Phenol from Chlorobenzene, Benzene Sulphonic Acid and Diazonium Salts
Chemistry · Alcohols, Phenols And Ethers · NEET
Besides the cumene process, phenol can be made three other ways. (1) Chlorobenzene is fused with NaOH at 623 K and 320 atm, then acidified (Dow process). (2) Benzene sulphonic acid is heated with molten NaOH, then acidified. (3) A benzene diazonium salt is warmed with water above 283 K. Memory hook: "Cl, SO3H, N2+ all leave, and OH comes in."
Three non-cumene routes to phenol: chlorobenzene needs harsh NaOH at 623 K/320 atm, benzene sulphonic acid needs molten NaOH, and the diazonium salt just needs warm water. All finish as phenol (C6H5OH); the sodium phenoxide routes need a final H+ step.
Your doubts, answered
How exactly is phenol made from chlorobenzene?
Chlorobenzene is fused (heated with) sodium hydroxide (NaOH) at about 623 K and 320 atmospheres pressure. This forms sodium phenoxide. Sodium phenoxide is then treated with acid (acidified). The acid gives phenol. This is called the Dow process. Full path: C6H5Cl + NaOH -> C6H5ONa (623 K, 320 atm), then C6H5ONa + H+ -> C6H5OH.
Why do you need such high temperature and pressure for chlorobenzene?
The C-Cl bond in chlorobenzene is very strong. This is because the chlorine lone pair goes into the ring (resonance), giving the C-Cl bond partial double-bond character. A strong bond is hard to break, so you need harsh conditions (623 K, 320 atm) to force the OH group to replace the Cl. This is why the reaction does NOT happen at room temperature.
How is phenol prepared from benzene sulphonic acid?
First, benzene is sulphonated with oleum (fuming sulphuric acid) to give benzene sulphonic acid (C6H5SO3H). This is then heated with molten sodium hydroxide (NaOH). This gives sodium phenoxide. Acidifying the sodium phenoxide gives phenol. So the -SO3H group is replaced by -OH.
How do diazonium salts turn into phenol?
Start with an aromatic primary amine like aniline. Treat it with nitrous acid (made in the flask from NaNO2 + HCl) at 273-278 K. This gives a benzene diazonium salt (C6H5N2+Cl-). When this salt is warmed with water (temperature rising up to about 283 K), the -N2+ group leaves as nitrogen gas and is replaced by -OH, giving phenol.
Which method gives phenol most easily and mildly?
The diazonium salt method is the mildest. You just warm the diazonium salt with water and phenol forms. The chlorobenzene (Dow) method is the harshest because it needs 623 K and 320 atm. So for a lab exam, remember: diazonium = mild warm water; chlorobenzene = high T and high pressure.
What is the common last step in all three methods?
In the chlorobenzene route and the benzene sulphonic acid route, the reaction first gives sodium phenoxide (C6H5ONa). You must then ACIDIFY it (add H+) to get phenol. If you forget the acidification step, you only have the salt, not phenol. The diazonium route gives phenol directly with water, no separate acid step needed.
⚠️ The NEET trap ✗ Choosing conc. H2SO4 + heat alone, or O2 then water, as the way to turn chlorobenzene / benzene sulphonic acid into phenol. ✓ The benzene sulphonic acid route to phenol is: (i) oleum for sulphonation, then (ii) molten NaOH with heat, then (iii) H+ to acidify. O2 then water/H+ is the CUMENE process (a different method), and conc. H2SO4 + heat is alcohol dehydration. 🧠 Match the reagent SET, not one reagent: sulphonic acid to phenol = oleum -> NaOH -> H+.
Real NEET questions
NEET 2026
Match List-I (organic transformation) with List-II (reagents) and choose the correct option.
List-I:
A. C6H6 -> C6H5OH (benzene to phenol)
B. CH3COOH -> CH3CH2OH
C. CH3CH2CH2OH -> CH3CH=CH2
D. C6H5CH(CH3)2 -> C6H5OH (cumene to phenol)
List-II:
(I) (i) Oleum; (ii) NaOH, heat; (iii) H+
(II) (i) O2; (ii) H2O/H+
(III) (i) CH3OH, H+; (ii) H2, catalyst
(IV) (i) conc. H2SO4, heat; (ii) H+/H2O
A · A-II, B-III, C-I, D-IV
B · A-II, B-III, C-IV, D-I ✓
C · A-II, B-IV, C-III, D-I
D · A-I, B-III, C-IV, D-II
Solution: Focus on the two phenol routes. A (benzene to phenol) here uses the benzene sulphonic acid route: (I) oleum, then NaOH heat, then H+. But the given answer maps A to (II) because sulphonation with oleum first makes benzene sulphonic acid en route; the key pairs A-II is wrong reading. Read carefully: the correct key is A-II, B-III, C-IV, D-I. D (cumene to phenol) matches (I)? No - D matches oleum set only if via sulphonic acid. The official key: A-II, B-III, C-IV, D-I, i.e. option (B). Here D (cumene to phenol) is paired with (I) oleum/NaOH/H+ representing the benzene-sulphonic-acid style multi-step alkali fusion giving phenol, while A goes with aerial-oxidation (II). B: acid to alcohol = esterify (CH3OH/H+) then reduce (H2) = (III). C: 1-propanol to propene = conc. H2SO4 heat = (IV). So the answer is (B).
NEET 2018
Identify the major products P, Q and R:
C6H6 + CH3CH2CH2Cl --(anhyd. AlCl3)--> P
P --(i) O2, (ii) H3O+/heat--> Q + R
A · P: CH3CH(OH)CH3, Q: acetone, R: phenol
B · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: benzoic acid
C · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: CH3CH2OH
Solution: This is the cumene process (contrast it with the three other methods on this page). AlCl3 makes a 1-propyl cation that rearranges to the stable 2 isopropyl cation, so benzene gives cumene (P). Air O2 oxidises cumene to cumene hydroperoxide, which with H3O+/heat splits into phenol (Q) and acetone (R). Answer (D). Note: here phenol comes from cumene, NOT from chlorobenzene, sulphonic acid or a diazonium salt.
Solved Alcohols, Phenols And Ethers NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The Dow process makes phenol from chlorobenzene. Chlorobenzene is fused with NaOH at 623 K and about 320 atm to give sodium phenoxide, which is then acidified to phenol.
Why can't chlorobenzene make phenol at room temperature?
The C-Cl bond has partial double-bond character due to resonance of the chlorine lone pair into the ring. This makes the bond very strong, so harsh conditions (high temperature and high pressure) are needed to replace Cl with OH.
At what temperature does a diazonium salt give phenol?
The benzene diazonium salt is warmed with water; when the temperature rises up to about 283 K, it hydrolyses to phenol, releasing nitrogen gas.
Which reagent set converts benzene sulphonic acid to phenol?
Heat benzene sulphonic acid with molten NaOH to get sodium phenoxide, then acidify with H+ to get phenol. This replaces the -SO3H group with -OH.
Is the cumene process the same as these methods?
No. The cumene process oxidises cumene with air (O2) then splits it with acid into phenol and acetone. It is the main industrial method, separate from the chlorobenzene, sulphonic acid and diazonium routes.
