1. Classification, nomenclature and structure
Alcohols: mono-, di-, trihydric; 1°, 2°, 3° by the carbon bearing –OH; allylic and benzylic alcohols; vinylic/aryl –OH are enols/phenols. Phenols: –OH on an aromatic carbon (phenol, cresols, catechol, resorcinol, quinol). Ethers: R–O–R (symmetrical) or R–O–R′ (unsymmetrical).
IUPAC: alkan-n-ol (propan-2-ol, 2-methylpropan-2-ol, propane-1,2,3-triol); substituted phenols (2-methylphenol, benzene-1,2-diol); ethers as alkoxyalkanes (methoxyethane, methoxybenzene = anisole, 2-methoxy-2-methylpropane).
Structure: O is sp³ with two lone pairs. C–O–H in methanol 108.9°; C–O–C in methoxymethane about 111.7° (bulky groups repel). In phenol the C–O bond (136 pm) is shorter than in methanol (142 pm): partial double-bond character from resonance and the sp² carbon.
Key terms
- Primary alcohol
An alcohol whose –OH carbon is bonded to one other carbon.
- Phenol
A compound with –OH directly attached to an aromatic ring carbon.
- Unsymmetrical ether
An ether whose two groups on oxygen are different.
2. Preparation of alcohols and phenols
Alcohols:
- Acid-catalysed hydration of alkenes (Markovnikov): propene → propan-2-ol.
- Hydroboration–oxidation (B₂H₆; H₂O₂/OH⁻): anti-Markovnikov, propene → propan-1-ol.
- Reduction: aldehydes → 1°, ketones → 2° (H₂/Pd, Ni; NaBH₄; LiAlH₄); acids and esters → 1° (LiAlH₄).
- Grignard reagents + carbonyl compounds, then hydrolysis: HCHO → 1°, RCHO → 2°, R₂CO → 3°.
Phenol:
- From chlorobenzene: NaOH at 623 K, 300 atm, then acid.
- From benzenesulfonic acid: fuse the sodium salt with NaOH, then acid.
- From diazonium salts: warm with water, .
- Cumene process: ; (phenol + propanone).
| Grignard + carbonyl | Alcohol after hydrolysis |
|---|---|
| RMgX + HCHO | RCH₂OH (1°) |
| RMgX + R′CHO | RR′CHOH (2°) |
| RMgX + R′COR″ | RR′R″COH (3°) |
Key terms
- Hydroboration–oxidation
Addition of B₂H₆ to an alkene followed by H₂O₂/OH⁻, giving the anti-Markovnikov alcohol.
- Cumene
Isopropylbenzene, oxidised by air in the industrial production of phenol and propanone.
3. Physical and chemical properties of alcohols and phenols
Physical: hydrogen bonding gives high boiling points (ethanol 351 K vs methoxymethane 248 K and propane 231 K); lower alcohols are miscible with water; bp rises with chain length and falls with branching.
Acidity: alcohols < water < phenol (pKa: ethanol 15.9, water 15.7, phenol 10.0). Phenoxide is resonance-stabilised; electron-withdrawing groups (–NO₂ at o/p) increase acidity (4-nitrophenol 7.1, picric acid 0.5); electron-releasing groups (–CH₃) decrease it. Alcohols and phenols react with Na; phenol also with NaOH; neither with NaHCO₃.
Alcohols: esterification ; with HX (3° > 2° > 1°; Lucas test); with PX₃, SOCl₂; dehydration (conc. H₂SO₄: ethanol → ethene at 443 K, → ethoxyethane at 413 K; ease 3° > 2° > 1°); oxidation 1° → aldehyde (PCC or Cu, 573 K) → acid (KMnO₄, K₂Cr₂O₇), 2° → ketone, 3° resist (alkene over hot Cu).
Phenol: –OH activates the ring (o/p): bromine water → 2,4,6-tribromophenol (white ppt); Br₂ in CS₂ at 273 K → mono-bromophenols; dilute HNO₃ → o- and p-nitrophenol (o-isomer steam-volatile, intramolecular H-bond); conc. HNO₃ → picric acid. Kolbe: sodium phenoxide + CO₂ → salicylic acid. Reimer–Tiemann: CHCl₃ + NaOH → salicylaldehyde. Zn dust → benzene; Na₂Cr₂O₇/H₂SO₄ → benzoquinone. Neutral FeCl₃ gives a violet colour.
| Compound | pKa |
|---|---|
| Ethanol | 15.9 |
| Water | 15.7 |
| Phenol | 10.0 |
| 4-Nitrophenol | 7.1 |
| Picric acid | 0.5 |
Key terms
- Lucas test
Distinguishes 1°, 2° and 3° alcohols by the rate of turbidity with conc. HCl/ZnCl₂.
- Kolbe’s reaction
Carboxylation of sodium phenoxide with CO₂ to give salicylic acid.
- Reimer–Tiemann reaction
Formylation of phenol with CHCl₃ and NaOH to give salicylaldehyde.
4. Commercially important alcohols
Methanol (wood spirit): (573–673 K, 200–300 atm). Solvent for paints and varnishes; used to make methanal. Highly poisonous: small quantities cause blindness, larger ones death.
Ethanol: fermentation of molasses or starch. ; . Solvent, sanitisers, beverages, fuel blended with petrol. Industrial alcohol is denatured (methanol and a dye such as pyridine added) to make it unfit to drink.
| Alcohol | Made from | Use | Hazard |
|---|---|---|---|
| Methanol | CO + H₂ | Solvent, methanal | Blindness, death |
| Ethanol | Fermentation of sugars | Solvent, fuel, sanitiser | Intoxicating |
Key terms
- Denatured alcohol
Ethanol made unfit to drink by adding methanol and other substances.
- Fermentation
Slow breakdown of sugars by enzymes of yeast to ethanol and carbon dioxide.
5. Ethers
Preparation: Williamson synthesis — alkoxide/phenoxide + primary alkyl halide (SN2): . With 3° halides, elimination occurs, so the bulky group must be on the alkoxide. Dehydration of 1° alcohols at 413 K gives symmetrical ethers.
Physical: bp close to alkanes of similar mass (no H-bonding between molecules); solubility in water similar to alcohols of similar size (they H-bond to water).
Reactions: cleavage by HI (or HBr): methyl alkyl ethers → CH₃I + alcohol (SN2 at methyl); tertiary alkyl ethers → tertiary halide + methanol (SN1); aryl alkyl ethers → phenol + alkyl halide (). Anisole: –OCH₃ activates the ring, o/p-directing — bromination in ethanoic acid gives mainly 4-bromoanisole; nitration and Friedel–Crafts reactions give mainly para products.
| Ether + HI | Products |
|---|---|
| CH₃OC₂H₅ | CH₃I + C₂H₅OH |
| (CH₃)₃COCH₃ | (CH₃)₃CI + CH₃OH |
| C₆H₅OCH₃ | C₆H₅OH + CH₃I |
Key terms
- Williamson synthesis
Preparation of ethers by reaction of an alkoxide or phenoxide with a primary alkyl halide.
Exam tips
Common mistakes
- Using a tertiary alkyl halide in a Williamson synthesis (elimination occurs).
- Writing iodobenzene as a product of anisole + HI (phenol + CH₃I form).
- Saying phenol reacts with NaHCO₃ (it is weaker than carbonic acid).
- Forgetting "then H₃O⁺/acidify" in Grignard, Kolbe and Reimer–Tiemann reactions.
- Saying tertiary alcohols oxidise to ketones.
How marks are usually awarded
- 1-mark items: product, reagent, name or order.
- Conversions (2–3 marks): reagent and condition per step (½–1) + correct product.
- Acidity explanations: compare the stability of the conjugate bases (resonance, +I/−I/−R) (1–2 marks).
- Tests: reagent (½–1) + observation for each compound (½–1).