AQA A-Level Chemistry: Alcohols (7405) – Revision Notes

These notes condense section 3.3.5 Alcohols of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. They cover 3.3.5.1 to 3.3.5.3 and Required practical 5. Everything here is AS content, so none of it is A-level only. It is examined on Paper 2 at AS and A-level, and A-level Paper 3 can test any content. For full explanations and worked examples, read the alcohols study guide. To test yourself, use the alcohols practice questions.

Course hub: AQA A-Level Chemistry. Tick off each point on the printable checklist, and use the free 10-minute diagnostics to see where to focus.

Key definitions

Term	Meaning
Primary alcohol	The C bonded to –OH is bonded to one other carbon (methanol: none)
Secondary alcohol	The C bonded to –OH is bonded to two other carbons
Tertiary alcohol	The C bonded to –OH is bonded to three other carbons
Hydration	Addition of water (steam) across C=C to make an alcohol
Fermentation	Conversion of glucose to ethanol and CO₂ by yeast, without air
Biofuel	A fuel made from biological material (biomass) that has been grown recently and can be regrown
Carbon neutral	No net emission of CO₂ to the atmosphere
Elimination (dehydration)	Removal of H₂O from an alcohol to form an alkene
[O]	Oxygen supplied by the oxidising agent, allowed in oxidation equations

3.3.5.1 Alcohol production

Two industrial routes for ethanol

	Hydration of ethene	Fermentation of glucose
Equation	CH₂=CH₂ + H₂O → CH₃CH₂OH	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Catalyst	Acid, e.g. phosphoric acid on a solid support	Enzymes in yeast
Conditions	Steam, about 300 °C, high pressure	Around 35 °C, anaerobic, aqueous
Raw material	Ethene from crude oil (finite)	Sugar from crops (renewable)
Product	Fairly pure ethanol; unreacted ethene recycled	Aqueous ethanol; needs fractional distillation

Justify the fermentation conditions

• Temperature: too cold → slow; too hot → yeast enzymes denature. About 35 °C is a compromise.
• No air: oxygen would oxidise ethanol to ethanoic acid, and yeast would respire aerobically to CO₂ and water.
• Separation: fractional distillation (ethanol boils at about 78 °C), then the ethanol can be used as a biofuel.

Hydration mechanism in three arrows (plus loss of H⁺)

1  C=C electrons → H⁺            gives carbocation
2  lone pair on O of H₂O → C⁺     gives R–OH₂⁺
3  O–H bond electrons → O         releases H⁺ (catalyst back)

With an unsymmetrical alkene, the more stable carbocation (tertiary > secondary > primary) gives the major alcohol. Carbocation stability is explained in alkenes.

Worked reminder. But-1-ene + steam: H⁺ adds to C1, giving the secondary carbocation on C2. Major product: butan-2-ol. Minor product (via the primary carbocation): butan-1-ol. But-2-ene is symmetrical, so it gives butan-2-ol only.

Carbon-neutral equations: method in steps

1. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (6 CO₂ in).
2. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (2 out).
3. Combustion: 2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O (4 out).
4. Total 6 in and 6 out, so “carbon neutral” by these equations alone.
5. Not valid: fossil fuels burned for farm machinery, fertiliser production, distillation and transport.

Biofuel decisions: points to weigh

• Renewable; saves crude oil; can cut net CO₂.
• Land diverted from food (food prices: ethical); deforestation and habitat loss; fertiliser run-off; water use.

3.3.5.2 Oxidation of alcohols

Reagent: acidified potassium dichromate(VI) (K₂Cr₂O₇ / H₂SO₄). Colour change if oxidation happens: orange → green (Cr₂O₇²⁻ → Cr³⁺).

Alcohol	Product	Example equation
Primary (distil)	Aldehyde	CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O
Primary (reflux, excess oxidant)	Carboxylic acid	CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O
Secondary	Ketone	CH₃CH(OH)CH₂CH₃ + [O] → CH₃COCH₂CH₃ + H₂O
Tertiary	No reaction (stays orange)	—

Must-know distinctions

• Distil vs reflux. Distil off the aldehyde as it forms (alcohol in excess, gentle heat): the aldehyde has no hydrogen bonding between molecules, so it boils lowest and escapes before further oxidation. Reflux with excess dichromate: vapour returns to the flask until all is oxidised to the acid.
• Why tertiary alcohols resist oxidation. No H atom on the carbon bonded to –OH.
• Aldehyde vs ketone. An aldehyde has C=O at the end of the chain (–CHO); a ketone has C=O between two carbons. Only the aldehyde is easily oxidised further.

Method in steps: predicting an oxidation product

1. Find the carbon bonded to –OH and count the carbons joined to it.
2. Primary: keep the chain length; –CH₂OH becomes –CHO (distil) or –COOH (reflux). Name it -al or -oic acid.
3. Secondary: –CH(OH)– becomes –CO–. Name it -one, keeping the same position number.
4. Tertiary: no product; the dichromate stays orange.
5. Balance with [O]: one [O] per step; each step to an aldehyde or ketone also makes H₂O.

Tests for aldehydes

Reagent	Warm with aldehyde	Warm with ketone
Tollens’ ([Ag(NH₃)₂]⁺)	Silver mirror	No change
Fehling’s (blue, Cu²⁺)	Red precipitate	Stays blue

Warm in a water bath. The aldehyde is oxidised: RCHO + [O] → RCOOH.

3.3.5.3 Elimination

Reagent and conditions: heat with concentrated sulfuric acid or concentrated phosphoric acid.

Mechanism: (1) lone pair on O takes H⁺ → R–OH₂⁺; (2) C–O breaks, H₂O leaves → carbocation; (3) C–H electrons on the adjacent carbon form C=C, H⁺ released.

Counting products: list every carbon next to the C–OH carbon that carries an H. Each different one gives a different position of C=C; then check each alkene for E/Z isomers.

• Propan-1-ol or propan-2-ol: propene only.
• 2-methylbutan-2-ol: 2-methylbut-1-ene and 2-methylbut-2-ene (no E/Z, because one carbon of the double bond carries two methyl groups).

Use: ethanol from fermentation → ethene → poly(ethene), making addition polymers without crude oil.

Required practical 5: distillation of a product

Cyclohexene from cyclohexanol (suggested in the specification):

1. Heat cyclohexanol with concentrated phosphoric acid; distil off the crude product.
2. Separating funnel: shake with sodium carbonate solution (release CO₂ pressure), discard aqueous layer, wash with water.
3. Dry with anhydrous CaCl₂ or MgSO₄ until clear; decant.
4. Redistil; collect the fraction near the boiling point of cyclohexene.

Apparatus points: anti-bumping granules; thermometer bulb at the side arm; water in at the bottom of the condenser; never heat a sealed system; use an electric heater, not a flame, with flammable organic liquids.

Quick self-test

1. Classify 2-methylpropan-1-ol.
2. Give the conditions for the industrial hydration of ethene.
3. Write the equation for fermentation of glucose.
4. Why must air be kept out of a fermentation vessel?
5. Calculate the maximum mass of ethanol from 90.0 g of glucose (Mr 180.0; ethanol 46.0).
6. Write an equation, using [O], for oxidation of propan-2-ol.
7. State the colour change when a primary alcohol is oxidised by acidified potassium dichromate(VI).
8. Name the product of warming butan-1-ol under reflux with excess acidified dichromate.
9. What is seen when propanone is warmed with Fehling’s solution?
10. Name the alkenes from pentan-3-ol and concentrated sulfuric acid.
11. 6.00 g of cyclohexanol (Mr 100.0) gives 3.28 g of cyclohexene (Mr 82.0). Find the percentage yield.
12. Give one reason why bioethanol is not truly carbon neutral.

Answers

1. Primary: the C–OH carbon is joined to one other carbon.
2. Steam with a phosphoric acid catalyst on a solid support, about 300 °C, high pressure.
3. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
4. Oxygen would oxidise the ethanol to ethanoic acid (and yeast would respire aerobically).
5. 90.0 ÷ 180.0 = 0.500 mol glucose → 1.00 mol ethanol → 46.0 g.
6. CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
7. Orange to green.
8. Butanoic acid.
9. No change: the solution stays blue.
10. (E)-pent-2-ene and (Z)-pent-2-ene.
11. 0.0600 mol in, 3.28 ÷ 82.0 = 0.0400 mol out; 0.0400 ÷ 0.0600 × 100 = 66.7 %.
12. Fossil fuels are burned to power farm machinery, make fertiliser, distil the ethanol or transport it.

Where marks are usually lost

• Giving “high temperature” for fermentation, or forgetting that it must be anaerobic.
• Leaving yeast or enzymes out of the conditions.
• Stating that fermented ethanol is carbon neutral without the counter-argument when asked to evaluate.
• Starting the hydration curly arrow at H⁺ instead of at the C=C bond.
• Forgetting to show H⁺ being regenerated, so the acid is not shown as a catalyst.
• Giving “reflux” for making an aldehyde.
• Writing an aldehyde group as –COH instead of –CHO.
• Saying the dichromate “turns green” with a tertiary alcohol.
• Naming only one but-2-ene or pent-2-ene when E and Z both form.
• Writing “white” or “brown” for the Fehling’s precipitate.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.5 Alcohols (3.3.5.1 to 3.3.5.3), with Required practical 5.