AQA A-Level Chemistry: Alcohols (7405)

This guide teaches 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 from June 2016 onwards. It covers every point in 3.3.5.1 Alcohol production, 3.3.5.2 Oxidation of alcohols and 3.3.5.3 Elimination, plus Required practical 5. All of it is AS content, so nothing here is A-level only. The specification places organic chemistry (3.3.1 to 3.3.6) on Paper 2 at AS, organic chemistry (3.3) on Paper 2 at A-level, and A-level Paper 3 can test any content and any practical skill.

For quick recall, use the alcohols revision notes. To test yourself, use the alcohols practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. To find your weak spots across the course, try one of the free 10-minute diagnostics.

What this unit covers

Spec point	What you must be able to do	Status
3.3.5.1 Alcohol production	Hydration of alkenes with an acid catalyst and its mechanism; fermentation of glucose and its conditions; fractional distillation; define biofuel; carbon-neutral equations and why the claim fails; environmental and ethical issues	AS and A-level
3.3.5.2 Oxidation of alcohols	Classify alcohols; oxidise with acidified potassium dichromate(VI); write [O] equations; explain how the method gives an aldehyde or a carboxylic acid; Tollens’ and Fehling’s tests	AS and A-level
3.3.5.3 Elimination	Acid-catalysed elimination to alkenes and its mechanism; alkenes for polymers without crude oil	AS and A-level
Required practical 5	Distillation of a product from a reaction	AS and A-level

3.3.5.1 Alcohol production

Hydration of alkenes

Industrially, alcohols are made by adding steam to an alkene in the presence of an acid catalyst. For ethanol:

CH₂=CH₂(g) + H₂O(g) → CH₃CH₂OH(g)

A common industrial catalyst is phosphoric acid held on a solid support such as silica, used with steam at about 300 °C and high pressure. The reaction is reversible, so unreacted ethene is recycled. You can see this equilibrium treated with Kp in the Kp study guide.

Mechanism (electrophilic addition, acid catalysed).

1. The electrons in the C=C double bond attack an H⁺ ion from the acid (shown as H⁺ or as H₃O⁺). One carbon gains the H and the other becomes a carbocation.
2. A lone pair on the oxygen of a water molecule attacks the positive carbon. This gives a protonated alcohol (an oxonium ion, with O carrying the + charge).
3. An O–H bond in the oxonium ion breaks, its electrons going to the oxygen, and H⁺ is released. The catalyst is regenerated.

CH₂=CH₂ + H⁺ → CH₃–CH₂⁺
CH₃–CH₂⁺ + H₂O → CH₃–CH₂–OH₂⁺
CH₃–CH₂–OH₂⁺ → CH₃–CH₂–OH + H⁺

Worked example 1. Predict the major product when propene reacts with steam and an acid catalyst.

• Step 1: H⁺ can add to either carbon of the double bond.
• Step 2: adding H to the end carbon (C1) gives a secondary carbocation, CH₃–C⁺H–CH₃. Adding H to C2 gives a primary carbocation, ⁺CH₂–CH₂–CH₃.
• Step 3: the secondary carbocation is more stable, so it gives the major product.
• Answer: propan-2-ol, CH₃CH(OH)CH₃, is the major product; propan-1-ol is the minor product.

Fermentation of glucose

Ethanol is also made industrially by fermentation of glucose using yeast:

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

Conditions and why they are used:

• Yeast: its enzymes catalyse the reaction.
• A warm temperature, around 35 °C: a compromise. Lower temperatures give a slow rate; much higher temperatures denature the enzymes in the yeast.
• Anaerobic conditions (air excluded): with oxygen present, the ethanol is oxidised to ethanoic acid, and the yeast respires aerobically to carbon dioxide and water instead of making ethanol.

Fermentation gives an aqueous mixture. The ethanol is separated by fractional distillation; ethanol boils at about 78 °C, below water. The purified ethanol can then be used as a biofuel.

Worked example 2. Calculate the maximum mass of ethanol from 360 g of glucose. (Mr: glucose 180.0, ethanol 46.0)

n(glucose) = 360 ÷ 180.0 = 2.00 mol
n(ethanol) = 2 × 2.00 = 4.00 mol
mass       = 4.00 × 46.0 = 184 g

Answer: 184 g of ethanol (with 176 g of carbon dioxide).

Biofuels and carbon neutrality

A biofuel is a fuel produced from biological material (biomass), such as crops, that has been grown recently and can be grown again. Ethanol from fermented sugar is a biofuel.

A carbon-neutral fuel is one with no net carbon dioxide emission to the atmosphere. The claim for bioethanol rests on three equations:

Photosynthesis:  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Fermentation:    C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Combustion:      2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O

Worked example 3. Per mole of glucose, photosynthesis absorbs 6 mol CO₂. Fermentation releases 2 mol and burning the 2 mol of ethanol releases 4 mol: 6 in, 6 out, so no net CO₂ by these three equations.

Why the claim is not valid. Fossil fuels are burned, releasing CO₂, for energy used in farm machinery, making fertilisers, the fractional distillation and transporting the fuel. So the overall process releases more CO₂ than the crop absorbed.

Environmental and ethical issues

Weigh both sides when discussing biofuel decisions:

• For: the crop is renewable; less finite crude oil is used; net CO₂ emissions can be lower than for petrol.
• Against: land for fuel crops is not used for food, which can raise food prices (an ethical issue); clearing forest destroys habitats and releases stored carbon; fertiliser run-off pollutes water.

3.3.5.2 Oxidation of alcohols

Classifying alcohols

Look at the carbon atom bonded to the –OH group and count the carbon atoms joined to it.

Class	Carbons on the C–OH carbon	Example
Primary	one (or none, in methanol)	butan-1-ol, CH₃CH₂CH₂CH₂OH
Secondary	two	butan-2-ol, CH₃CH(OH)CH₂CH₃
Tertiary	three	2-methylpropan-2-ol, (CH₃)₃COH

The oxidising agent

Acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid, is a suitable oxidising agent. When it oxidises an alcohol, the orange Cr₂O₇²⁻ ions are reduced to green Cr³⁺ ions. You may write equations with [O] for the oxygen from the oxidising agent.

• Primary alcohol → aldehyde → carboxylic acid

  CH₃CH₂OH + [O] → CH₃CHO + H₂O         (ethanal)
  CH₃CHO + [O] → CH₃COOH                (ethanoic acid)
  CH₃CH₂OH + 2[O] → CH₃COOH + H₂O       (in one step)

• Secondary alcohol → ketone

  CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O   (propanone)

• Tertiary alcohol: not easily oxidised. The carbon bonded to –OH has no hydrogen atom to lose, so the dichromate stays orange.

Worked example 4. For butan-1-ol, butan-2-ol and 2-methylpropan-2-ol, warmed with excess acidified potassium dichromate(VI) under reflux, give the organic product and the colour seen.

• Butan-1-ol (primary): butanoic acid, CH₃CH₂CH₂COOH; orange to green.
• Butan-2-ol (secondary): butanone, CH₃COCH₂CH₃; orange to green.
• 2-methylpropan-2-ol (tertiary): no reaction; stays orange.

How the method decides aldehyde or carboxylic acid

Wanted product	Method	Why it works
Aldehyde	Heat gently with a limited amount of oxidising agent (alcohol in excess) and distil the aldehyde off as it forms	The aldehyde has a lower boiling point than the alcohol, because there is no hydrogen bonding between aldehyde molecules, so it leaves the flask before it can be oxidised further
Carboxylic acid	Heat under reflux with excess oxidising agent, then distil	Vapours condense and run back into the flask, so the aldehyde stays in contact with the oxidising agent until it is fully oxidised

Telling aldehydes from ketones

Aldehydes are oxidised easily; ketones are not. Two mild oxidising agents use this difference:

Test	Method	Aldehyde	Ketone
Tollens’ reagent (silver nitrate in aqueous ammonia, containing [Ag(NH₃)₂]⁺)	Warm in a water bath	Silver mirror forms; Ag⁺ is reduced to Ag	No change
Fehling’s solution (blue, contains Cu²⁺)	Warm	Blue solution gives a red precipitate (Cu₂O)	Stays blue

In both tests the aldehyde is oxidised to a carboxylic acid (as its salt in the alkaline reagent): RCHO + [O] → RCOOH. Use a water bath, not a flame: organic liquids are flammable.

3.3.5.3 Elimination

Alkenes form when alcohols are heated with a concentrated acid catalyst, such as concentrated sulfuric acid or concentrated phosphoric acid. Water is eliminated:

CH₃CH₂OH → CH₂=CH₂ + H₂O

Mechanism (acid-catalysed elimination):

1. A lone pair on the oxygen of the –OH group attacks H⁺, giving a protonated alcohol, R–OH₂⁺.
2. The C–O bond breaks, its electrons going to oxygen, and a water molecule leaves. A carbocation forms.
3. The electrons in a C–H bond on a carbon next to the positive carbon move to form the C=C bond. H⁺ is released, so the acid is a catalyst.

Worked example 5. Name every alkene formed when butan-2-ol is heated with concentrated phosphoric acid.

• Step 1: the carbocation is CH₃–C⁺H–CH₂CH₃ (on C2).
• Step 2: H can be lost from C1 or from C3.
• Step 3: loss from C1 gives but-1-ene, CH₂=CHCH₂CH₃. Loss from C3 gives but-2-ene, CH₃CH=CHCH₃, which has E/Z isomers.
• Answer: three alkenes: but-1-ene, (E)-but-2-ene and (Z)-but-2-ene.

Why this matters. Ethanol from fermentation can be dehydrated to ethene, and that ethene can be polymerised to poly(ethene). This makes addition polymers without using monomers derived from crude oil.

Required practical 5: distillation of a product from a reaction

The specification suggests preparing cyclohexene from cyclohexanol, purified with a separating funnel and by distillation.

1. Warm cyclohexanol with concentrated phosphoric acid and anti-bumping granules. Distil the impure cyclohexene out as it forms (thermometer bulb level with the side arm, condenser water in at the bottom).
2. Shake the distillate in a separating funnel with sodium carbonate solution to neutralise acid. Release the pressure often, because carbon dioxide forms. Run off and discard the lower aqueous layer.
3. Wash with water, separate again, and transfer the organic layer to a dry flask.
4. Add an anhydrous drying agent such as calcium chloride or magnesium sulfate, swirl until the liquid is clear, then decant or filter.
5. Redistil and collect only the fraction boiling close to the boiling point of cyclohexene.

Worked example 6. 20.0 g of cyclohexanol (Mr 100.0) gives 11.5 g of cyclohexene (Mr 82.0). Calculate the percentage yield.

n(cyclohexanol)        = 20.0 ÷ 100.0 = 0.200 mol
theoretical n(alkene)  = 0.200 mol (1 : 1)
actual n(cyclohexene)  = 11.5 ÷ 82.0 = 0.1402 mol
percentage yield       = 0.1402 ÷ 0.200 × 100 = 70.1 %

Answer: 70.1 %. Product is lost in transfers, on the drying agent and through incomplete reaction.

Common errors

• Writing the fermentation equation with one CO₂ instead of two.
• Justifying the temperature without both rate and enzyme denaturation.
• Drawing the first curly arrow of hydration from H⁺ to the double bond: it starts at the C=C bond.
• Distilling to get a carboxylic acid, or refluxing to get an aldehyde.
• Saying tertiary alcohols “cannot react”: they are not easily oxidised and the dichromate stays orange.
• Missing E/Z isomers among elimination products.
• Calling the Fehling’s precipitate “brown” instead of red.

Where this links

Carbocations and polymers: alkenes. Combustion calorimetry: energetics. Dichromate(VI) half-equations: redox equations. Exam technique: exam preparation.

Next: revision notes and practice questions.

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 Alcohol production, 3.3.5.2 Oxidation of alcohols, 3.3.5.3 Elimination), with Required practical 5.