Organic Chemistry: Alcohols, Phenols and Ethers
1. Introduction
Alcohols, phenols, and ethers are oxygen-containing organic compounds. Alcohols and phenols have the -OH group (alcoholic and phenolic respectively), while ethers have the -O- linkage.
2. Alcohols
2.1 Classification
Primary (1°): -OH on C with two H atoms (ethanol). Secondary (2°): -OH on C with one H atom (propan-2-ol). Tertiary (3°): -OH on C with no H atoms (2-methylpropan-2-ol).
2.2 Preparation
- Hydration of alkenes (acid-catalysed).
- Reduction of aldehydes/ketones.
- From Grignard reagent.
- Hydrolysis of alkyl halides.
2.3 Reactions
- Dehydration (to alkenes): With conc. H₂SO₄ at 170°C.
- Oxidation: 1° → aldehyde → carboxylic acid; 2° → ketone; 3° → resistant.
- Esterification: R-OH + R'COOH → RCOOR' + H₂O.
- Reaction with HX: R-OH + HX → R-X + H₂O. Reactivity: 3° > 2° > 1°.
3. Phenols
-OH directly bonded to benzene ring. Weakly acidic.
3.1 Preparation
- From benzene sulphonic acid: C₆H₅SO₃H + NaOH → C₆H₅ONa → C₆H₅OH.
- From diazonium salts: C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl.
- Dow process: Chlorobenzene + NaOH (high pressure, 300°C).
3.2 Acidity
Phenol is more acidic than alcohols but less acidic than carboxylic acids. Electron-withdrawing groups increase acidity; electron-donating groups decrease it.
3.3 Reactions
- Electrophilic substitution (activating —OH): Bromination, nitration, Friedel-Crafts.
- Kolbe's reaction: Phenol + CO₂ + NaOH → Salicylic acid.
- Reimer-Tiemann reaction: Phenol + CHCl₃ + NaOH → Salicylaldehyde.
- Ferric chloride test: Violet colouration confirms phenol.
5.3 Distinction Tests for Alcohols and Phenols
| Test | Primary Alcohol | Secondary Alcohol | Tertiary Alcohol | Phenol |
|---|---|---|---|---|
| Lucas test | No reaction (at RT) | Cloudy in 5 min | Cloudy immediately | - |
| Ceric ammonium nitrate | Red colour | Red colour | Red colour | Brown precipitate |
| FeCl₃ test | No colour | No colour | No colour | Violet colour |
| Iodoform test | Only ethanol gives | CH₃CO- gives | NO | NO |
| Na metal test | H₂ gas evolved | H₂ gas evolved | H₂ gas evolved | H₂ gas evolved |
| NaOH solubility | Insoluble | Insoluble | Insoluble | Soluble (acidic) |
'Lucas test distinguishes 1°, 2°, and 3° alcohols based on the stability of the carbocation intermediate. Primary alcohols do not react at room temperature.'
4. Ethers
R-O-R'. General formula similar to alcohols (isomeric).
4.1 Preparation
- Williamson's synthesis: R-ONa + R'-X → R-O-R' + NaX.
- Dehydration of alcohols: 2ROH → ROR + H₂O (at 140°C with conc. H₂SO₄).
4.2 Reactions
- Cleavage with HI: R-O-R' + HI → R-I + R'-OH (the smaller alkyl group forms iodide).
- Electrophilic substitution (for aromatic ethers).
5. Commercially Important Compounds
Ethanol: Beverages, fuel, solvent. Methanol: Solvent, formaldehyde production. Phenol: Disinfectant, plastic (bakelite) production. Diethyl ether: Anaesthetic, solvent.
6. Worked Problems
Problem 1: Distinguish between ethanol and phenol using FeCl₃ test. Solution: Phenol gives violet colour with FeCl₃. Ethanol gives no colour.
Problem 2: Write the product of CH₃CH₂ONa + CH₃CH₂Br. Solution: Williamson's synthesis: CH₃CH₂OCH₂CH₃ (diethyl ether) + NaBr.
7. Common Mistakes
'Students often forget that the -OH group in phenol is directly attached to the sp² carbon, making it more acidic than alcohols where -OH is attached to sp³ carbon.'
8. ISC Exam Focus
| Topic | Theory Marks | Practical Marks |
|---|---|---|
| Alcohols | 4 | 2 |
| Phenols | 4 | 2 |
| Ethers | 3 | 1 |
| Distinction tests | 2 | 2 |
9. Self-Test Questions
- Describe the preparation of ethanol from ethene.
- Why is phenol more acidic than ethanol? Explain with resonance.
- Write the mechanism of dehydration of ethanol to ethene.
- Explain Williamson's synthesis with an example.
- How can you distinguish between: (i) 1°, 2°, 3° alcohols (Lucas test) (ii) Phenol and ethanol.
10. Additional Worked Problems
Problem A: Predict the product when propan-2-ol is oxidised with K₂Cr₂O₇/H₂SO₄.
Solution: Propan-2-ol is a secondary alcohol. Oxidation gives propanone (acetone). The orange colour of dichromate turns green as Cr³⁺ is formed. CH₃CH(OH)CH₃ → CH₃COCH₃.
Problem B: How would you convert phenol to salicylic acid? Give the reaction conditions.
Solution: Kolbe's reaction. Phenol is treated with NaOH to form sodium phenoxide, which reacts with CO₂ at 125°C under pressure to form sodium salicylate. Acidification gives salicylic acid (2-hydroxybenzoic acid).
Problem C: Write the mechanism of cleavage of diethyl ether with excess HI.
Solution: Step 1: Protonation of ether oxygen by HI. Step 2: I⁻ attacks the less hindered alkyl carbon (SN2). Step 3: CH₃CH₂I and CH₃CH₂OH are formed. The alcohol further reacts with HI to give a second molecule of CH₃CH₂I. Final products: 2 moles of ethyl iodide + H₂O.
Problem D: Explain why o-nitrophenol is more acidic than phenol.
Solution: The nitro group (-NO₂) is strongly electron-withdrawing by both inductive and resonance effects. It stabilises the phenoxide ion by delocalising the negative charge through resonance, making o-nitrophenol more acidic than phenol. Electron-withdrawing groups at ortho and para positions increase the acidity of phenols.
