OCR GCSE Chemistry A: Organic chemistry (J248) – Revision Notes

These notes condense sub-topic C6.2 Organic chemistry in J248, the OCR GCSE (9–1) Chemistry A (Gateway Science) specification, Version 4.0 of August 2026. They cover learning outcomes C6.2a to C6.2q, the mathematical learning outcome CM6.2i and the practical suggestions on molecular models and making nylon. Only C6.2e and C6.2f are Higher Tier only. Topic C6 is assessed on Paper 2 for Foundation candidates and Paper 4 for Higher.

Explanations from scratch and worked examples are in the Organic chemistry study guide. For exam-style questions with marked answers, use the practice set. The course hub links every J248 topic, the printable J248 checklist tracks each statement, and our free diagnostics, each about 10 minutes, help you choose what to revise first.

Key words

Term	Meaning
Hydrocarbon	Compound of hydrogen and carbon only
Homologous series	Family with one general formula and functional group; next member adds CH₂
Functional group	Atom or group that decides how the compound reacts
Saturated	Only C–C single bonds (alkanes)
Unsaturated	Contains a C=C double bond (alkenes)
Displayed formula	Shows every atom and every bond
Monomer / polymer	Small repeating molecule / long chain made from many monomers
Feedstock	Raw material for an industrial process
Cracking	Breaking long alkanes into shorter alkanes and alkenes with heat and a catalyst
Fuel cell	Cell fed continuously with fuel and oxygen to give a potential difference

C6.2a, C6.2b, C6.2i: the four series

Series	Functional group	General formula	1 C	2 C	3 C	4 C
Alkane	C–C only	CₙH₂ₙ₊₂	methane	ethane	propane	butane
Alkene	C=C	CₙH₂ₙ	—	ethene	propene	butene
Alcohol	–OH	CₙH₂ₙ₊₁OH	methanol	ethanol	propanol	butanol
Carboxylic acid	–COOH	CₙH₂ₙ₊₁COOH	methanoic	ethanoic	propanoic	butanoic

The fourth alkene is pentene, C₅H₁₀. In an acid, the –COOH carbon counts in the name: ethanoic acid is CH₃COOH.

Method: identify the series from a molecular formula

1. Ends in –COOH: carboxylic acid. Ends in –OH: alcohol.
2. Only C and H: count hydrogens. 2n + 2 means alkane; 2n means alkene.

Method: draw a displayed formula

1. Draw the carbon chain in a line.
2. Add the functional group (C=C, –O–H or the C=O plus O–H of –COOH).
3. Fill every remaining carbon bond with H, so each carbon has four bonds.
4. Check: H has one bond, O two, C four.

Ethanoic acid, displayed:

    H   O
    |   ‖
H — C — C — O — H
    |
    H

C6.2i in one line: compounds with the same functional group react the same way, whatever the chain length.

CM6.2i: 3D and 2D

• Displayed formulae are flat; real carbon atoms with four single bonds are tetrahedral, about 109.5°.
• Diamond: 3D network, each carbon bonded to four others. Graphite: flat hexagonal layers.
• Molecular model kits link the two: build from a drawing, draw from a model.

C6.2c: reactions

Reaction	Reagent / conditions	What happens	Example
Complete combustion	plenty of oxygen	CO₂ + H₂O	C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
Bromine addition	bromine water, room temperature	orange → colourless (alkenes only)	ethene → 1,2-dibromoethane
Hydrogen addition	hydrogen, nickel catalyst, heat	alkene → alkane	butene → butane
Alcohol oxidation	acidified potassium manganate(VII), warm	–CH₂OH → –COOH; purple → colourless	ethanol → ethanoic acid

Method: balance a combustion equation

1. Carbons → that number of CO₂.
2. Hydrogens ÷ 2 → that number of H₂O.
3. Count oxygens on the right, subtract any in the fuel, halve for O₂.
4. A fraction such as 4½ means you multiply every coefficient by 2.

Worked reminder: propan-1-ol, C₃H₇OH. 3CO₂ and 4H₂O give 10 O atoms; subtract 1 in the alcohol, leaving 9, so 4½O₂. Doubled: 2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O.

C6.2d, C6.2g: addition polymers

• Monomer has a C=C; the double bond opens and monomers join.
• Only one product: the polymer. The written form is [repeat unit]ₙ.
• Monomer → repeat unit: C=C becomes C–C, bonds stick out of both sides, brackets, n.
• Repeat unit → monomer: put the C=C back between the two main-chain carbons.
• Number of monomers in a chain = Mr of polymer ÷ Mr of monomer.

Polymer	Made from	Repeat unit
PTFE	tetrafluoroethene, CF₂=CF₂	–[CF₂–CF₂]–
poly(but-1-ene)	but-1-ene, CH₂=CHC₂H₅	–[CH₂–CH(C₂H₅)]–
poly(ethene)	ethene, CH₂=CH₂	–[CH₂–CH₂]–

C6.2e, C6.2f: condensation polymers (Higher Tier only)

• Each monomer has two functional groups; one group per monomer would end the chain.
• Each link releases a small molecule, usually water.
• Polyester: diol + dicarboxylic acid, ester links. Polyamide (nylon): diamine + dicarboxylic acid, amide links.
• Block diagram, polyester: n HO–●–OH + n HOOC–■–COOH → –[O–●–O–OC–■–CO]ₙ– + 2n H₂O.
• Making nylon: 1,6-diaminohexane in water, with hexanedioyl dichloride in cyclohexane layered on top. Nylon forms where the layers meet; pull it out with tweezers and wind it on a rod. The small molecule here is hydrogen chloride. Gloves and eye protection; no flames near cyclohexane.

C6.2h: natural polymers

Polymer	Monomers
DNA	four nucleotides, named by base: adenine, thymine, cytosine, guanine
Starch, cellulose	sugars (glucose)
Proteins	amino acids

C6.2j–C6.2n: crude oil

• Crude oil: a mixture of hydrocarbons, mostly alkanes, CₙH₂ₙ₊₂.
• Fractional distillation: vaporise, feed into a column hot at the bottom and cool at the top, each fraction condenses where the temperature drops below its boiling point.
• Fractions, top to bottom: refinery gases, petrol, kerosene, diesel, fuel oil, bitumen.
• Bigger molecules: stronger intermolecular forces, higher boiling point, collected lower.
• Crude oil supplies fuels and is the feedstock for plastics, solvents and many other products.
• It is finite: it formed over millions of years and is used much faster.

C6.2o: cracking

• Long alkanes → shorter alkanes + alkenes.
• Conditions: vapour over a hot catalyst (zeolite in industry, about 500 °C; aluminium oxide in the lab).
• Reasons: demand for short-chain fuels exceeds supply; alkenes make polymers.

Method: complete a cracking equation

1. Subtract the carbons and hydrogens you are given.
2. Share what is left between the missing molecules.
3. Check the hydrogen-to-carbon ratio: 2n means alkene.

C6.2p, C6.2q: cells and fuel cells

• Chemical cell: gives a potential difference until the reactants run out.
• Fuel cell: fuel and oxygen are fed in continuously.
• Hydrogen/oxygen (acidic electrolyte): H₂ → 2H⁺ + 2e⁻ at the negative electrode; O₂ + 4H⁺ + 4e⁻ → 2H₂O at the positive; overall 2H₂ + O₂ → 2H₂O.
• For: only water at the point of use, continuous running, quiet.
• Against: hydrogen storage and flammability, hydrogen often made from fossil fuels, high cost, few filling stations.

Must-know distinctions

This	Not this
Alkene decolourises bromine water	Alkane leaves it orange
Addition polymer: one product	Condensation polymer: polymer plus small molecule
Fractional distillation separates (physical)	Cracking breaks bonds (chemical)
Intermolecular forces overcome when boiling	Covalent bonds stay intact
Fuel cell runs while fuel is supplied	Chemical cell stops when reactants are used up

Quick self-test

1. An alkane has nine carbon atoms. Write its molecular formula.
2. Name CH₃CH₂CH₂COOH.
3. Which series is C₅H₁₀ in?
4. State the colour change when an alkene is shaken with bromine water.
5. Name the product when propene reacts with hydrogen, and the catalyst.
6. Name the acid formed when methanol is oxidised by acidified potassium manganate(VII).
7. Draw the repeat unit of the polymer made from CH₂=CHCH₃.
8. A chain of poly(butene) has Mr 56 000. How many C₄H₈ monomers joined? (C = 12, H = 1)
9. Name the four DNA nucleotides by their bases.
10. Why do larger alkane molecules condense lower in the column?
11. (Higher Tier only) Why must a condensation monomer have at least two functional groups?
12. Write the overall equation for the hydrogen/oxygen fuel cell.

Answers

1. C₉H₂₀ (2 × 9 + 2 = 20).
2. Butanoic acid (four carbons, including the –COOH carbon).
3. Alkene: 10 = 2 × 5, so CₙH₂ₙ.
4. Orange to colourless.
5. Propane; nickel catalyst.
6. Methanoic acid, HCOOH.
7. –[CH₂–CH(CH₃)]–, with n outside the brackets.
8. Mr of C₄H₈ = 48 + 8 = 56; 56 000 ÷ 56 = 1000.
9. Adenine, thymine, cytosine, guanine.
10. Stronger intermolecular forces, so higher boiling points; they condense where the column is hotter.
11. One group joins the chain on each side; with only one, the chain cannot grow past it.
12. 2H₂ + O₂ → 2H₂O

Where marks are usually lost

• Writing “goes clear” for bromine water instead of colourless.
• Counting three carbons in butanoic acid by ignoring the –COOH carbon.
• Missing hydrogens on a displayed formula, leaving a carbon with three bonds.
• Leaving the C=C inside an addition polymer’s repeat unit, or missing the n.
• Saying intermolecular forces are covalent bonds being broken during boiling.
• Describing cracking without giving both heat and a catalyst.
• Calling cracking a separation; it is a chemical reaction.
• (Higher Tier) Forgetting the small molecule in condensation polymerisation.
• Listing fuel-cell advantages with no disadvantages when the command word is “evaluate”.

Official syllabus

Published by OCR: Version 4.0 (August 2026) of specification J248, for the GCSE (9–1) in Chemistry A, Gateway Science, first assessed in 2018. Content taken from it: sub-topic C6.2 Organic chemistry, statements C6.2a–C6.2q, maths outcome CM6.2i, and the practical suggestions “Use of models” and “Making nylon”.