1. Chapter at a Glance
Alcohols (R–OH), phenols (Ar–OH) and ethers (R–O–R′) are oxygen-containing organic compounds used as fuels, solvents, antiseptics and starting materials. This chapter covers their classification, naming, preparation, physical properties, acidity, and characteristic reactions, plus tests that distinguish them. NEET asks one or two questions, often on acidity of phenols, Lucas test, dehydration, Reimer–Tiemann and Kolbe reactions, and cleavage of ethers by HI.
- Preparation of alcohols: hydration, hydroboration–oxidation, Grignard reagents, reduction.
- Acidity order of alcohols and substituted phenols.
- Lucas test; dehydration of alcohols and carbocation rearrangement.
- Named reactions of phenol: Kolbe, Reimer–Tiemann, bromination, nitration.
- Williamson ether synthesis and cleavage of ethers by HI.
2. Classification and Structure
- Alcohols are monohydric, dihydric (glycol) or trihydric (glycerol), and 1°, 2° or 3° depending on the carbon bearing –OH.
- In alcohols, the O is sp³ and the C–O–H angle is about 108.9°. In phenol, the C–O bond is shorter (136 pm vs 142 pm) because of partial double-bond character from resonance with the ring.
- In ethers, the C–O–C angle (111.7° in methoxymethane) is larger than tetrahedral because of repulsion between the bulky alkyl groups.
- Alcohols and phenols have high boiling points and dissolve in water (lower members) because of hydrogen bonding. Ethers cannot hydrogen-bond with each other, so their boiling points are close to those of alkanes of similar mass.
3. Preparation of Alcohols
| Method | Reaction | NEET Note |
|---|---|---|
| Acid-catalysed hydration | CH₃CH=CH₂ + H₂O (H⁺) → CH₃CH(OH)CH₃ | Markovnikov product |
| Hydroboration–oxidation | CH₃CH=CH₂ + B₂H₆, then H₂O₂/OH⁻ → CH₃CH₂CH₂OH | Anti-Markovnikov product, no rearrangement |
| Reduction of aldehydes/ketones | RCHO → RCH₂OH; R₂CO → R₂CHOH (NaBH₄, LiAlH₄, H₂/Pd) | Aldehyde → 1°, ketone → 2° |
| Reduction of acids/esters | RCOOH → RCH₂OH (LiAlH₄) | NaBH₄ does not reduce acids |
| Grignard reagents | HCHO → 1°; other RCHO → 2°; ketone → 3° alcohol | Classic question |
| Fermentation | Sugar → ethanol + CO₂ (zymase in yeast) | Anaerobic |
4. Preparation and Acidity of Phenols
- From chlorobenzene (NaOH, 623 K, 300 atm), from benzenesulphonic acid (fuse with NaOH), from diazonium salts (warm with water), and from cumene (air oxidation to cumene hydroperoxide, then dilute acid — gives phenol and acetone).
- Acidity: phenol (pKa ≈ 10) is more acidic than alcohols (pKa ≈ 16) and water, but weaker than carboxylic acids. The phenoxide ion is stabilised by resonance, spreading the negative charge into the ring.
- Phenol reacts with Na and NaOH but not with NaHCO₃ (carboxylic acids do).
- Electron-withdrawing groups (–NO₂) at ortho/para increase acidity: 2,4,6-trinitrophenol (picric acid) is almost as strong as a mineral acid. Electron-donating groups (–CH₃, –OCH₃) decrease acidity.
- Among alcohols, acidity order is 1° > 2° > 3° (alkyl groups push electrons into O–H).
5. Reactions of Alcohols
- As acids: 2ROH + 2Na → 2RONa + H₂.
- Esterification: ROH + R′COOH ⇌ R′COOR + H₂O (conc. H₂SO₄); faster with acid chlorides or anhydrides.
- With HX: ROH + HX → RX + H₂O; reactivity 3° > 2° > 1°.
- Lucas test (conc. HCl + ZnCl₂): 3° alcohols turn cloudy immediately, 2° in about 5 minutes, and 1° do not react at room temperature.
- Dehydration with conc. H₂SO₄ or H₃PO₄: 1° needs 443 K, 2° less, 3° about 358 K (3° easiest, via carbocation). At lower temperature (413 K), ethanol gives diethyl ether.
- Oxidation: 1° → aldehyde (PCC stops here) → carboxylic acid (acidified K₂Cr₂O₇ or KMnO₄); 2° → ketone; 3° resist oxidation. Heated copper at 573 K dehydrogenates 1° → aldehyde, 2° → ketone, and dehydrates 3° → alkene.
6. Reactions of Phenol
- –OH strongly activates the ring towards electrophilic substitution at ortho and para positions.
- Bromination: with bromine water, phenol gives a white precipitate of 2,4,6-tribromophenol. In CS₂ at low temperature it gives mono-bromophenols (o and p).
- Nitration: dilute HNO₃ gives o- and p-nitrophenol (separated by steam distillation — the ortho isomer is steam-volatile because of intramolecular H-bonding). Concentrated HNO₃ gives picric acid.
- Kolbe's reaction: sodium phenoxide + CO₂ (400 K, 4–7 atm), then acid, gives salicylic acid (2-hydroxybenzoic acid), used to make aspirin.
- Reimer–Tiemann reaction: phenol + CHCl₃ + NaOH, then acid, gives salicylaldehyde (2-hydroxybenzaldehyde); dichlorocarbene is the electrophile.
- With zinc dust, phenol is reduced to benzene. Oxidation with chromic acid gives benzoquinone.
- FeCl₃ test: phenols give a violet colour with neutral ferric chloride.
7. Ethers
- Williamson synthesis: RX + R′ONa → R–O–R′ + NaX. Use a primary alkyl halide; with tertiary halides the alkoxide causes elimination instead (alkene forms). For methyl tert-butyl ether, use (CH₃)₃CONa + CH₃Br.
- Cleavage by HI: R–O–R′ + HI → ROH + R′I. With a methyl and a primary alkyl, I⁻ attacks the smaller group (SN2), so the methyl group leaves as CH₃I. With a tertiary alkyl, the tertiary halide forms (SN1). Alkyl aryl ethers give phenol + alkyl iodide (never aryl iodide).
- Anisole (methoxybenzene) undergoes electrophilic substitution at ortho/para positions (–OCH₃ is activating).
- Diethyl ether was used as an anaesthetic; ethers are good solvents but can form explosive peroxides on standing.
8. Formula and Reaction Master Table
| Name / type | Reaction |
|---|---|
| Hydroboration–oxidation | RCH=CH₂ → RCH₂CH₂OH (anti-Markovnikov) |
| Grignard + HCHO | RMgX + HCHO → RCH₂OH (1°) |
| Grignard + ketone | RMgX + R′₂CO → R′₂C(OH)R (3°) |
| Cumene process | Cumene → cumene hydroperoxide → phenol + acetone |
| Kolbe's reaction | C₆H₅ONa + CO₂ → salicylic acid |
| Reimer–Tiemann | C₆H₅OH + CHCl₃ + NaOH → salicylaldehyde |
| Williamson synthesis | RONa + R′X (1°) → ROR′ |
| Ether cleavage | C₆H₅OCH₃ + HI → C₆H₅OH + CH₃I |
| Acidity order | picric acid > p-nitrophenol > phenol > water > ethanol |
9. Master Summary Table
| Topic | Key idea | NEET Note |
|---|---|---|
| Boiling points | H-bonding in alcohols | Ethers much lower |
| Lucas test | 3° instant, 2° ~5 min, 1° none | Distinguishes 1°, 2°, 3° |
| Dehydration | 3° easiest | Low T with ethanol → ether |
| Phenol acidity | Resonance-stabilised phenoxide | No reaction with NaHCO₃ |
| Bromine water + phenol | White precipitate | 2,4,6-tribromophenol |
| Kolbe / Reimer–Tiemann | Salicylic acid / salicylaldehyde | Both ortho products |
| Williamson | Needs 1° halide | 3° halide eliminates |
| Anisole + HI | Phenol + CH₃I | Not iodobenzene |
10. Mind Map
- Hydration, hydroboration
- Grignard
- Lucas, oxidation
- Cumene process
- Acidity
- Kolbe, Reimer–Tiemann
- Williamson
- HI cleavage
- Anisole
- Na metal
- FeCl₃ violet
- Bromine water
11. Tips and Tricks
- Grignard product type: count R groups on the carbonyl carbon plus one new R.
- For substituted phenols, rank acidity by the stability of the phenoxide: –NO₂ at o/p ≫ at m > H > –CH₃.
- o-Nitrophenol is steam-volatile (intramolecular H-bond); p-nitrophenol is not (intermolecular H-bond).
- In Williamson synthesis, put the bulky group on the alkoxide, not on the halide.
- Writing iodobenzene as a product of anisole + HI.
- Expecting phenol to release CO₂ from NaHCO₃.
- Using hydration (H⁺) when the question wants an anti-Markovnikov alcohol.
- Thinking 3° alcohols are easily oxidised.
12. Important One-liners
- Ethanol is made industrially by fermentation of molasses.
- Methanol is poisonous and can cause blindness.
- Phenol was once called carbolic acid.
- Salicylic acid is used to make aspirin.
- Picric acid is 2,4,6-trinitrophenol.
- Phenol gives a violet colour with neutral FeCl₃.
- The C–O–C bond angle in ethers is slightly greater than tetrahedral.
13. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
Which alcohol gives immediate turbidity with Lucas reagent?
- Propan-1-ol
- Propan-2-ol
- 2-Methylpropan-2-ol
- Methanol
Show solution
2-Methylpropan-2-ol is a tertiary alcohol. It forms a stable carbocation quickly, so the alkyl chloride separates at once. Propan-2-ol (2°) takes minutes, and the 1° alcohols do not react at room temperature.
Answer: (C)
Phenol is more acidic than ethanol because:
- Phenol is a solid
- The phenoxide ion is stabilised by resonance
- Ethanol has hydrogen bonding
- Phenol has more hydrogen atoms
Show solution
In the phenoxide ion, the negative charge spreads into the benzene ring, so the ion is more stable. In ethoxide, the charge stays on the oxygen, and the ethyl group pushes even more electron density onto it.
Answer: (B)
Propene is converted into propan-1-ol by:
- H₂O/H⁺
- B₂H₆, then H₂O₂/OH⁻
- HBr, then aqueous KOH
- KMnO₄
Show solution
Hydroboration–oxidation adds OH to the less substituted carbon (anti-Markovnikov), giving propan-1-ol. Acid-catalysed hydration gives propan-2-ol. HBr without peroxide also leads to the 2-substituted product.
Answer: (B)
Phenol reacts with CHCl₃ and aqueous NaOH, followed by acidification, to give:
- Salicylic acid
- Salicylaldehyde
- Benzaldehyde
- Benzoic acid
Show solution
In the Reimer–Tiemann reaction, a –CHO group is introduced mainly at the ortho position, giving 2-hydroxybenzaldehyde. Salicylic acid comes from Kolbe's reaction with CO₂.
Answer: (B)
Anisole on heating with HI gives:
- Iodobenzene and methanol
- Phenol and methyl iodide
- Benzene and CH₃OH
- Iodobenzene and methyl iodide
Show solution
The O–aryl bond has partial double-bond character, so I⁻ attacks the methyl carbon (SN2). This gives CH₃I and C₆H₅OH. An SN2 attack on an aryl carbon does not occur.
Answer: (B)
A ketone reacts with methylmagnesium bromide, and the product is hydrolysed. The alcohol obtained is:
- Primary
- Secondary
- Tertiary
- Phenol
Show solution
A ketone carbonyl carbon already carries two R groups. Adding a third R from the Grignard reagent makes a tertiary alcohol. HCHO gives a 1° alcohol and other aldehydes give 2° alcohols.
Answer: (C)
The correct order of acidity is:
- p-cresol > phenol > p-nitrophenol
- p-nitrophenol > phenol > p-cresol
- phenol > p-nitrophenol > p-cresol
- p-cresol > p-nitrophenol > phenol
Show solution
–NO₂ (−I, −R) stabilises the phenoxide ion, which raises acidity. –CH₃ in p-cresol (+I, hyperconjugation) destabilises it, which lowers acidity. So the order is p-nitrophenol > phenol > p-cresol.
Answer: (B)
To prepare tert-butyl methyl ether by Williamson synthesis, the best pair is:
- (CH₃)₃CBr + CH₃ONa
- (CH₃)₃CONa + CH₃Br
- (CH₃)₃COH + CH₃OH
- (CH₃)₃CCl + CH₃OH
Show solution
The halide must be primary (here, methyl), because a strong base with a tertiary halide gives elimination (isobutylene) instead of the ether. So the bulky group goes on the alkoxide. Pairs (C) and (D) are not Williamson conditions.
Answer: (B)
14. Quick Revision Summary
- Alcohols from hydration (Markovnikov), hydroboration (anti-M), reduction, Grignard (HCHO 1°, RCHO 2°, ketone 3°).
- Lucas test; dehydration easiest for 3°; oxidation 1° → acid, 2° → ketone, 3° resist.
- Phenol acidity via resonance; –NO₂ increases, –CH₃ decreases; no reaction with NaHCO₃.
- Phenol: bromine water (white ppt), Kolbe (salicylic acid), Reimer–Tiemann (salicylaldehyde), FeCl₃ violet.
- Williamson needs 1° halide; anisole + HI → phenol + CH₃I.