OCR A Level Chemistry A: Organic synthesis (H432) – Revision Notes

These revision notes condense section 4.2.3, Organic synthesis, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover every learning outcome, 4.2.3(a) to (c), in Module 4, Core organic chemistry, including the PAG5 techniques. H432 is the full A level. The Organic synthesis study guide explains each idea in full, with longer examples.

Test yourself afterwards with the Organic synthesis practice questions. The OCR chemistry hub links the other H432 topics, and the printable checklist lists each outcome. A quick check across the course is in the free diagnostics.

4.2.3(a)(i): Quickfit set-ups

Quickfit glassware joins by standard ground-glass joints. Clamp the flask and condenser. Add anti-bumping granules. Heat flammable liquids with a heating mantle or water bath, not a flame.

Feature	Heating under reflux	Distillation
Condenser position	Vertical, above the flask	Sloping down, from a still head to a receiver
Top of apparatus	Open to air, never stoppered	Thermometer in the still head
Thermometer	Not needed	Bulb level with the side arm
Water flow	In at the bottom, out at the top	In at the bottom, out at the top
What happens to vapour	Condenses and returns to the flask	Condenses and is collected
Use	Long heating without losing volatile substances	Removing a product as it forms; separating by boiling point

Oxidising a primary alcohol with K₂Cr₂O₇ / H₂SO₄:

• Aldehyde: distil as it forms, so it escapes before further oxidation.
• Carboxylic acid: reflux with excess oxidising agent.
• Colour change for both: orange to green.

4.2.3(a)(ii): Purifying an organic liquid

Method in steps

1. Heat the reactants under reflux.
2. Distil off the crude organic product.
3. Shake with water in a separating funnel; vent through the tap often.
4. Identify layers by density: the denser liquid is at the bottom. Keep the organic layer.
5. Neutralise leftover acid by shaking with sodium hydrogencarbonate solution, releasing the CO₂ through the tap; finish with a water wash.
6. Dry with an anhydrous salt, e.g. anhydrous MgSO₄ or CaCl₂, until the liquid is clear and some solid moves freely.
7. Decant or filter off the drying agent.
8. Redistil; collect the fraction within a narrow range around the boiling point.

Must-know distinctions

• Washing removes water-soluble impurities (acid, salts). Drying removes the traces of water left after washing.
• Distillation of the reaction mixture gives a crude product. Redistillation of the dried liquid gives the pure product.
• Density, not identity, decides the layer order. Many bromo and iodo compounds sink below water; most hydrocarbons and alcohols float.

Worked reminder: mass from volume

Mass (g) = volume (cm³) × density (g cm⁻³). For 6.00 cm³ of a liquid of density 0.785 g cm⁻³, the mass is 4.71 g. Then n = mass ÷ M, as in 2.1.3.

4.2.3(b)(i): Spotting functional groups

Group	Look for	Classify by
Alkene	C=C	Position in the chain
Alcohol	C–OH	Primary (–CH₂OH), secondary (>CHOH), tertiary (no H on the C–OH carbon)
Haloalkane	C–F, C–Cl, C–Br, C–I	The halogen
Aldehyde	–CHO at a chain end	–
Ketone	C=O with a carbon on each side	–
Carboxylic acid	–COOH	–

In a structural formula, the carboxyl group is written COOH. A tertiary alcohol carbon looks like C(OH) with three carbon groups attached, for example (CH₃)₂C(OH)CH₂–.

4.2.3(b)(ii): Predicting properties and reactions

Properties

• O–H present: hydrogen bonding, so higher boiling point than an alkane of similar Mr, and some water solubility.
• C=C present: reactive, because the π-bond has a relatively low bond enthalpy.
• C–X present: polar bond; the carbon is attacked by nucleophiles such as OH⁻.

Reactions: test each reagent against each group

Reagent	Reacts with	Observation or product
Br₂ (bromine water)	C=C	Orange to colourless; dibromo product
H₂ / Ni	C=C	Alkane chain
HBr	C=C	Bromoalkane (Markownikoff)
K₂Cr₂O₇ / H₂SO₄	1° and 2° OH, CHO	Orange to green; 3° OH unchanged; ketones unchanged
H₃PO₄, heat	OH	Alkene, often as a mixture
NaBr / H₂SO₄	OH	Bromoalkane
NaOH(aq), heat	C–X	Alcohol

4.2.3(c): Two-stage routes

The Module 4 map

• Alkane → haloalkane: Cl₂ or Br₂, UV
• Haloalkane → alcohol: NaOH(aq), heat under reflux
• Alkene → haloalkane: HBr or HCl
• Alkene → alcohol: steam, H₃PO₄ catalyst
• Alcohol → alkene: H₃PO₄ or H₂SO₄ catalyst, heat
• Alkene → alkane: H₂, Ni catalyst
• Alcohol → haloalkane: NaBr, H₂SO₄
• Primary alcohol → aldehyde: Cr₂O₇²⁻ / H⁺, distil; aldehyde → carboxylic acid: Cr₂O₇²⁻ / H⁺, reflux
• Secondary alcohol → ketone: Cr₂O₇²⁻ / H⁺, reflux

Method in steps

1. Mark which groups change and confirm the carbon chain is unchanged.
2. Work back one step from the target.
3. Check the intermediate is one step from the starting material.
4. State reagent and conditions for both steps, and give the intermediate’s structure.
5. Check the reagents do not destroy another group you need.
6. If the question supplies a reaction, use it as written.

Small worked reminders

• Haloalkane to ketone: the haloalkane must be secondary. Hydrolyse with NaOH(aq), then oxidise the secondary alcohol under reflux.
• Alkene to alkane via an alcohol is a wasted step: H₂ / Ni does it directly.
• Alcohol to a different-position alcohol: dehydrate, then add steam. Markownikoff’s rule decides the main product, so check which carbocation is more stable.

Oxidation outcomes at a glance

Alcohol type	Distil with K₂Cr₂O₇ / H₂SO₄	Reflux with excess K₂Cr₂O₇ / H₂SO₄
Primary	Aldehyde	Carboxylic acid
Secondary	Ketone	Ketone
Tertiary	No reaction; stays orange	No reaction; stays orange

Using a reaction the question gives you

Exam papers may give a reaction that is not in Module 4. Read it as a new row in your toolkit:

1. Note the functional group it starts from and the group it makes.
2. Copy the reagents and conditions exactly as stated.
3. Fit it into the route as either step 1 or step 2, then add a Module 4 step to complete the route.
4. Check whether its product needs Markownikoff’s rule or gives a mixture.

Quick self-test

1. Give the position of the thermometer bulb in a distillation.
2. Why is the top of a reflux condenser left open?
3. State the purpose of anti-bumping granules.
4. Name two anhydrous salts used to dry an organic liquid.
5. How do you know an organic liquid is dry?
6. A product has density 0.88 g cm⁻³. Which layer is it in when shaken with water?
7. Why is sodium hydrogencarbonate solution used during washing, and why must you vent?
8. Name the groups in HOCH₂CH₂CH=CHCOOH.
9. Which group in (CH₃)₃COH resists oxidation, and why does that matter in a route?
10. Give a two-stage route from 2-chloropropane to propanone.
11. 0.0400 mol of propan-2-ol gives 3.10 g of 2-bromopropane (M = 122.9 g mol⁻¹). Calculate the percentage yield.
12. Why is a heating mantle used instead of a Bunsen burner to heat the reaction flask?

Answers

1. Level with the side arm of the still head.
2. To avoid heating a sealed system, which could build pressure and shatter.
3. To make the liquid boil smoothly, without bumping.
4. Anhydrous magnesium sulfate and anhydrous calcium chloride.
5. The cloudy liquid turns clear and some drying agent moves freely without clumping.
6. The upper layer, because it is less dense than water.
7. It neutralises and removes acid. The reaction gives off CO₂, so pressure builds in the stoppered funnel.
8. Primary alcohol (–CH₂OH), alkene (C=C) and carboxylic acid (–COOH).
9. The tertiary alcohol group. It is not oxidised by acidified dichromate, so no route can turn it into an aldehyde, ketone or acid by oxidation.
10. NaOH(aq), heat under reflux, gives propan-2-ol; then K₂Cr₂O₇ / H₂SO₄, reflux, gives propanone.
11. Theoretical mass = 0.0400 × 122.9 = 4.916 g; yield = 3.10 ÷ 4.916 × 100 = 63.1%.
12. Organic liquids and their vapours are flammable, and a mantle has no naked flame that could ignite any vapour that escapes.

Where marks are usually lost

• Drawing a stoppered reflux condenser, or showing cooling water entering at the top.
• Writing “heat” for an alcohol oxidation without saying reflux or distil, which leaves the product unclear.
• Leaving out “acidified” or “excess” when oxidising a primary alcohol to a carboxylic acid.
• Calling the top layer organic without using the density data in the question.
• Saying “drying removes impurities” instead of “removes water”.
• Missing the redistillation step, or not saying which fraction to collect.
• Spotting only two of three functional groups in a multi-group molecule.
• Giving a route that oxidises a tertiary alcohol, or ignores Markownikoff’s rule with HBr.
• Giving a reagent with no conditions, such as “NaOH” without “aqueous” and “heat”.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 4.2.3 Organic synthesis, learning outcomes (a) to (c). Published by OCR.