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🧪 Chemistry · Class 12 · NEET & JEE

Alcohols, Phenols & Ethers

Oxygen-containing organic compounds. Understand the acidic nature of phenols, reactions of alcohols (dehydration, oxidation, esterification), and properties of ethers. Includes industrial applications and named reactions.

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Reading time~7 min

Revision time~2 min

Last updated2026-07-19

1 Read the chapter ~7 min

🎯 Key Points

1° alcohol → aldehyde → carboxylic acid on oxidation; 2° → ketone; 3° resists oxidation
Acidity order: picric acid > o-nitrophenol > phenol > water > ethanol — phenol is acidic due to phenoxide resonance with the ring
Lucas test (ZnCl₂/HCl): 3° instant turbidity, 2° turbid in minutes, 1° no reaction at room temp
FeCl₃ test: phenols give violet colour, alcohols don't
Williamson synthesis: R-O⁻ + R'-X → ether, SN2, works best with 1° R'-X (3° gives elimination instead)
Industrial phenol: from cumene (air oxidation then acid treatment), giving phenol + acetone as byproduct

The phenoxide ion's negative charge spreads into the aromatic ring through resonance, lowering its energy and making phenol give up its proton more easily; an alkoxide ion (from a plain alcohol) has nowhere to delocalise the charge, making alcohols far less acidic.

Alcohols (R-OH)

Contains hydroxyl group (-OH). Classified as primary (1°), secondary (2°), or tertiary (3°) based on carbon attached to -OH.

Preparation: hydration of alkene, hydrolysis of alkyl halide, reduction of carbonyl
Dehydration: gives alkene (acid catalyst, 170°C)
Oxidation: 1° alcohol → aldehyde → carboxylic acid; 2° → ketone; 3° resists oxidation
Esterification: alcohol + carboxylic acid ⇌ ester + water (acid catalyst)
Lucas test: distinguishes 1°, 2°, 3° alcohols using ZnCl₂/conc. HCl

Phenols (ArOH)

Hydroxyl group directly attached to benzene ring; more acidic than alcohols due to resonance stabilisation of phenoxide ion.

Acidic nature: phenol > water > alcohol
Electron-withdrawing groups on ring increase acidity; electron-donating groups decrease it
Reactions: FeCl₃ test (violet colour), bromination (white ppt with Br₂ water), nitration
Kolbe-Schmitt reaction: phenol + CO₂ → salicylic acid (aspirin precursor)

Ethers (R-O-R')

Relatively inert under normal conditions
Diethyl ether: common solvent; anaesthetic properties
Cleavage with HI: R-O-R' + HI → RI + R'OH
Used as solvents in Grignard reactions and other sensitive organic reactions

Important Preparation Methods

From Grignard reagent: RMgX + HCHO → 1° alcohol; RMgX + RCHO → 2° alcohol; RMgX + R₂CO → 3° alcohol (followed by hydrolysis)
Reduction of carbonyl compounds: Aldehydes/ketones reduced by NaBH₄ or LiAlH₄ give 1°/2° alcohols
Williamson synthesis (for ethers): R-O⁻Na⁺ + R'-X → R-O-R' + NaX; an SN2 reaction, works best with 1° alkyl halides
Phenol from diazonium salt: ArN₂⁺ + H₂O → ArOH + N₂ + heat (hydrolysis)
Phenol from cumene (industrial): Cumene oxidised by air then treated with dilute acid to give phenol + acetone

Distinguishing Tests

Lucas test: 3° alcohol gives turbidity immediately; 2° alcohol turns turbid in a few minutes; 1° alcohol shows no turbidity at room temperature
Ferric chloride test: Phenols give violet/purple colour with neutral FeCl₃; alcohols do not respond
Iodoform test: Ethanol and secondary alcohols with a CH₃CH(OH)- group give a yellow precipitate of CHI₃ with I₂/NaOH
Victor Meyer's test: distinguishes 1°, 2°, 3° alcohols using HI, AgNO₂, and HNO₂ via colour of products (red, blue, colourless respectively)

Acidity Order and Reasoning

Order of acidity: picric acid > o-nitrophenol > phenol > water > ethanol (3° < 2° < 1° alcohol)
Phenol is more acidic than alcohols because the phenoxide ion is stabilised by resonance with the benzene ring
Alkyl groups push electron density onto -OH (+I effect), making alcohols less acidic as branching increases

Williamson Synthesis Limitation

Using a 3° alkyl halide with sodium alkoxide favours E2 elimination over substitution, giving an alkene instead of the desired ether, since 3° carbocations are sterically hindered for SN2 attack.

Nomenclature and Classification

Alcohols named by replacing the -e of the parent alkane with -ol (ethanol, propan-1-ol); position of -OH gets the lowest locant
Monohydric / dihydric / trihydric: one, two, or three -OH groups (ethanol; ethane-1,2-diol/glycol; propane-1,2,3-triol/glycerol)
Allylic (-OH on carbon next to C=C), benzylic (-OH on carbon attached to ring), vinylic (-OH on doubly bonded C) alcohols
Phenols: hydroxybenzene = phenol; methylphenols = cresols; benzene-1,2-diol = catechol, 1,3 = resorcinol, 1,4 = quinol
Ethers named as alkoxyalkanes (CH₃OCH₃ = methoxymethane); symmetrical (R-O-R) vs unsymmetrical (R-O-R')

Preparation of Alcohols

Acid-catalysed hydration of alkenes: follows Markovnikov's rule (-OH on more substituted carbon)
Hydroboration-oxidation (B₂H₆ then H₂O₂/OH⁻): anti-Markovnikov and syn addition, gives 1° alcohol from a terminal alkene without rearrangement
Oxymercuration-demercuration: Markovnikov -OH addition without carbocation rearrangement
Phenols specifically: from haloarenes (Dow process), from benzenesulphonic acid (fused NaOH), from diazonium salts, and industrially from cumene

Electrophilic Substitution and Special Reactions of Phenol

-OH is strongly activating and o/p-directing, so phenol undergoes ring substitution far more readily than benzene
Bromination: with Br₂ water gives 2,4,6-tribromophenol (white ppt); in CS₂ at low T gives mainly p-bromophenol
Nitration: dilute HNO₃ gives o- and p-nitrophenol; conc. HNO₃ gives 2,4,6-trinitrophenol (picric acid)
Kolbe's reaction: sodium phenoxide + CO₂ (under pressure) then H⁺ → salicylic acid (o-hydroxybenzoic acid)
Reimer-Tiemann reaction: phenol + CHCl₃ + NaOH → salicylaldehyde (via dichlorocarbene :CCl₂ intermediate)
Reaction with Zn dust: phenol → benzene; oxidation gives benzoquinone

🚀 JEE Advanced Edge

Why o/p-nitrophenol is much more acidic than phenol: The -NO₂ group withdraws electron density via both induction AND resonance, and crucially can directly stabilise the negative charge on the phenoxide oxygen through extended conjugation when positioned ortho or para (not meta) — this is why o- and p-nitrophenol are dramatically more acidic than m-nitrophenol, a classic "explain the exception" JEE question.

Reimer-Tiemann and Kolbe-Schmitt as ortho-directing reactions: Both reactions on phenol proceed through the phenoxide ion, where the negative charge density is highest at the ortho and para positions (resonance structures), explaining why salicylaldehyde (Reimer-Tiemann) and salicylic acid (Kolbe-Schmitt) form preferentially over their para isomers under the standard reaction conditions.

Worked problem: Predict the major product of acid-catalysed dehydration of 2-methylcyclohexanol. Approach: Dehydration proceeds via a carbocation (E1) and follows Zaitsev's rule (more substituted/stable alkene is major product). The carbocation can lose a proton from either side; losing H from the MORE substituted side gives the more substituted (more stable, more heavily conjugated where applicable) alkene as the major product — 1,2-dimethylcyclohexene-type extra substitution wins over the less-substituted alternative.

2 Revise ~2 min before the exam

🔑 Key Reactions

Alcohol from alkene: acid-catalysed hydration (Markovnikov) or hydroboration–oxidation (anti-Markovnikov)
Oxidation: 1° alcohol → aldehyde → carboxylic acid; 2° alcohol → ketone; 3° alcohol resists oxidation
Dehydration: alcohol + conc. H₂SO₄ →(heat) alkene (E1, Saytzeff product favoured)
Lucas test: distinguishes 1°/2°/3° by turbidity speed with HCl/ZnCl₂ (3° fastest)
Phenol acidity: phenol is more acidic than alcohols (phenoxide is resonance-stabilised)
Kolbe reaction: phenol + CO₂/NaOH → salicylic acid
Williamson ether synthesis: R–X + R'–O⁻ → R–O–R' (use a primary halide to avoid elimination)

3 Practice apply it

✍️ Worked Examples

Example 1 — Lucas test
Q: Three unlabelled bottles hold 1-butanol, 2-butanol and tert-butanol. How does the Lucas test tell them apart?
Step 1 — Lucas reagent (conc. HCl + ZnCl₂) reacts fastest with the most stable carbocation.
Step 2 — tert-butanol (3°) turns cloudy immediately; 2-butanol (2°) within about 5 minutes; 1-butanol (1°) only on heating.
Step 3 — Turbidity speed thus ranks them.
Answer: immediate cloudiness = tert-butanol, ~5 min = 2-butanol, slow/none = 1-butanol. Key idea: reactivity follows carbocation stability, 3° > 2° > 1°.

Example 2 — Why phenol is more acidic than ethanol
Q: Explain why phenol is a stronger acid than ethanol.
Step 1 — Both lose H⁺ to form an alkoxide/phenoxide; acidity depends on how stable that anion is.
Step 2 — The phenoxide's negative charge is delocalised into the aromatic ring by resonance, spreading it out.
Step 3 — Ethoxide has no such delocalisation, so its charge is concentrated and less stable.
Answer: resonance stabilisation of phenoxide makes phenol more acidic. Note: electron-withdrawing groups (e.g. −NO₂) on the ring make phenols more acidic still.

Example 3 — Product of oxidation
Q: What is formed when (a) ethanol and (b) propan-2-ol are oxidised by acidified KMnO₄?
Step 1 — Ethanol is a primary alcohol, so it oxidises first to ethanal, then on to ethanoic acid.
Step 2 — Propan-2-ol is a secondary alcohol, oxidising to the ketone propanone.
Step 3 — A ketone resists further oxidation under normal conditions.
Answer: ethanol → ethanoic acid; propan-2-ol → propanone. Key idea: the oxidation product reveals whether the alcohol was primary or secondary.