Chapters

Everything NCERT Class 12 Unit 7 (Alcohols, Phenols and Ethers) says about phenols that can be asked: their naming, preparation, acidity and reactions; alcohols and ethers, the rest of the unit, are in their own file. JEE Main material beyond the book is marked Beyond NCERT. The short note stays for the last week; this is for a proper revision pass.

Formula sheet

Every order and rule in the chapter on one screen, for the last pass. Details and examples are in the sections below.

Rule	Order or result	Watch
Acid strength	phenol > water > ethanol	pKa 10.0 vs 15.9 (a million times)
Lower pKa	stronger acid	Order by pKa is the reverse of acidity
–NO₂ on phenol	o, p raise acidity a lot; m only a little	p (7.1) ≈ o (7.2) > m (8.3)
–CH₃, –OCH₃ at p	lower acidity	Cresols weaker than phenol
Phenol + NaOH	dissolves (phenoxide)	Alcohols don't
Phenol + NaHCO₃	no CO₂	Picric acid does · Beyond NCERT
–OH on the ring	strongly activating, o/p	No Lewis acid needed for Br₂
Br₂ water	2,4,6-tribromophenol (white ppt)
Br₂ in CS₂ or CHCl₃, low T	4-bromophenol (major) + 2-	Low polarity
dil. HNO₃, 298 K	o- + p-nitrophenol	Separate by steam distillation (o goes over)
Kolbe	NaOH; CO₂ (400 K, 4–7 atm · Beyond NCERT); H⁺	Salicylic acid (ortho)
Reimer–Tiemann	CHCl₃ + aq NaOH; H⁺	Salicylaldehyde (ortho)
Cumene process	O₂ (air); then H⁺, H₂O	Phenol + acetone
Zn dust, heat	benzene	The only C–O break of phenol

0 · What JEE Main asks from this chapter

77 questions in JEE Main 2019–2026 (71 MCQ, 6 numerical), about 1 in every 2 papers. Grouped by type (counts are approximate):

Type	Share	Typical question
Acidity: pKa order, substituent effects, NaOH/NaHCO₃	~30%	Ascending acidity: BuOH, p-nitro-, p-methoxyphenol, phenol, 2,4-dinitrophenol (2024)
Named reactions: Kolbe, Reimer–Tiemann, cumene, Zn, oxidation	~27%	Match phenol → salicylic acid, salicylaldehyde, benzoquinone, anisole (2024)
Ring substitution: bromination, nitration, picric acid	~20%	Br₂ in CHCl₃ needs a Lewis acid? (2024)
Numericals: masses, % composition, atom counts	~8%	4.7 g phenol + Zn at 60% → moles of benzene (2026)
Tests and identification: FeCl₃, phthalein, solubility	~8%	Acidic, dissolves in NaOH not NaHCO₃, violet with FeCl₃ (2022)
Ethers of phenols and other sequences	~7%	Ph–O–CH(CH₃)₂ + HI, then Zn dust (2022)

1 · Classification and nomenclature

• Phenols carry –OH on an sp² carbon of an aromatic ring. Phenol (C₆H₅OH) is both its common and its accepted IUPAC name; it is also called carbolic acid.
• By number of –OH groups: monohydric, dihydric, trihydric.

Structure	Common	IUPAC
C₆H₅OH	phenol	phenol
o-CH₃C₆H₄OH	o-cresol	2-methylphenol
m-CH₃C₆H₄OH	m-cresol	3-methylphenol
p-CH₃C₆H₄OH	p-cresol	4-methylphenol
1,2-(OH)₂C₆H₄	catechol	benzene-1,2-diol
1,3-(OH)₂C₆H₄	resorcinol	benzene-1,3-diol
1,4-(OH)₂C₆H₄	hydroquinone (quinol)	benzene-1,4-diol
1,2,3-(OH)₃C₆H₃	pyrogallol	benzene-1,2,3-triol

Common names use o- (1,2), m- (1,3) and p- (1,4).

2 · Structure

• O is bonded to an sp² carbon of the ring; C–O–H ≈ 109°.
• C–O is 136 pm, shorter than methanol's 142 pm, for two reasons:
  • partial double-bond character from O's lone pair conjugating with the ring;
  • the sp² carbon holds the bond pair more tightly.

Example (2020) · Increasing C–OH bond length: phenol < p-ethoxyphenol < methanol. The para –OEt pushes electrons into the ring, so O's lone pair conjugates a little less, and the C–O gets slightly longer.

3 · Preparation of phenols

Phenol was first isolated from coal tar in the early 19th century; it is now made synthetically.

1. From haloarenes: chlorobenzene fused with NaOH at 623 K and 320 atm gives sodium phenoxide; acid then frees phenol. NCERT's haloalkane unit gives the same reaction as 623 K and 300 atm, and JEE used 300 atm (2021, 2024). Accept either; no Cu.

2. From benzenesulphonic acid: benzene is sulphonated with oleum; the sulphonic acid, heated with molten NaOH, gives sodium phenoxide; acid gives phenol.

3. From diazonium salts: a 1° aromatic amine + nitrous acid (NaNO₂ + HCl) at 273–278 K gives the diazonium salt. Warming with water, or treating with dilute acid, hydrolyses it to phenol.

4. From cumene (isopropylbenzene), the industrial route: most of the world's phenol is made this way.

• Air oxidises cumene to cumene hydroperoxide, C₆H₅C(CH₃)₂–O–OH.
• Dilute acid splits it into phenol + acetone. Acetone, the by-product, is also made in large amounts this way.

Example (2022) · Benzene + CH₃CH(Cl)CH₃/AlCl₃ → cumene → O₂ → H⁺, H₂O → P = phenol. P + Na₂Cr₂O₇/H₂SO₄ → 1,4-benzoquinone; P + Br₂ in CS₂ → 4-bromophenol.

Example (2024) · Chlorobenzene → NaOH, 623 K, 300 atm → X (sodium phenoxide) → HCl → Y (phenol) → conc. HNO₃ → Z = 2,4,6-trinitrophenol.

Aryl halides can't be made from phenol with HX, PX₃ or SOCl₂: phenol's C–O bond has partial double-bond character. Phenol + HCl → chlorobenzene is not possible (2024). The reason given, "phenols react violently with HX", is false.

4 · Physical properties

• Boiling points rise with the number of carbons and are higher than those of hydrocarbons, ethers and haloarenes of similar mass: intermolecular H-bonding of –OH.
• Solubility in water comes from H-bonds with water and falls as the aryl (hydrophobic) part grows.
• o-Nitrophenol has an intramolecular H-bond (–OH to its own –NO₂), so it can't associate. It has a lower b.p. and is steam volatile.
• p-Nitrophenol H-bonds between molecules, so it associates and is not steam volatile. This is how steam distillation separates them (2022).
• o-Nitrophenol does not have a high melting point (2021).

Example (2020) · Increasing b.p.: 4-methoxyphenol < 4-methylphenol < 4-nitrophenol < 4-aminophenol. More H-bonding sites, and more polar groups, mean higher b.p.

5 · Acidity of phenols

Phenols react with active metals (Na, Al) and, unlike alcohols, with aqueous NaOH:

So phenols are stronger acids than alcohols and water. Among C₂H₅OH, C₆H₅OH, C₆H₅CH₂OH and (CH₃)₃COH, only phenol reacts readily with dilute NaOH (2024). "Phenols are weaker acids than water" is false (2022).

Why phenol is more acidic than an alcohol:

1. The sp² carbon is more electronegative and pulls electron density from O, so the O–H bond is more polar and ionises more easily.
2. In the phenoxide ion the negative charge is delocalised over O and the o- and p-carbons of the ring (five resonance structures). In an alkoxide it sits on O alone.
3. Phenol itself is resonance-stabilised too, but its structures need charge separation, so phenoxide gains more than phenol does.

The greater the pKa, the weaker the acid.

Compound	pKa
p-Nitrophenol	7.1
o-Nitrophenol	7.2
m-Nitrophenol	8.3
Phenol	10.0
m-Cresol	10.1
o-Cresol, p-cresol	10.2
Ethanol	15.9

Phenol (10.0) against ethanol (15.9): about a million times more acidic. ("Ethanol is the stronger acid" is false; 2024.)

Substituents:

• Electron-withdrawing groups (–NO₂) raise acidity. At o and p the effect is strong: the –NO₂ takes the phenoxide's negative charge by resonance. At m it works only through –I, so less.
• Electron-releasing groups (alkyl) lower acidity: cresols are weaker than phenol.
• More –NO₂ groups, stronger acid: 2,4,6-trinitrophenol (picric acid) is a strong acid.

Example (NCERT Example 7.4) · Increasing acid strength: propan-1-ol < 4-methylphenol < phenol < 3-nitrophenol < 3,5-dinitrophenol < 2,4,6-trinitrophenol.

More substituents · Beyond NCERT

Phenol	pKa	Why
2,4,6-Trinitrophenol	≈ 0.4	three o/p –NO₂
2,4-Dinitrophenol	≈ 4.1	two o/p –NO₂
o-, m-, p-Chlorophenol	≈ 8.5, 9.1, 9.4	–I only; fades with distance
m-Methoxyphenol	≈ 9.6	–I only (no +R at m)
p-Methoxyphenol	≈ 10.2	+R beats –I

• Carboxylic acids (benzoic ≈ 4.2) beat every phenol except di- and trinitrophenols.
• NaHCO₃ test: a phenol releases CO₂ from NaHCO₃ only if it is stronger than carbonic acid (pKa ≈ 6.4). Phenol, cresols and mononitrophenols don't; 2,4-dinitrophenol and picric acid do.

Example (2024) · Ascending acidity: BuOH < p-methoxyphenol < phenol < p-nitrophenol < 2,4-dinitrophenol.

Example (2026) · Descending acidity: 4-nitrobenzoic acid > benzoic acid > 4-nitrophenol > phenol > 4-methoxyphenol.

Example (2023) · Increasing pKa: 2,4-dinitrophenol < 4-nitrophenol < 3-chlorophenol < phenol < 2,4,5-trimethylphenol. Strongest of phenol, 3-chloro-, 3-methyl-, 3-nitrophenol: 3-nitrophenol.

6 · Esterification

Phenols, like alcohols, form esters with carboxylic acids or anhydrides (a little conc. H₂SO₄) and with acid chlorides (in pyridine, which takes up HCl). Putting a CH₃CO group on –OH is acetylation.

Aspirin (acetylsalicylic acid) is made by acetylating salicylic acid:

Benzoylation with C₆H₅COCl in aqueous NaOH (Schotten–Baumann) gives phenyl benzoate · Beyond NCERT.

7 · Electrophilic substitution

The –OH group activates the ring and directs ortho and para: its lone pair puts extra electron density there (the resonance structures under acidity). Phenoxide, with a full negative charge, is even more reactive.

Nitration:

• Dilute HNO₃ at 298 K → o-nitrophenol + p-nitrophenol. They separate by steam distillation (section 4).
• Concentrated HNO₃ → 2,4,6-trinitrophenol (picric acid), but the yield is poor (much of the phenol is oxidised).
• Picric acid is now made by first heating phenol with conc. H₂SO₄ to phenol-2,4-disulphonic acid, then conc. HNO₃, which replaces both –SO₃H groups and adds a third –NO₂.
• Picric acid is 2,4,6-trinitrophenol, not trinitrotoluene (2024).

Example (2026, numerical) · The steam-volatile product X is o-nitrophenol, C₆H₅NO₃ (139). %O: phenol 16/94 = 17.02%; X 48/139 = 34.53%. Increase = 17.51% → 175 (× 10⁻¹).

Example (2024, numerical) · Phenol → conc. H₂SO₄ → A (phenol-2,4-disulphonic acid, C₆H₆O₇S₂: 1 + 2 × 3 = 7 O) → conc. HNO₃ → B (picric acid, 1 + 3 × 2 = 7 O). Sum = 14.

Halogenation:

Conditions	Product
Br₂ in CHCl₃ or CS₂ (low polarity), low T	4-bromophenol (major) + 2-bromophenol
Bromine water	2,4,6-tribromophenol, white precipitate

• No Lewis acid is needed: –OH activates the ring enough to polarise Br₂ by itself. Benzene needs FeBr₃. "Bromination of phenol in CHCl₃ needs a Lewis acid" is false (2024).
• The difference between the two conditions is the polarity of the solvent (2022). In water phenol partly ionises to the far more reactive phenoxide, and Br₂ is more polarised, so all three o/p positions fill.

Example (2025) · Br₂ needed for 2 g phenol → tribromophenol: 2/94 = 0.02128 mol × 3 Br₂ × 160 = 10.2 g.

Kolbe's reaction: phenoxide is reactive enough to be attacked by CO₂, a weak electrophile, at the ortho position.

The product is salicylic acid (2-hydroxybenzoic acid), the starting material for aspirin.

Example (2026) · X has 76.6% C, 6.38% H, vapour density 47 (M = 94) → C₆H₆O, phenol. Kolbe gives Y, salicylic acid (violet with FeCl₃). The incorrect statement: "X is more acidic than Y". Y is a carboxylic acid and dissolves in NaHCO₃ with CO₂.

Reimer–Tiemann reaction: phenol + CHCl₃ + aqueous NaOH puts –CHO at the ortho position.

• The intermediate is a substituted benzal chloride: the sodium phenoxide with an ortho –CHCl₂ (2022). Alkali hydrolyses it to the aldehyde.
• The product is salicylaldehyde (2-hydroxybenzaldehyde). Ortho is major, para minor ("para major" is false; 2026). The two can be separated by steam distillation (o has the intramolecular H-bond).
• The electrophile is dichlorocarbene, :CCl₂, made from CHCl₃ by the base · Beyond NCERT. With CCl₄ in place of CHCl₃, the product is salicylic acid.

Example (2020, numerical) · P = salicylaldehyde, C₇H₆O₂ (122). %C = 84/122 × 100 = 69.

Example (2021) · A (C₆H₆O, colour with FeCl₃) is phenol. CHCl₃/KOH, then acid → B, salicylaldehyde. B also comes from C, 2-hydroxybenzyl alcohol, with PCC.

8 · Other reactions

Zinc dust: phenol + Zn dust, heated → benzene (+ ZnO). This is the one reaction in which phenol's C–O bond breaks.

Example (2026, numerical) · 4.7 g phenol = 0.05 mol; 60% → 0.03 mol benzene = 3 (× 10⁻²).

Example (2022) · Ph–O–CH(CH₃)₂ + conc. HI, heat → A = phenol (+ isopropyl iodide) → Zn dust → B = benzene.

Oxidation: chromic acid (Na₂Cr₂O₇/H₂SO₄) → benzoquinone, a conjugated diketone. In air, phenols slowly turn into dark mixtures containing quinones; that is why old phenol is pink to brown.

Phenyl ethers: sodium phenoxide + an alkyl halide (Williamson) → alkyl aryl ether; phenol → anisole needs NaOH then CH₃Cl or CH₃I. HI cleaves them back to phenol + alkyl iodide (Alcohols and Ethers).

9 · Telling phenols apart · Beyond NCERT

Test	Phenol	Alcohol
Neutral FeCl₃	violet (or green, blue) colour	none
Bromine water	white ppt of tribromophenol	none
Aqueous NaOH	dissolves	doesn't
NaHCO₃	no CO₂ (picric acid: yes)	no CO₂

• Phthalein test: phenol + phthalic anhydride + conc. H₂SO₄ → phenolphthalein, pink in NaOH (2023, 2021).
• A compound that is acidic, dissolves in NaOH but not in NaHCO₃, and turns FeCl₃ violet is a phenol (2022).

10 · Named reactions

Name	Reagents	Product
Dow (from chlorobenzene)	NaOH, 623 K, 300–320 atm; H⁺	phenol
Cumene process	O₂ (air); H⁺, H₂O	phenol + acetone
Diazonium hydrolysis	NaNO₂ + HCl, 273–278 K; warm H₂O	phenol
Kolbe	NaOH; CO₂ (400 K, 4–7 atm · Beyond NCERT); H⁺	salicylic acid
Reimer–Tiemann	CHCl₃, aq NaOH; H⁺	salicylaldehyde
Acetylation	(CH₃CO)₂O or CH₃COCl	aryl acetate; aspirin from salicylic acid
Zn dust distillation	Zn, heat	benzene
Oxidation	Na₂Cr₂O₇, H₂SO₄	benzoquinone
Williamson (aryl ether)	ArO⁻Na⁺ + RX	ArOR
Phthalein · Beyond NCERT	phthalic anhydride, conc. H₂SO₄	phenolphthalein

11 · Traps

Trap	What goes wrong	Do this
pKa vs acidity	Ordering pKa like acidity	Low pKa = strong acid; reverse the order
o- vs p-nitrophenol	Calling ortho the strongest	p 7.1 ≈ o 7.2; both well above m 8.3
Methoxy group	Treating –OCH₃ as withdrawing at p	p-OCH₃ lowers acidity (+R); m-OCH₃ raises it (–I)
NaHCO₃	Thinking phenol gives CO₂	Only phenols with two or three o/p –NO₂
Br₂ conditions	Writing tribromophenol in CS₂	Low polarity, low T → monobromo (p major)
Lewis acid	Adding FeBr₃ to brominate phenol	Not needed
Kolbe vs Reimer–Tiemann	Swapping CO₂ and CHCl₃	CO₂ → acid (Kolbe); CHCl₃ → aldehyde (R–T); both ortho
R–T position	Para as major	Ortho major
Phenol → ArCl	Using HCl, PCl₅ or SOCl₂	Impossible: go through the diazonium salt
Anisole + HI	Iodobenzene + methanol	Phenol + CH₃I
Picric acid	"trinitrotoluene"; by direct conc. HNO₃ in good yield	2,4,6-trinitrophenol; via the disulphonic acid
Steam distillation	Expecting p-nitrophenol to distil	o-Nitrophenol distils (intramolecular H-bond)
Dow pressure	Rejecting an option over 300 vs 320 atm	NCERT gives both; decide on 623 K, NaOH and no Cu

12 · Numbers worth knowing

• pKa: phenol 10.0, ethanol 15.9, p-nitrophenol 7.1, o-nitrophenol 7.2, m-nitrophenol 8.3, cresols 10.1–10.2.
• C–O in phenol: 136 pm (methanol 142 pm).
• Dow: 623 K, 300–320 atm. Diazotisation: 273–278 K. Kolbe: 400 K, 4–7 atm (Beyond NCERT: NCERT gives no conditions). Nitration with dilute HNO₃: 298 K.
• Phenol 94 g mol⁻¹; salicylic acid 138; salicylaldehyde 122; picric acid 229; 2,4,6-tribromophenol 331; o-nitrophenol 139.

13 · For the boards only

JEE Main doesn't ask this, but the CBSE paper can.

• Phenol is also called carbolic acid; it was first isolated from coal tar in the early 19th century.
• Most of the world's phenol is made from cumene, which also supplies a large share of acetone.
• Aspirin is analgesic (relieves pain), anti-inflammatory and antipyretic (lowers fever).
• Phenols are the basic compounds for antiseptics (alcohols for detergents, ethers for fragrances).