Alcohols
1. What are Alcohols?
Definition: organic compounds in which one or more hydrogens are replaced by an –OH group. Numbering starts from the end that gives the OH the lowest number (it does not always have to be C1).
| Class | Definition | Board examples |
|---|---|---|
| Primary (1°) | C carrying OH is attached to 1 alkyl group (has 2 H) | ethanol · propan-1-ol · 2-methylpropan-1-ol |
| Secondary (2°) | C carrying OH is attached to 2 alkyl groups (has 1 H) | propan-2-ol · butan-2-ol · pentan-3-ol · 2-methylpentan-3-ol |
| Tertiary (3°) | C carrying OH is attached to 3 alkyl groups (has 0 H) | 2-methylpropan-2-ol · 2-methylbutan-2-ol · 3-methylpentan-3-ol |
2. Physical Properties
Alcohols have much higher b.p. than alkanes of similar size because they have:
- hydrogen bonding (the biggest factor);
- permanent dipoles (polar C–O and O–H);
- more electrons than the alkane with the same carbon chain (stronger temporary dipoles).
B.p. increases with chain length: more electrons ⇒ stronger temporary dipole forces. (Board graph: alkane vs alcohol b.p. for 1–5 carbons — the alcohol bar is always higher.)
Solubility in water
Alcohols are polar and can form hydrogen bonds with water ⇒ soluble. But only small alcohols dissolve well — solubility decreases as the carbon chain gets longer (the long non-polar chain can’t hydrogen-bond, so bonding to water is inefficient — e.g. ethanol dissolves fully, hexan-1-ol hardly).
3. Chemical Properties — the 5 Reactions
4. ① Reaction with Sodium Metal
- Products: sodium ethoxide (the ion is CH₃CH₂O⁻, the ethoxide ion — remember it) and hydrogen gas (effervescence). Propoxide naming isn’t required.
- The O–H bond is weakly acidic; Na replaces the H. (Board wrote the equations unbalanced — balance H₂ as 2 : 2 : 2 : 1.)
5. ② Oxidation of Alcohols
Reagent: acidified potassium dichromate(VI): K₂Cr₂O₇ + dil. H₂SO₄ (KMnO₄ also works — it’s a stronger oxidant). Just show [O] in the equation; balance O and you’re done.
Observation: orange → green (Cr₂O₇²⁻ → Cr³⁺). The board wrote “green to orange” — it is orange to green.
Oxidation of a primary alcohol
| Product | Conditions | Equation |
|---|---|---|
| → aldehyde | keep alcohol in excess and distil the aldehyde off as soon as it forms (aldehyde has lower b.p., no H-bonding) | CH₃CH₂OH + [O] → CH₃CHO + H₂O (ethanal) CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O (propanal) |
| → carboxylic acid | keep oxidising agent in excess and heat under reflux until complete oxidation | CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (ethanoic acid) CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O (propanoic acid) |
Secondary and tertiary
6. ③ Substitution — Making Halogenoalkanes
The OH group is replaced by a halogen (nucleophilic substitution). Water is a by-product with HX.
Substituting Cl
| Reagent | Conditions | Equation (ethanol) |
|---|---|---|
| HCl | HCl made in situ from NaCl + conc. H₂SO₄; heat with the alcohol | CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O |
| PCl₅ | No conditions; used as a test for the –OH group: steamy fumes of HCl (also reacts with water, carboxylic acids) | CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + HCl + POCl₃ |
| SOCl₂ | No conditions; steamy fumes (SO₂ + HCl) — best method: both by-products are gases | CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + HCl + SO₂ |
| PCl₃ | heat under reflux | 3CH₃CH₂OH + PCl₃ → 3CH₃CH₂Cl + H₃PO₃ (phosphorous acid) |
Substituting Br and I
| Reagent | Conditions / comment | Equation |
|---|---|---|
| HBr | made in situ from NaBr + conc. H₂SO₄, heat — not usually used: conc. H₂SO₄ oxidises HBr ⇒ SO₂ and other messy products | CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O |
| PBr₃ | made in situ: red phosphorus + bromine, heat under reflux (alcohol in the mixture) | P₄ + 6Br₂ → 4PBr₃ ; 3CH₃CH₂OH + PBr₃ → 3CH₃CH₂Br + H₃PO₃ |
| HI | not used — NaI + conc. H₂SO₄ gives other products (SO₂, S, H₂S): messy, I⁻ is a good reducing agent | — |
| PI₃ | made in situ: red phosphorus + iodine, heat under reflux (board wrote bromine by mistake) | P₄ + 6I₂ → 4PI₃ ; 3CH₃CH₂OH + PI₃ → 3CH₃CH₂I + H₃PO₃ |
7. ④ Dehydration (Elimination of Water)
Dehydration of an alcohol ⇒ alkene (OH and an H from the neighbouring C are removed).
| Reagent | Conditions | Equation |
|---|---|---|
| Al₂O₃ catalyst | heating required | CH₃CH₂OH → CH₂=CH₂ + H₂O |
| conc. H₂SO₄ | heating required | CH₃CH₂OH → CH₂=CH₂ + H₂O |
Lab set-up (with Al₂O₃)
Dehydrating a “complex” alcohol
The C=C can form on either side of the carbon carrying OH — not necessarily only one place. Butan-2-ol ⇒ but-1-ene (50 %) + cis-but-2-ene (25 %) + trans-but-2-ene (25 %).
8. ⑤ Esterification
Alcohol + carboxylic acid ⇒ ester + water; conc. H₂SO₄ catalyst + heat (reversible, ⇌).
Ester group: R–C(=O)–O–R′. Name: alkyl (from the alcohol) + alkanoate (from the acid). The alkanoate part has the C=O carbon; read it as the carboxylic acid with the –OH replaced by –O–alkyl.
| Ester | Name | Made from |
|---|---|---|
| CH₃CH₂–C(=O)–O–CH₂CH₃ | ethyl propanoate | ethanol + propanoic acid |
| CH₃–C(=O)–O–CH₂CH₂CH₃ | propyl ethanoate (board wrote “propyl methanoate” — the acyl part has 2 C) | propan-1-ol + ethanoic acid |
| CH₃–O–C(=O)–CH₂CH₃ | methyl propanoate | methanol + propanoic acid |
- Separating the ester: add water — the alcohol and carboxylic acid dissolve (hydrogen bonding) but the ester does not (only temporary dipoles).
- Uses: esters give pleasant smells — perfumes, flavourings, glues (their temporary dipoles stick paper well).
- Structural formulae: “CO₂” in the middle like CH₃CO₂CH₂CH₃ is accepted as an ester group.
9. Past-Paper MCQs (board)
| Q | Question | Working / answer |
|---|---|---|
| 2 | decolourises acidified KMnO₄ (warm) but not bromine water | alcohol oxidises, no C=C ⇒ ethanol ⇒ B ✓ (ethene decolourises both; butane neither) |
| 4 | X → NaOH(aq) → C₄H₁₀O → Cr₂O₇²⁻/H⁺ → C₄H₈O only | X = 2-bromobutane ⇒ butan-2-ol (2°) ⇒ butanone only ⇒ B ✓ (a 1° alcohol would also give an acid) |
| 5 | hex-3-en-1-ol + excess hot conc. acidic KMnO₄ | C=C cut: CH₃CH₂COOH; other half HOCH₂CH₂… oxidised too ⇒ HOOC–CH₂–COOH ⇒ D ✓ |
| 8 | alcohol giving only one oxidation product with warm dichromate | 2° ⇒ ketone only ⇒ butan-2-ol ⇒ B ✓ (1° gives aldehyde and acid; 3° none) |
| 10 | X (C₁₀H₁₄O) unreactive to mild oxidising agents; dehydration product? | unreactive ⇒ tertiary alcohol ⇒ dehydrate with H from adjacent C ⇒ D ✓ (PhC(CH₃)=CHCH₃) |
| 13 | reacts with its own oxidation product (no C lost) ⇒ sweet-smelling liquid | propan-1-ol + propanoic acid ⇒ ester ⇒ D ✓ |
| 21 | reagent for primary alcohol giving product with the same number of O atoms | Na ⇒ RCH₂ONa (1 O) ⇒ D ✓ (Al₂O₃ loses O; CH₃CO₂H gives ester, 2 O; HBr loses O) |
| 22 | X oxidised to Y; Y + ethanol + conc. H₂SO₄ ⇒ liquid Z | X = 1° alcohol, Y = carboxylic acid, Z = ethyl ester ⇒ C ✓ |
| 25 | X + ethanoic acid (H⁺) ⇒ CH₃C(=O)CH₂CH₂OCOCH₃ | remove the ethanoate group ⇒ X = CH₃COCH₂CH₂OH = C₄H₈O₂ ⇒ D ✓ |
| 27 | propan-1-ol → 2-bromopropane | conc. H₂SO₄ ⇒ propene; then HBr (H to carbon with more H) ⇒ B ✓ |
| 31 | cholesterol: change in no. of chiral C with cold dilute / hot conc. KMnO₄ | cold dilute: both C=C carbons get OH ⇒ +2; hot conc.: the 2° C–OH ⇒ ketone (loses a chiral C) ⇒ −1 ⇒ D ✓ (+2, −1) |
| 32 | geraniol — which statement is NOT correct | orange→green ✓; decolourises Br₂ water ✓; counting CH₃/CH₂ ✓; “two pairs of cis-trans” ✗ (one C=C has two CH₃ on one carbon) ⇒ D ✓ |
| 38 | X unaffected by hot acidified KMnO₄; reacts with ethanoic acid + conc. H₂SO₄ | alcohol that can’t oxidise ⇒ tertiary ⇒ (CH₃)₃COH ⇒ C ✓ |
| 50 | alcohol with a chiral centre AND oxidised to a ketone | 2° and chiral ⇒ pentan-2-ol ⇒ A ✓ (pentan-3-ol not chiral; 3-methylhexan-1-ol is 1°; 3-methylhexan-3-ol is 3°) |
| 55 | C₅H₁₂O alcohols — numbers of 1°, 2°, 3° | 4 primary, 3 secondary, 1 tertiary ⇒ C ✓ |
| 59 | Q + excess HBr (one C=C, two OH) | HBr adds across C=C and replaces both OH ⇒ 3 Br in all ⇒ C ✓ |
| 67 | name of the skeletal alcohol (OH on C2, two methyls on C3) | 3,3-dimethylpentan-2-ol ⇒ C ✓ (lowest number to OH; longest chain) |
| 68 | retinol with acidified KMnO₄ — conditions and final products | break C=C ⇒ hot, concentrated; ends: ketones and carboxylic acids (primary alcohol ⇒ acid) ⇒ D ✓ |
10. Long CIE-style Questions (board)
A = (CH₃)₃COH (tertiary, no reaction with dichromate) · B = butan-2-ol · C = butan-1-ol · D = 2-methylpropan-1-ol. Linked pairs: A–B chain isomerism · B–C position isomerism · C–D chain isomerism.
(b) B + hot conc. H₂SO₄ ⇒ but-1-ene and but-2-ene. but-2-ene shows geometric (cis–trans) isomerism: each C=C carbon carries 2 different groups and the π bond restricts rotation. Draw displayed formulae of cis- and trans-but-2-ene.
(a) molecular formula C₁₀H₂₀O, Mr = 156 (as on the board). (b) functional groups: alkene and primary alcohol. (c) circle the chiral C (the C bearing H, CH₃, and two different chains).
(d)(i) the other cis-trans isomer of geraniol: swap the groups on one C=C carbon. (ii) no optical isomers — no chiral carbon. (e) aqueous bromine: brown ⇒ colourless.
(f) geraniol with: (i) excess H⁺/Cr₂O₇²⁻ reflux ⇒ the alcohol → carboxylic acid, C=C intact · (ii) ethanoic acid + acid catalyst ⇒ ethanoate ester (R–CH₂–O–CO–CH₃) · (iii) HBr ⇒ addition across C=C and substitution of OH (both Br).
(a)(i) Br₂ ⇒ CH₂BrCHBrCH₂OH · (ii) reflux with Cr₂O₇²⁻/H⁺ ⇒ CH₂=CHCO₂H. (b) with MnO₂ ⇒ CH₂=CHCHO: redox. (c) CH₃CH₂CHO from ruthenium(IV): they’re isomers ⇒ functional-group (structural) isomerism.
(d) without Ru: step I H₂ + Ni catalyst, 180 °C ⇒ propan-1-ol; step II acidified K₂Cr₂O₇, heat and distil off the aldehyde immediately ⇒ propanal. (e) unusual: the organic product was both reduced and oxidised in one step (C=C reduced, OH oxidised). (f) cold dilute MnO₄⁻ ⇒ CH₂(OH)CH(OH)CH₂OH; hot acidified MnO₄⁻ reflux ⇒ HOOC–CH₂–COOH.
(a)(i) propene → diol: KMnO₄(aq), acidified, cold, dilute; (ii) diol → lactic acid: redox / oxidation. (b)(i) CH₃CO₂H → ClCH₂CO₂H: Cl₂ gas, UV light (free-radical substitution); (ii) ClCH₂CO₂H → HOCH₂CO₂H: NaOH(aq), heat under reflux (nucleophilic substitution).
(c) heated under reflux with acidified dichromate: lactic acid (2° alcohol) ⇒ CH₃COCO₂H; glycolic acid (1° alcohol) ⇒ HO₂C–CO₂H (full oxidation of CH₂OH).
(d) lactic acid is chiral: draw the two optical isomers (mirror line, wedge/dash), asterisk on the chiral C. (e) lactic + glycolic acid ⇒ two different esters (each acid can supply the –OH or the –COOH). (f) soluble stitches: polyester breaks up by hydrolysis at pH 5–6 in the body; products are soluble because they can form hydrogen bonds with water.
- R–OH; 1° / 2° / 3° = alkyl groups on the C–OH. Number so OH gets the lowest number.
- Higher b.p. than alkanes (H-bonding + permanent dipoles + electrons). Small alcohols soluble (H-bond with water), solubility ↓ with chain length. Draw H-bonds with δ+/δ–, lone pair, straight O–H···O.
- Na ⇒ alkoxide + H₂. Oxidation (acidified K₂Cr₂O₇, orange → green): 1° ⇒ aldehyde (excess alcohol, distil) or acid (excess oxidant, reflux); 2° ⇒ ketone; 3° ⇒ none.
- Halogenation: Cl — HCl (NaCl + conc. H₂SO₄), PCl₅, SOCl₂, PCl₃ (reflux); Br/I — PBr₃ / PI₃ made in situ from red P + Br₂/I₂, reflux (HBr/HI unreliable). PCl₅ + OH ⇒ steamy HCl fumes.
- Dehydration: Al₂O₃ + heat or conc. H₂SO₄ + heat ⇒ alkene (butan-2-ol ⇒ 3 alkenes); suck-back safety.
- Esterification: alcohol + carboxylic acid ⇌ ester + H₂O, conc. H₂SO₄, heat. Name: alkyl (alcohol) + alkanoate (acid). Ester insoluble in water.