1. Classification, structure, names and physical properties
Mono-, di-, trihydric; monohydric CₙH₂ₙ₊₁OH are 1°, 2°, 3°. IUPAC: alkan-n-ol (3-methylbutan-1-ol, 2-methylpropan-2-ol); diols keep -e (ethane-1,2-diol). O is sp³; C–O–H ≈ 108.9°; O–H polar.
Physical: hydrogen bonding → b.p. much higher than alkanes/ethers of similar mass; b.p. rises with chain length, falls with branching; lower alcohols miscible with water, solubility falls as the alkyl group grows.
Key terms
- Monohydric alcohol
An alcohol with one –OH group.
- Hydrogen bonding
Attraction between the δ+ H of an O–H group and a lone pair on another electronegative atom.
2. Preparation
- Alkenes: direct hydration (H₂O/H⁺) or indirect (cold conc. H₂SO₄ then boil with water) — Markovnikov; hydroboration–oxidation (B₂H₆; H₂O₂/OH⁻) — anti-Markovnikov.
- Alkyl halides: aqueous KOH or moist Ag₂O.
- Carbonyl reduction: aldehyde → 1°, ketone → 2° (H₂/Ni, NaBH₄, LiAlH₄); esters/acids → 1° (LiAlH₄).
- Primary amines + HNO₂: RNH₂ → ROH + N₂ + H₂O.
- Grignard: HCHO → 1°; RCHO → 2°; R₂CO → 3° (then H₃O⁺).
| Carbonyl + RMgX | Alcohol |
|---|---|
| HCHO | 1° |
| R′CHO | 2° |
| R′COR″ | 3° |
Formulas
- Grignard addition to methanal (then H₃O⁺ gives ethanol)
Key terms
- Hydroboration–oxidation
Addition of B₂H₆ to an alkene followed by H₂O₂/OH⁻; gives anti-Markovnikov alcohols.
3. Manufacture (outline)
Methanol (Bosch): (water gas, 1273 K); over ZnO–Cr₂O₃, 573–673 K, 200–300 atm. Ethanol (fermentation): molasses (sucrose) + yeast; invertase: sucrose → glucose + fructose; zymase: .
| Product | Catalyst |
|---|---|
| Methanol | ZnO–Cr₂O₃ |
| Ethanol | Invertase, zymase |
Formulas
- Methanol synthesis
Key terms
- Zymase
Yeast enzyme complex converting glucose into ethanol and CO₂.
- Water gas
A mixture of CO and H₂ made from steam and hot coke.
4. Acidity and reactions of the O–H and C–O bonds
Acidity: 2ROH + 2Na → 2RONa + H₂; alcohols are weaker acids than water (+I of alkyl); CH₃OH > 1° > 2° > 3°.
C–O cleavage: HX (HI > HBr > HCl; ZnCl₂ for HCl); PCl₃, PCl₅ (white HCl fumes — test for –OH); SOCl₂ (gaseous by-products).
Esterification: RCOOH + R′OH ⇌ RCOOR′ + H₂O (conc. H₂SO₄); acid chlorides (pyridine) and anhydrides give esters irreversibly. Mechanism: protonate acid C=O → alcohol O attacks → proton transfer → lose H₂O → lose H⁺; ester O comes from the alcohol.
| Reagent | Product |
|---|---|
| Na | RONa + H₂ |
| PCl₅ | RCl + POCl₃ + HCl |
| SOCl₂ | RCl + SO₂ + HCl |
| CH₃COCl | Ester + HCl |
Formulas
- Esterification
Key terms
- Esterification
Formation of an ester from an alcohol and a carboxylic acid (or acid chloride/anhydride).
5. Oxidation and dehydration
Oxidation: 1° → aldehyde (PCC; Cu, 573 K) → acid (K₂Cr₂O₇/H⁺, KMnO₄); 2° → ketone; 3° resist (Cu, 573 K → alkene). Dichromate turns orange → green with 1° and 2°.
Dehydration (conc. H₂SO₄): ethanol → ethene at 443 K; → ethoxyethane at 413 K (excess ethanol). Ease 3° > 2° > 1°. Mechanism: ; (slow); .
| Alcohol | Oxidation product |
|---|---|
| 1° | Aldehyde → acid |
| 2° | Ketone |
| 3° | None (mild) |
Formulas
- Dehydration
Key terms
- PCC
Pyridinium chlorochromate, a mild oxidant that converts 1° alcohols into aldehydes.
6. Conversions, Lucas test and uses
Ascent: ROH → RCl → RMgCl → + HCHO, H₃O⁺ → RCH₂OH. Descent: RCH₂OH → RCOOH → RCONH₂ → RNH₂ (Br₂/KOH, Hofmann) → ROH (HNO₂).
Lucas test (anhydrous ZnCl₂ + conc. HCl, room temperature): 3° turbid at once; 2° about 5 min; 1° no turbidity.
Uses: methanol — solvent, methanal manufacture, fuel; highly poisonous. Ethanol — solvent, medicines, sanitisers, beverages, fuel blended with petrol.
| Alcohol class | Lucas test |
|---|---|
| 3° | Immediate |
| 2° | ≈ 5 min |
| 1° | None at room temperature |
Key terms
- Lucas reagent
Anhydrous ZnCl₂ in conc. HCl, used to distinguish 1°, 2° and 3° alcohols.
- Hofmann bromamide reaction
RCONH₂ + Br₂ + KOH → RNH₂ with one carbon fewer.
Exam tips
Common mistakes
- Saying alcohols are stronger acids than water.
- Placing the ¹⁸O label in the water during esterification (it stays in the ester).
- Forgetting temperatures for dehydration (443 K ethene; 413 K ether).
- Grignard + HCHO giving 2° alcohols (it gives 1°).
How marks are usually awarded
- Mechanisms (2–3 marks): every step with the intermediate.
- Conversions (1 mark per step): reagent and intermediate.
- Tests (1–2 marks): reagent and observation for each class.