group from carbon to oxygen and ultimately splitting the molecule. Water captures the resulting cation, and the fragment collapses to phenol and acetone. You are not asked to draw every arrow at NEET level; you are asked to give the three balanced equations and identify acetone as the second product.

Laboratory versus Industrial Methods

A frequent source of marks lost is mislabelling which route is "industrial." NCERT and NIOS together draw a clear line. The diazonium and alkali-fusion routes are described as laboratory or older commercial methods; the Dow process is an industrial route now largely superseded; and the cumene route is the modern industrial workhorse.

Common NEET Confusions

Beyond the temperature swap in the diazonium method, two further traps appear repeatedly. The first concerns the by-product of the cumene process; the second concerns which method genuinely scales to industry.

The second is a labelling trap. Although phenol can be prepared from chlorobenzene, the question of which method is used "to manufacture phenol" or "industrially today" has a single intended answer: the cumene process. Reserve "Dow process" for the chlorobenzene route, and remember it is the older industrial method that cumene has displaced.