Preparation of Phenol
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Direct answer
World phenol supply begins with benzene and propene becoming cumene (isopropylbenzene), which air oxidation converts to cumene hydroperoxide; dilute sulphuric acid then rearranges and cleaves it into phenol and acetone in one pot. The mass ledger is exact — cumene (120 g) splits into phenol (94 g) plus acetone (58 g) — so every tonne of phenol arrives with about 0.62 tonnes of acetone beside it. Older and laboratory routes persist in exams: fused alkali on sodium benzenesulphonate, the Dow process (chlorobenzene + NaOH at 623 K and 300 atm), and hydrolysis of benzenediazonium salts with warm water. Coal tar's middle (carbolic) oil was the classical primary source before all of these.
What you must remember
- Cumene sequence: benzene + propene (acid catalyst) → cumene; O2 → cumene hydroperoxide; dilute H2SO4 → phenol + acetone (the Hock rearrangement).
- Carbon and mass accounting: C9H12O2 hydroperoxide → C6H5OH + CH3COCH3; 120 = 94 + 58, so acetone co-production fixes the economics of the process.
- Dow process: chlorobenzene + NaOH at 623 K and 300 atm → sodium phenoxide, then acidification — brutal conditions because the aryl C-Cl bond has partial double-bond character (benzyne mechanism at Advanced level).
- Sulphonate fusion: C6H5SO3Na + 2NaOH (fused) → C6H5ONa + Na2SO3 + H2O, then H+ — the classical laboratory synthesis.
- Diazonium route: C6H5N2^+Cl^- + warm water → phenol + N2 + HCl; inside conversion chains, this is how aniline hands you phenol.
- Conversion pair on phenol itself: Kolbe-Schmitt (phenoxide + CO2 at 400 K → salicylate → salicylic acid) and Reimer-Tiemann (CHCl3 + NaOH → salicylaldehyde) — reactions of phenol, frequently mislabelled as preparations.
- Coal tar origin: the middle fraction (carbolic oil) around 170-270°C carries phenol, cresols and xylenols — the historical answer to "where did phenol first come from".
Working the cumene arithmetic
Charge 240 kg of cumene (2 kmol). Perfect conversion gives 2 kmol phenol = 188 kg and 2 kmol acetone = 116 kg; at 85% yield, 159.8 kg of phenol leaves the plant per batch. The exercise matters because JEE integer questions dress exactly this stoichiometry in industrial clothing, and because it explains a real economic quirk: phenol plants live and die by acetone demand, since the co-product cannot be switched off.
Now the synthesis-chain version. From benzene, reach phenol in two steps: nitration (HNO3/H2SO4) to nitrobenzene, reduction (Sn/HCl or H2/Pd) to aniline, diazotisation (NaNO2/HCl at 273-278 K), then warm water to phenol. Three steps after nitration — and every step is a named reagent set JEE expects you to carry without hesitation. The chain also runs backwards in reverse questions (phenol to aniline goes via reduction with Zn dust to benzene, then forward again).
The examiner's angle
Confusion between preparation and reaction is the deliberate trap: Kolbe-Schmitt and Reimer-Tiemann start from phenol (they prepare salicylic acid and salicylaldehyde), so an option offering them as phenol syntheses is bait. The second probe is the Dow process's harsh conditions — the question "why 623 K and 300 atm?" tests the aryl C-Cl bond's strength, attributable to resonance delocalisation of the chlorine lone pair into the ring (and, at Advanced level, to the benzyne elimination-addition mechanism, which p-chlorotoluene with sodamide betrays by giving two aniline isomers). Third, know the functional-group tolerance of each route: the diazonium route needs aniline; the sulphonate route needs benzene plus sulphonation; the cumene route needs an acid-catalysed alkylation. When a multi-step question lands, pick the route whose first step the starting material already permits.
Frequently asked questions
Why does the cumene process dominate industrial phenol?
Benzene and propene are cheap, the sequence is short, and the acetone co-product is itself valuable — the 94:58 mass split delivers two sellable chemicals at once.
Why does the Dow process need such extreme conditions?
The carbon-chlorine bond of chlorobenzene is shortened and strengthened by partial double-bond character from resonance, so nucleophilic substitution demands 623 K and 300 atm.
How is phenol obtained from aniline?
Diazotise with NaNO2 and HCl at 273-278 K, then warm the diazonium salt with water — nitrogen leaves and the ring hydroxyl gives phenol.
What does Kolbe-Schmitt actually prepare?
Salicylic acid, from sodium phenoxide and carbon dioxide at about 400 K followed by acidification — a conversion of phenol, not a synthesis of it.
What evidence supports the benzyne mechanism in aryl substitution?
p-Chlorotoluene with sodamide gives both p- and m-toluidine — the symmetrical benzyne intermediate can be attacked at either adjacent carbon.