JEE Mains Chemistry · Alcohols, Phenols and Ethers

Ring Substitution of Phenols and Phenol Tests

The OH group makes the ring so reactive that bromine water gives 2,4,6-tribromophenol at once while bromine in CS₂ gives mainly p-bromophenol; dilute nitric acid gives o- and p-nitrophenol, concentrated nitric acid gives picric acid, and phthalic anhydride gives phenolphthalein.

All Alcohols, Phenols and Ethers notes JEE Mains Chemistry strategy

Why this matters

Seventeen PYQs, fourteen of them multiple choice, and two from 2026. Seven are about bromination: the solvent, the product and the mass of bromine used. Five are about nitration, steam-volatile o-nitrophenol and picric acid, and three of those ask for a number, usually a count of oxygen atoms or a percentage. Five test the phthalein dye test and the colours of phenolphthalein.

Concept 1 of 3: Bromination of phenol: the solvent decides

The OH group pushes so much electron density into the ring that bromine is polarised without any Lewis acid. In water, phenol is partly ionised to the even more reactive phenoxide, and every free ortho and para position is brominated at once. In a solvent of low polarity at low temperature, the reaction stops after one bromine, mostly at para.

Definition

Bromine water (polar): 2,4,6-tribromophenol, a white precipitate. Three $\mathrm{Br_2}$ per phenol; three HBr are released.
$\mathrm{Br_2}$ in $\mathrm{CS_2}$ or $\mathrm{CHCl_3}$ at about 273 K (low polarity): monobromophenols, mainly p-bromophenol with some o-bromophenol.
No $\mathrm{FeBr_3}$ is needed for phenol. For benzene the Lewis acid is needed: it polarises $\mathrm{Br_2}$ to give the electrophile $\mathrm{Br^+}$ .
The difference between the two solvents comes from solvent polarity, not from hyperconjugation or free radicals.

Bromine water on phenol

\mathrm{C_6H_5OH + 3Br_2 \xrightarrow{H_2O} C_6H_2Br_3OH\downarrow + 3HBr}

Worked example

9.4 g of phenol is treated with bromine water until no more reacts. Find the mass of bromine used and the mass of 2,4,6-tribromophenol formed (C 12, H 1, O 16, Br 80).

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The same idea in a real exam question:

JEE Mains · 2022 · 26 July 2022 · Q42Moderate

Example 1 · Alcohols, Phenols and Ethers · Ring Substitution of Phenols and Phenol Tests

The difference in the reaction of phenol with bromine in chloroform and bromine in water medium is due to:

CS₂ and CHCl₃ both count as low polarity

Either solvent, at low temperature, gives mainly p-bromophenol. Only water (or another polar medium) gives the tribromo product.

Count bromine molecules, not bromine atoms

Three

\mathrm{Br_2}

(160 g mol

^{-1}

each) are used per phenol. Three Br atoms go onto the ring and three leave as HBr.

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Concept 2 of 3: Nitration of phenol and picric acid

Dilute nitric acid is enough to nitrate phenol, and it gives both ortho and para nitrophenol. The ortho isomer holds its OH to its own nitro group, so it cannot hydrogen-bond to its neighbours: it boils low and is carried over by steam. Concentrated nitric acid nitrates all three positions but also oxidises phenol, so picric acid is made by a safer route.

Definition

Dilute $\mathrm{HNO_3}$ , 298 K: o-nitrophenol + p-nitrophenol. Steam distillation carries off the ortho isomer (intramolecular hydrogen bond); the para isomer stays behind (intermolecular hydrogen bonds).
p-Nitrophenol melts higher (about 114 °C) than o-nitrophenol (about 45 °C).
Conc. $\mathrm{HNO_3}$ : 2,4,6-trinitrophenol, called picric acid, but in poor yield because the acid oxidises phenol.
Better route: conc. $\mathrm{H_2SO_4}$ first gives phenol-2,4-disulphonic acid; conc. $\mathrm{HNO_3}$ then replaces both $\mathrm{SO_3H}$ groups and adds a third nitro group.
Picric acid is a trinitroPHENOL (yellow, pKa about 0.4). TNT is trinitrotoluene.
Oxygen atoms: phenol 1, a nitrophenol 3, phenol-2,4-disulphonic acid 7, picric acid 7.

Percentage of oxygen in a compound

\%\,\mathrm{O} = \dfrac{16 \times (\text{number of O atoms})}{M} \times 100

Worked example

Find the percentage by mass of oxygen in picric acid, the yellow product of phenol with concentrated nitric acid (C 12, H 1, N 14, O 16).

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JEE Mains · 2026 · 5 Apr 2026 Shift 1 · Q48Moderate

Example 2 · Alcohols, Phenols and Ethers · Ring Substitution of Phenols and Phenol Tests

One mole of phenol is treated with dilute

{HNO}_{3}

at 298 K to give a mixture of products. The mixture is separated by steam distillation. The steam volatile compound ( X ) is separated. The increase in percentage of oxygen in (X) with respect to phenol is

\_\_\_\_

10^{- 1}\%

(Given molar mass in

g{mol}^{- 1}H:1,C:12,\text{ }N:14

, O:16)

Picric acid is a phenol, not TNT

2,4,6-Trinitrotoluene (TNT) has a methyl group. Picric acid has an OH group, which is why it is a strong acid.

Steam carries off the ortho isomer

The hydrogen bond in o-nitrophenol is inside one molecule, so its molecules are held together weakly. p-Nitrophenol, bonded to its neighbours, stays in the flask.

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Concept 3 of 3: The phthalein dye test for phenols

Two phenol molecules join one carbonyl carbon of phthalic anhydride, each through the ring carbon para to its OH. The product, phenolphthalein, is a colourless lactone in acid. Dilute alkali opens the lactone to a coloured ion; a large excess of strong alkali takes the colour away again. A phenol whose para position is blocked cannot form the dye.

Definition

2 phenol + phthalic anhydride, conc. $\mathrm{H_2SO_4}$ , heat → phenolphthalein + water.
Phenolphthalein is colourless in acid, pink in dilute NaOH, and colourless again in excess concentrated NaOH.
The ring carbon para to OH must carry an H; p-cresol fails the test.
The test is specific to phenols. Lucas is for alcohols, Tollens' for aldehydes and the carbylamine test for primary amines.

Phenol	Position para to OH	Product with phthalic anhydride	Colour in alkali
Phenol	Free	Phenolphthalein	Pink in dilute NaOH; colourless in acid and in excess strong alkali
o-Cresol	Free	o-Cresolphthalein	Purple-red
p-Cresol	Blocked by $\mathrm{CH_3}$	No phthalein dye	No colour The usual answer to 'which phenol gives no colour'.
Resorcinol (benzene-1,3-diol)	Free	Fluorescein	Yellow-green fluorescence

Look for the ring carbon para to OH: if it carries a substituent, no phthalein forms.

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The same idea in a real exam question:

JEE Mains · 2023 · 8 Apr 2023 · Q140Moderate

Example 3 · Alcohols, Phenols and Ethers · Ring Substitution of Phenols and Phenol Tests

A compound '

X

' when treated with phthalic anhydride in presence of concentrated

H_{2}SO_{4}

yields '

Y

'. '

Y

' is used as an acid/base indicator. '

X

' and '

Y

' are respectively:

Excess alkali removes the pink colour

Phenolphthalein is pink only in dilute alkali. In a large excess of concentrated NaOH it turns colourless again, so 'colourless' can be the right answer at the end of a sequence.

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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 (2)

Bromination of phenol: the solvent decides
Bromine water on phenol
$\mathrm{C_6H_5OH + 3Br_2 \xrightarrow{H_2O} C_6H_2Br_3OH\downarrow + 3HBr}$
Nitration of phenol and picric acid
Percentage of oxygen in a compound
$\%\,\mathrm{O} = \dfrac{16 \times (\text{number of O atoms})}{M} \times 100$

Reference tables (1)

The phthalein dye test for phenols4 rows

Phenol	Position para to OH	Product with phthalic anhydride	Colour in alkali
Phenol	Free	Phenolphthalein	Pink in dilute NaOH; colourless in acid and in excess strong alkali
o-Cresol	Free	o-Cresolphthalein	Purple-red
p-Cresol	Blocked by $\mathrm{CH_3}$	No phthalein dye	No colour The usual answer to 'which phenol gives no colour'.
Resorcinol (benzene-1,3-diol)	Free	Fluorescein	Yellow-green fluorescence

Look for the ring carbon para to OH: if it carries a substituent, no phthalein forms.

Watch out for (5)

CS₂ and CHCl₃ both count as low polarity
→ Bromination of phenol: the solvent decides
Count bromine molecules, not bromine atoms
→ Bromination of phenol: the solvent decides
Picric acid is a phenol, not TNT
→ Nitration of phenol and picric acid
Steam carries off the ortho isomer
→ Nitration of phenol and picric acid
Excess alkali removes the pink colour
→ The phthalein dye test for phenols

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