JEE Main Chemistry Alcohols, Phenols & Ethers 2027: Lucas Test, Acidity & Reimer-Tiemann
Identification of alcohols, dehydration mechanism, acidity order of phenols and alcohols, electrophilic substitution in phenol and the Reimer-Tiemann reaction.
Edurack
September 28, 2026
An alcohol, phenol and water look similar on paper, yet their acidity differs by many orders of magnitude. This chapter is about understanding why, and about a few must-know reactions that JEE Main returns to almost every year.
Phenol is acidic because its conjugate base is stabilised by resonance. Alcohols have no such help.
Chapter at a Glance
| Snapshot | Detail |
|---|---|
| NTA unit | Unit 17 (part 1) of 20: Organic Compounds Containing Oxygen: Alcohols, Phenols & Ethers |
| Priority (trend-based) | Moderate |
| Typical question style | Acidity-order MCQs, product prediction and identification tests |
| Best first step | Learn the Lucas test and the acidity order first |
Priority reflects past-paper trends, not an official NTA weightage.
What the NTA Syllabus Covers
- Alcohols: identification of primary, secondary and tertiary alcohols, mechanism of dehydration
- Phenols: acidic nature, electrophilic substitution (halogenation, nitration, sulphonation), Reimer-Tiemann reaction
- Ethers: structure
Master These Topics
1. Identifying 1°, 2° and 3° Alcohols
Lucas test uses ZnCl₂/conc. HCl:
- Tertiary alcohols turn cloudy immediately, since they form stable carbocations.
- Secondary alcohols turn cloudy in about 5 minutes.
- Primary alcohols show no reaction at room temperature.
Oxidation also distinguishes them: primary alcohols give aldehydes then acids, secondary alcohols give ketones, and tertiary alcohols resist oxidation under mild conditions.
2. Dehydration of Alcohols
With conc. H₂SO₄ at 443 K, alcohols dehydrate to alkenes, and at 413 K to ethers. The mechanism for 2° and 3° alcohols is E1: protonation of OH, loss of water to give a carbocation, then loss of H⁺.
Ease of dehydration is 3° > 2° > 1°, and the major product follows Zaitsev's rule. Worked example: Butan-2-ol dehydrates mainly to but-2-ene rather than but-1-ene.
3. Acidity: Phenols versus Alcohols
Approximate pKa values: ethanol about 16, water about 15.7, phenol about 10. So the order of acidity is phenol > water > alcohols.
Phenoxide is stabilised by resonance, spreading the negative charge into the ring. Electron-withdrawing groups increase phenol's acidity, and electron-donating groups decrease it, so p-nitrophenol > phenol > p-cresol.
Trap: Phenol is a weak acid, and it does not liberate CO₂ from sodium bicarbonate (unlike carboxylic acids). It does react with NaOH.
Phenol also gives a violet colour with neutral FeCl₃, a common identification test.
4. Reactions of Phenol
The −OH group is strongly activating and ortho/para directing.
- Halogenation: bromine water gives a white precipitate of 2,4,6-tribromophenol.
- Nitration: dilute HNO₃ gives o- and p-nitrophenol, while conc. HNO₃ gives 2,4,6-trinitrophenol (picric acid).
- Sulphonation: conc. H₂SO₄ gives o-hydroxybenzenesulphonic acid at low temperature and the para isomer at higher temperature.
- Reimer-Tiemann: phenol with CHCl₃ and aqueous NaOH, followed by acidification, gives salicylaldehyde (2-hydroxybenzaldehyde). The reacting species is dichlorocarbene,
:CCl₂.
5. Ethers
Ethers have an R-O-R structure with a bent C-O-C angle of about 111°. They are much less reactive than alcohols and are used as solvents. They have lower boiling points than isomeric alcohols, because they cannot form hydrogen bonds with themselves.
Common Traps to Avoid
- Expecting phenol to react with sodium bicarbonate.
- Forgetting that tertiary alcohols react fastest in the Lucas test.
- Assuming the major dehydration product is the less substituted alkene.
- Mixing up the reagents of Reimer-Tiemann (CHCl₃ and NaOH) with other named reactions.
60-Second Revision Sheet
- Lucas: 3° immediately, 2° in about 5 minutes, 1° none at room temperature
- Acidity:
p-nitrophenol > phenol > water > alcohols - Phenol with bromine water gives 2,4,6-tribromophenol
- Reimer-Tiemann: phenol + CHCl₃ + NaOH gives salicylaldehyde
Your Study Plan
- Day 1: alcohol classification, preparation and Lucas test.
- Day 2: dehydration mechanism and Zaitsev's rule.
- Day 3: phenol acidity and substituent effects.
- Day 4: electrophilic substitution, Reimer-Tiemann and a timed set.
Practice Alcohols, Phenols & Ethers Questions Free → (opens in a new tab)
Continue Your Chemistry Journey
- Previous chapter: Organic Compounds Containing Halogens
- Next chapter: Aldehydes, Ketones & Carboxylic Acids
- All JEE Main Chemistry chapters
- Complete JEE Main Syllabus 2027 guide
Frequently Asked Questions
Why is phenol more acidic than ethanol?
Phenoxide ion is stabilised by delocalising its negative charge into the benzene ring, while ethoxide has no such resonance stabilisation.
What is the Reimer-Tiemann reaction?
Phenol reacts with chloroform in aqueous NaOH to give salicylaldehyde, through electrophilic attack by dichlorocarbene at the ortho position.