MHT-CET Chemistry · Alcohols, Phenols and Ethers

Phenols: Preparation and Reactions

Phenol is made industrially from cumene (air oxidation, then acid) and from chlorobenzene (Dow); its ring is so activated by OH that bromine water gives 2,4,6-tribromophenol and mixed acid gives picric acid, while the phenoxide gives salicylic acid with CO₂ (Kolbe) and salicylaldehyde with chloroform (Reimer–Tiemann).

All Alcohols, Phenols and Ethers notes MHT-CET Chemistry strategy

Why this matters

26 PYQs, 2 HARD. Eight are ring substitution — the reagent for picric acid (mixed acid), dilute HNO₃ giving the o/p mixture, bromine water giving the tribromo product; ten are named reactions — Kolbe's substrate and product, Reimer–Tiemann's reagent, CrO₃ to p-benzoquinone, zinc dust to benzene, H₂/Ni to cyclohexanol; five are the cumene and Dow preparations; three are natural phenols — gallic acid, eugenol, the phloroglucinol drawing. Four cards.

Concept 1 of 4

Making Phenol: Cumene and Dow

Intuition

Two routes, both from benzene derivatives. Cumene (isopropylbenzene) is oxidised by air with cobalt naphthenate to cumene hydroperoxide, which dilute acid splits into phenol AND acetone. Chlorobenzene with fused NaOH at 623 K and 300 atm gives sodium phenoxide through a phenoxide-ion intermediate; acid then frees phenol.

Definition

Cumene process: $\text{C}_6\text{H}_5\text{CH(CH}_3)_2 \xrightarrow{\text{O}_2,\ \text{Co-naphthenate},\ 423\text{ K}} \text{cumene hydroperoxide} \xrightarrow{\text{dil. HCl}} \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{COCH}_3$ . Substrate X = isopropylbenzene; product = phenol + acetone (not phenol + CO₂).
Cumene with $\text{KMnO}_4/\text{KOH}$ instead gives potassium benzoate → benzoic acid on acidification (side-chain oxidation, not phenol).
Dow process: chlorobenzene + NaOH, 623 K / 300 atm → sodium phenoxide (intermediate: phenoxide ion) $\xrightarrow{\text{H}_3\text{O}^+}$ phenol; the phenol then gives 2,4,6-tribromophenol with bromine water.
Also: benzene sulphonic acid + fused NaOH; benzene diazonium chloride + warm water.

Cumene route

\text{C}_6\text{H}_5\text{CH(CH}_3)_2 \xrightarrow{\text{O}_2} \text{C}_6\text{H}_5\text{C(CH}_3)_2\text{OOH} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH} + (\text{CH}_3)_2\text{CO}

Worked example

Identify A and B: benzene

\xrightarrow{\text{CH}_3\text{CH=CH}_2,\ \text{H}^+}

\xrightarrow{\text{O}_2;\ \text{H}_3\text{O}^+}

B + acetone.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Alcohols, Phenols and EthersMODERATE

What is the product formed when cumene is air oxidised in presence of Co-naphthenate and further treated with dilute acid?

[Q87 · Shift 1 · 2022]

Stopping at cumene hydroperoxide

The hydroperoxide is the intermediate; the question asks for the products after dilute acid — phenol AND acetone, both of them, which is why the process pays.

Drill 4 more on making phenol: cumene and dow

Concept 2 of 4

Nitration and Bromination of Phenol

Intuition

OH activates the ring strongly at ortho and para. So mild reagents go far: dilute HNO₃ at low temperature gives a MIXTURE of o- and p-nitrophenol, and bromine WATER goes all the way to 2,4,6-tribromophenol as a white precipitate without any catalyst. Only concentrated HNO₃ with H₂SO₄ (mixed acid) reaches picric acid; in CS₂ at low temperature bromine stops at monobromination.

Definition

Dilute $\text{HNO}_3$ , 298 K: o- + p-nitrophenol (mixture; ortho separated by steam distillation).
Conc. $\text{HNO}_3$ + conc. $\text{H}_2\text{SO}_4$ : picric acid (2,4,6-trinitrophenol). Dilute nitric acid alone cannot trinitrate.
$\text{Br}_2$ in WATER: 2,4,6-tribromophenol (white precipitate). $\text{Br}_2$ in $\text{CS}_2$ or $\text{CHCl}_3$ , 273 K: o- and p-bromophenol (para major).
Phenol + $\text{H}_2\text{SO}_4$ : o-hydroxybenzenesulphonic acid at 288 K, para at 373 K.

How far the reaction goes

\text{dil. HNO}_3 \to o/p\text{-nitrophenol};\quad \text{HNO}_3/\text{H}_2\text{SO}_4 \to \text{picric acid};\quad \text{Br}_2(\text{aq}) \to 2,4,6\text{-tribromophenol}

Worked example

Phenol is treated with (i) bromine in CS₂ at 0 °C and (ii) bromine water. Name the major product of each.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Alcohols, Phenols and EthersEASY

Identify the product obtained when phenol is reacted with dilute nitric acid at low temperature.

[Q59 · 11th May Shift 1 · 2024]

Trinitrating with dilute acid

Dilute HNO₃ gives the mononitro mixture; 2,4,6-trinitrophenol needs the mixed acid. Bromine is the opposite case — WATER gives the tri-substituted product, the organic solvent the mono.

Drill 7 more on nitration and bromination of phenol

Concept 3 of 4

Kolbe, Reimer–Tiemann, Oxidation and Reduction of Phenol

Intuition

The phenoxide ion is a better nucleophile than phenol, so the named reactions start from it. Kolbe: sodium phenoxide + CO₂ at 398 K and 6 atm gives sodium salicylate, acidified to salicylic acid. Reimer–Tiemann: phenol + CHCl₃ + aqueous NaOH puts CHO ortho — salicylaldehyde. Chromic oxide oxidises phenol to p-benzoquinone; zinc dust strips the OH to give benzene; H₂ over nickel saturates the ring to cyclohexanol.

Definition

Kolbe (Kolbe–Schmitt): $\text{C}_6\text{H}_5\text{ONa} \xrightarrow{\text{CO}_2,\ 398\text{ K},\ 6\text{ atm}} \text{sodium salicylate} \xrightarrow{\text{H}_3\text{O}^+} \text{salicylic acid}$ . Substrate A = sodium phenoxide; product B = salicylic acid.
Reimer–Tiemann: reagent $\text{CHCl}_3$ , aq. NaOH, then $\text{H}_3\text{O}^+$ → salicylaldehyde (2-hydroxybenzaldehyde).
Oxidation $\text{CrO}_3$ (or $\text{Na}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4$ ): p-benzoquinone. $\text{KMnO}_4$ oxidises the side chain of cumene, not the ring.
Zn dust, heat: phenol → benzene (the OH is removed). H₂/Ni, 433 K: phenol → cyclohexanol.
Phenol + acyl chloride/pyridine → aryl ester (O-acylation); phenol + FeCl₃ → violet colour (test).

Kolbe and Reimer–Tiemann

\text{ArONa} + \text{CO}_2 \to o\text{-HOC}_6\text{H}_4\text{COONa} \to \text{salicylic acid};\qquad \text{ArOH} + \text{CHCl}_3/\text{NaOH} \to o\text{-HOC}_6\text{H}_4\text{CHO}

Worked example

Starting from phenol, how would you make (i) salicylic acid and (ii) salicylaldehyde?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Alcohols, Phenols and EthersMODERATE

Identify product 'B' in the following reaction. Sodium phenoxide

\xrightarrow{\text{CO}_2,\ 398\text{K},\ 6\ \text{atm}} A \xrightarrow{\text{H}_3\text{O}^+} B

[Q100 · 11th May Shift 2 · 2023]

Starting Kolbe from phenol itself

The substrate is SODIUM PHENOXIDE — the phenoxide ion is what attacks CO₂. 'Phenol' is offered as option (a) and is wrong.

Drill 9 more on kolbe, reimer–tiemann, oxidation and reduction of phenol

Concept 4 of 4

Phenols in Nature: Eugenol, Gallic Acid, Curcumin and the Polyols

Intuition

A short list of naturally occurring phenols with a source and a use each. Eugenol from clove oil is the dentist's analgesic and antimicrobial; gallic acid comes from gall nuts and Indian gooseberry (amla); curcumin from turmeric; methyl salicylate from wintergreen. Ascorbic acid is the one on the list that is NOT a phenol.

Definition

Eugenol (4-allyl-2-methoxyphenol): clove oil; analgesic and antimicrobial, local anaesthetic in dentistry.
Gallic acid (3,4,5-trihydroxybenzoic acid): gall nuts, Indian gooseberry; antioxidant.
Curcumin: turmeric; antioxidant, anti-inflammatory. Methyl salicylate: wintergreen oil.
Drawn structures to recognise: phloroglucinol (1,3,5-triol, symmetric), pyrogallol (1,2,3), hydroxyhydroquinone (1,2,4), resorcinol (1,3).

Phenol	Source	Property / use
Eugenol	Clove	Analgesic and antimicrobial Not 'antiseptic' alone — the paper keys the analgesic-and-antimicrobial pair.
Gallic acid	Indian gooseberry (amla), gall nuts	Antioxidant
Curcumin	Turmeric	Antioxidant, anti-inflammatory
Methyl salicylate	Wintergreen	Liniment
Ascorbic acid	Citrus fruits	Vitamin C — NOT a phenol No OH on a benzene ring.

Source and use are asked in both directions.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 4Alcohols, Phenols and EthersEASY

Identify the source of gallic acid from following

[Q95 · 22 April Shift I · 2025]

Confusing the 1,2,3 and 1,3,5 triol drawings

Phloroglucinol is the SYMMETRIC one — OH at alternate carbons. Three OH in a row is pyrogallol; the paper draws both and asks for one.

Drill 2 more on phenols in nature: eugenol, gallic acid, curcumin and the polyols

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Making Phenol: Cumene and Dow
Cumene route
$\text{C}_6\text{H}_5\text{CH(CH}_3)_2 \xrightarrow{\text{O}_2} \text{C}_6\text{H}_5\text{C(CH}_3)_2\text{OOH} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH} + (\text{CH}_3)_2\text{CO}$
Nitration and Bromination of Phenol
How far the reaction goes
$\text{dil. HNO}_3 \to o/p\text{-nitrophenol};\quad \text{HNO}_3/\text{H}_2\text{SO}_4 \to \text{picric acid};\quad \text{Br}_2(\text{aq}) \to 2,4,6\text{-tribromophenol}$
Kolbe, Reimer–Tiemann, Oxidation and Reduction of Phenol
Kolbe and Reimer–Tiemann
$\text{ArONa} + \text{CO}_2 \to o\text{-HOC}_6\text{H}_4\text{COONa} \to \text{salicylic acid};\qquad \text{ArOH} + \text{CHCl}_3/\text{NaOH} \to o\text{-HOC}_6\text{H}_4\text{CHO}$

Reference tables (1)

Phenols in Nature: Eugenol, Gallic Acid, Curcumin and the Polyols5 rows

Phenol	Source	Property / use
Eugenol	Clove	Analgesic and antimicrobial Not 'antiseptic' alone — the paper keys the analgesic-and-antimicrobial pair.
Gallic acid	Indian gooseberry (amla), gall nuts	Antioxidant
Curcumin	Turmeric	Antioxidant, anti-inflammatory
Methyl salicylate	Wintergreen	Liniment
Ascorbic acid	Citrus fruits	Vitamin C — NOT a phenol No OH on a benzene ring.

Source and use are asked in both directions.

Watch out for (4)

Stopping at cumene hydroperoxide
→ Making Phenol: Cumene and Dow
Trinitrating with dilute acid
→ Nitration and Bromination of Phenol
Starting Kolbe from phenol itself
→ Kolbe, Reimer–Tiemann, Oxidation and Reduction of Phenol
Confusing the 1,2,3 and 1,3,5 triol drawings
→ Phenols in Nature: Eugenol, Gallic Acid, Curcumin and the Polyols

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