Co-ordinated Sciences Chemistry C11: Organic Chemistry
Cambridge IGCSE Co-ordinated Sciences 0654 Chemistry C11 notes with worked examples, misconception checks and assessment guidance.
Chemistry C11 of Cambridge IGCSE Co-ordinated Sciences 0654 organises carbon compounds by structure, naming and reaction. Official sections C11.1 to C11.7 cover displayed formulae, saturation, homologous-series characteristics, unbranched alkanes and alkenes up to four carbons, ethanol, petroleum fractions, alkane combustion, cracking, alkene addition, ethanol combustion and uses, addition polymers, condensation polymers and nylon. Carboxylic acids, esters, fermentation, PET and proteins are outside this boundary.
Formulae and functional groups
A displayed formula shows every atom and every bond. A structural formula gives an unambiguous compact description of atom arrangement, such as CH₂=CH₂, CH₃CH₂OH or CH₃COOCH₃.
A homologous series is a family of similar compounds with similar chemical properties. Its members have the same general formula and show a trend in physical properties. Recall of specific general formulae is not required in C11.
Name and draw organic compounds
The prefix gives carbon-chain length: meth-, eth-, prop- and but- for one to four carbon atoms. The suffix identifies the required type: -ane, -ene or -ol. Name and draw methane, ethane, ethene and ethanol, plus unbranched alkanes and alkenes up to four carbon atoms. For butenes, distinguish but-1-ene and but-2-ene; cis-trans naming is not required.
Fossil fuels and hydrocarbons
Coal, natural gas and petroleum are fossil fuels. Methane is the main constituent of natural gas.
A hydrocarbon contains hydrogen and carbon only. Petroleum is a mixture of hydrocarbons rather than one pure compound.
Ethanol contains oxygen and is therefore not a hydrocarbon.
Fractional distillation of petroleum
Petroleum is heated so much of it vaporises. Vapours enter a fractionating column that is hot at the bottom and cooler at the top. Compounds condense at levels where temperature falls below their boiling ranges. Fractions are mixtures with similar boiling points.
From bottom to top of the column, average chain length decreases, volatility increases, boiling point decreases and viscosity decreases.
| Fraction | Required use |
| refinery gas | heating and cooking gas |
| gasoline or petrol | fuel for cars |
| naphtha | chemical feedstock |
| diesel oil or gas oil | diesel-engine fuel |
| bitumen | road making |
Fractional distillation is physical separation. It does not break covalent bonds or create shorter hydrocarbons.
Alkanes
Alkanes are saturated hydrocarbons with only single covalent bonds. They are generally unreactive except for combustion.
Complete combustion in excess oxygen forms carbon dioxide and water. Incomplete combustion can form carbon monoxide or carbon particulates plus water.
Cracking
Cracking breaks larger alkane molecules into smaller alkanes, alkenes and hydrogen using high temperature and a catalyst.
A possible equation is:
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Cracking makes more useful shorter-chain fuels and produces alkenes needed as chemical feedstocks and polymer monomers. Atom counts must balance, but product combinations can vary.
Cracking is chemical decomposition, not fractional distillation or combustion.
Alkenes and the bromine test
Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond.
Aqueous bromine distinguishes a typical alkene from a saturated hydrocarbon. Orange or brown bromine water is decolourised by an alkene because bromine adds across the C=C bond. An alkane shows no change under the ordinary test conditions.
The observation is decolourisation, not that the alkene itself becomes colourless.
Alkene addition reactions
In an addition reaction, two reactants form only one product as atoms add across the double bond.
- ethene plus bromine forms 1,2-dibromoethane
- ethene plus hydrogen forms ethane with a nickel catalyst
- ethene plus steam forms ethanol with an acid catalyst
The C=C becomes C-C in the product. Draw both new attachments on the former double-bond carbons and preserve every atom.
Hydrogen addition is hydrogenation. Steam addition is hydration. Conditions matter when the question requests a manufacturing route.
Ethanol combustion and uses
Complete ethanol combustion forms carbon dioxide and water:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Ethanol is used as a fuel because combustion releases energy and as a solvent because it dissolves a range of substances.
Calling bioethanol carbon neutral without qualification is too absolute because farming, processing and transport may use energy and cause emissions.
Addition polymers
A polymer is a large molecule built from many smaller monomer molecules. Plastics are made from polymers.
In addition polymerisation, many alkene monomers join and no small molecule is eliminated. The double bond opens to form single bonds in the backbone.
Ethene forms poly(ethene), whose repeat unit can be written [-CH₂-CH₂-]ₙ. The repeat-unit brackets cut through the two backbone bonds that continue to neighbouring units.
To deduce a polymer from an alkene, open C=C and retain substituents on the same carbons. To deduce the monomer from a repeat unit, identify two backbone carbons and restore their double bond.
Condensation polymers
Condensation polymerisation joins monomers with two functional groups and eliminates a small molecule such as water at each linkage.
A dicarboxylic acid plus a diamine forms a polyamide with -CO-NH- links. Nylon is the required synthetic polyamide example.
Identify the -CO-NH- links in the supplied nylon structure. The detailed monomer structures and other named condensation polymers are not required.
Addition polymerisation uses alkene double bonds, forms one polymer product and retains all monomer atoms. Condensation polymerisation uses bifunctional monomers and forms the polymer plus a small molecule.
Worked application: trace carbon through reactions and polymers
Decane from petroleum can be cracked as C₁₀H₂₂ → C₈H₁₈ + C₂H₄, producing a useful shorter alkane and ethene. Ethene decolourises aqueous bromine because bromine adds across its double bond. Hydrogen with a nickel catalyst makes ethane, while steam with an acid catalyst makes ethanol. Alternatively, ethene undergoes addition polymerisation to [-CH₂-CH₂-]ₙ, where the double bond opens and no small molecule forms. The same alkene therefore supports three addition reactions and polymer formation; every product must preserve the atoms supplied by its reactants.
Nylon follows condensation polymerisation instead: bifunctional monomers join through amide links and a small molecule is eliminated, unlike ethene addition polymerisation.
Common misconceptions and corrections
- Calling a displayed formula a molecular formula. Displayed formula shows every bond.
- Saying homologues have identical physical properties. They show a trend.
- Forgetting the
-CH₂-difference between neighbours. It is a defining pattern. - Calling every compound with C and H a hydrocarbon. It must contain only those elements.
- Numbering from the end giving the larger locant. Use the lower position.
- Calling petroleum a compound. It is a mixture of hydrocarbons.
- Saying fractions are pure compounds. They are boiling-range mixtures.
- Saying chain length increases up the column. It decreases.
- Calling fractional distillation cracking. It is physical separation.
- Calling alkanes unsaturated. They have only C-C single bonds.
- Calling cracking combustion. It decomposes large molecules.
- Producing only smaller alkanes in cracking. Alkenes or hydrogen are also formed.
- Saying bromine water tests any organic compound. It tests unsaturation in this context.
- Saying bromine water turns milky. It is decolourised.
- Leaving C=C in an addition product. It becomes C-C.
- Calling addition a two-product reaction. Only one product forms.
- Calling ethanol combustion an addition reaction. Complete combustion forms CO₂ and water.
- Leaving the alkene double bond in an addition-polymer repeat unit. It opens.
- Omitting continuation bonds through repeat brackets. The chain must continue.
- Saying addition polymerisation releases water. It produces polymer only.
- Using monomers with one functional group for condensation chains. They need two.
- Calling nylon an addition polymer. It is a condensation polyamide.
Assessment guidance
Draw displayed formulae with every atom and bond and check carbon valency of four. Name unbranched alkanes and alkenes up to four carbons, including but-1-ene and but-2-ene, without importing cis-trans notation. Petroleum answers should state temperature-gradient separation, decreasing chain length and lower boiling points up the column, and use only the five named fractions. Organic transformations need reactant, catalyst where stated, reaction type and product structure. For polymer deductions, mark the opened double bond, preserve substituents, show continuation bonds and distinguish addition from condensation. Recognise nylon as the required polyamide structure.
Retrieval practice
Name and draw every required unbranched alkane and alkene up to four carbons plus methane, ethane, ethene and ethanol. Rebuild the five petroleum fractions, two column trends and their uses. Complete the three alkene additions with catalysts where specified and balance ethanol combustion. Convert six alkene monomers to addition-polymer repeat units and back, then identify repeat units in supplied addition and condensation polymers, sketch the required nylon segment and compare the two polymerisation types.
Theory and practical ownership
This theory note owns displayed formulae, naming, petroleum, alkane combustion, cracking, alkene addition, ethanol uses and combustion, and addition or condensation polymer structures. The dedicated Co-ordinated Sciences practical series owns heating, gas and bromine tests, observations, purification, hazards, measurements and evaluation.
Official source
Cambridge International, Co-ordinated Sciences 0654 syllabus for examinations in 2025, 2026 and 2027.
Cambridge International, Co-ordinated Sciences (9-1) 0973 syllabus for examinations in 2025, 2026 and 2027.
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