9701 Chemistry · Topics 16 & 30 · AS + A Level
Alcohols and Phenol Cheat Sheet — A Level Chemistry 9701
Alcohols are the hub of every organic synthesis route in 9701, and phenol is the classic ‘compare and explain’ question. This sheet covers oxidation products for each class of alcohol and the conditions that decide between aldehyde and carboxylic acid, esterification and dehydration, the tri-iodomethane test, and why phenol is acidic enough to react with sodium hydroxide when ethanol is not.
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What’s on this cheat sheet
Alcohols and Phenol
01 · Physical properties
The –OH group hydrogen bonds, so alcohols boil far higher than alkanes of similar Mr and the short‑chain ones mix with water in all proportions.
Solubility falls as the non‑polar chain lengthens: the hydrocarbon part cannot hydrogen bond to water.
02 · Making alcohols
Hydration of an alkene — steam, H₃PO₄, 300 °C, 60 atm. Fast, pure product, continuous, but needs crude oil and high energy.
Fermentation — glucose with yeast, 35 °C, no air: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Renewable and low‑energy, but slow, batch, and gives dilute impure ethanol needing distillation.
03 · Oxidation — the key reaction
| Alcohol | Conditions | Product |
|---|---|---|
| primary | distil off | aldehyde |
| primary | reflux, excess | carboxylic acid |
| secondary | reflux | ketone |
| tertiary | — | no reaction |
Reagent: acidified K₂Cr₂O₇, orange → green. A tertiary alcohol has no H on the carbon bearing the –OH, so it cannot be oxidised without breaking a C–C bond.
04 · Other reactions of alcohols
| Reagent | Product |
|---|---|
| Na | alkoxide + H₂ (steady fizzing) |
| conc. H₂SO₄, 170 °C | alkene (dehydration) |
| PCl₅ or HCl / ZnCl₂ | halogenoalkane |
| carboxylic acid + H₂SO₄ | ester (reflux) |
| combustion | CO₂ + H₂O |
05 · The tri‑iodomethane test
Warm with I₂ and NaOH(aq). A pale yellow precipitate of CHI₃ with an antiseptic smell is positive.
Positive for a CH₃CH(OH)– group or a CH₃CO– group: ethanol, propan‑2‑ol, ethanal and all methyl ketones. Propan‑1‑ol and propanal give nothing.
06 · Distinguishing 1°, 2° and 3°
Orange stays orange → tertiary
Turns green → primary or secondary
Then distil the product and test with Fehling’s or Tollens’:
Positive → aldehyde → the alcohol was primary
Negative → ketone → it was secondary
07 · Phenol — structure A2
C₆H₅OH: the –OH is bonded straight to the ring. A lone pair on the oxygen overlaps with the delocalised π system, which has two consequences.
1 The O–H bond weakens and the phenoxide ion formed is stabilised by delocalisation, so phenol is acidic. 2 The ring becomes electron‑rich, so it is more reactive to electrophiles than benzene.
08 · Acidity compared A2
carboxylic acid > phenol > water > alcohol
Phenol reacts with NaOH to give sodium phenoxide, but not with Na₂CO₃ — it is too weak to displace carbonic acid, so no fizzing. A carboxylic acid does fizz, which distinguishes the two.
09 · Reactions of phenol A2
2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr
— white precipitate with bromine water, no catalyst needed
Nitration with dilute HNO₃ at room temperature also works, where benzene needs concentrated acids at 55 °C.
10 · Worked example — identify the alcohol
Oxidised → not tertiary
Tri‑iodomethane positive → has CH₃CH(OH)–
No aldehyde formed → secondary
→ butan‑2‑ol, oxidised to butanone.
11 · Equations to know
CH₃CHO + [O] → CH₃COOH
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 170 °C)
C₂H₅OH + PCl₅ → C₂H₅Cl + HCl + POCl₃
C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O
12 · Distinguishing the hydroxy compounds
| Test | Alcohol | Phenol | Acid |
|---|---|---|---|
| Na | fizz | fizz | fizz |
| NaOH | none | reacts | reacts |
| Na₂CO₃ | none | none | fizz |
| Br₂(aq) | none | white ppt | none |
Two tests separate all three: carbonate for the acid, bromine water for phenol.
13 · Uses
Ethanol — solvent, disinfectant, and a fuel. As a biofuel from fermentation it is close to carbon neutral, since the CO₂ released was absorbed by the crop.
Ethane‑1,2‑diol — antifreeze and a monomer for polyesters. Phenol — antiseptics, resins and the starting point for aspirin and many dyes.
Marks lost here
— Giving reflux when the aldehyde is wanted; it must be distilled off as it forms.
— Saying a tertiary alcohol resists oxidation “because it is bulky” rather than having no H on the carbon bearing the –OH.
— Claiming phenol fizzes with sodium carbonate.
— Forgetting the colour change of the oxidising agent when asked for observations.
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Alcohols and Phenol — Frequently Asked Questions
How do you stop the oxidation of a primary alcohol at the aldehyde?
Use acidified potassium dichromate(VI) and distil the aldehyde off as it forms, since aldehydes have lower boiling points than the parent alcohol. Heating under reflux with excess oxidising agent instead takes the reaction all the way to the carboxylic acid.
Why is phenol more acidic than ethanol?
The lone pair on the oxygen of phenol is delocalised into the benzene ring, which weakens the O–H bond and, more importantly, stabilises the phenoxide ion formed. Ethanol has no such delocalisation, so its alkoxide is much less stable.
What does a positive tri-iodomethane test show?
A pale yellow precipitate of CHI₃ with an antiseptic smell indicates a CH₃CH(OH)– or CH₃CO– group. Ethanol and propan-2-ol give a positive result; methanol and propan-1-ol do not.
