Explore how carbon's bonding versatility creates alkanes, alkenes, and alkynes — and how the presence or absence of a pi bond determines whether a hydrocarbon undergoes addition, substitution, or polymerisation.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Before diving into this lesson, pause and consider what you already know about this topic. What real-world examples can you think of?
Wrong: Chemical equations can be balanced by changing subscripts in formulas.
Right: Chemical equations must be balanced by changing coefficients only. Subscripts in chemical formulas define the identity of the compound — changing them creates a different substance. If you cannot balance an equation with whole-number coefficients, check that your formulas are correct.
To identify an organic compound from its structural formula, follow these steps:
The four major reaction types you need to master for Module 7 are:
| Reaction Type | Reactant class | Key condition | Products |
|---|---|---|---|
| Addition | Alkenes, alkynes | Varies by reagent | One organic product — nothing lost |
| Substitution | Alkanes | UV light (energy source) | Haloalkane + HX |
| Combustion | All hydrocarbons | O2, ignition | CO2 + H2O (complete) |
| Polymerisation | Alkenes (monomers) | Initiator, heat/pressure | Polymer chain |
A useful memory rule: alkenes add; alkanes substitute. Alkenes have a reactive pi bond that can open — alkanes have only sigma bonds and resist most reagents, but can be substituted by radicals under UV light.
Addition polymerisation joins many alkene monomers into a long polymer chain. The C=C double bond opens, and each monomer links to the next through new C-C single bonds. No atoms are lost — the polymer has the same empirical formula as the monomer.
Polymers are generally insoluble in water because they consist of long, nonpolar C-C chains with no sites for hydrogen bonding. Water has strong hydrogen bonds between molecules — these are not disrupted by nonpolar polymer chains, so water and polymer do not mix.
A carbon–carbon single bond (C–C) is a ____ bond formed along the bond axis. A C=C double bond contains one sigma bond and one ____ bond, which is the weaker bond in a double bond, forming above and below the bond axis. A C≡C triple bond contains one sigma and ____ pi bonds. The pi bond restricts rotation, making alkenes ____ around the double bond.
Complete the Learn phase to unlock Practice.
For each compound or description below, identify the functional group, name the compound using IUPAC nomenclature, and draw its structural formula.
Predict the products of the reactions below and explain the conditions required for each transformation.
1. Which of the following correctly distinguishes an addition reaction from a substitution reaction?
2. A student identifies an unknown compound as an alkene. Which evidence best supports this classification?
3. The addition polymerisation of ethene produces polyethylene. Which statement about this process is correct?
4. Which functional group is responsible for alkenes undergoing addition reactions?
5. Isomers are compounds that:
1. Define the term "functional group" and explain why identifying the functional group is the first step in predicting the reactions of an organic compound. Give one example. (3 marks)
2. Write the balanced equation for the addition polymerisation of propene. Include the repeat unit in the correct format. Explain why polypropylene is insoluble in water. (4 marks)
3. A student proposes to balance the equation: C₂H₄ + O₂ → CO₂ + H₂O by writing C₂H₄ + O₂ → CO + H₂O, arguing that this uses smaller coefficients. Evaluate this approach. What is the correct balanced equation for complete combustion of ethene? (3 marks)
1. B — Addition: two reactants → one product (nothing lost, double bond opens). Substitution: one group replaced by another → two products (haloalkane + HX). Option A has these reversed.
2. B — Decolourising bromine water confirms unsaturation AND CnH₂n formula confirms alkene class (not alkyne, which would be CnH₂n₋₂).
3. B — No atoms are lost in addition polymerisation. Each C=C opens to allow chain extension. Polyethylene is nonpolar and insoluble.
4. B — The pi bond is the reactive site in alkenes — it sits above and below the molecular plane, is electron-rich, and can be broken by approaching reagents at room temperature.
5. C — Isomers share the same molecular formula (same atoms) but have different structural arrangements.
Q1 (3 marks): A functional group is a specific atom or group of atoms that gives a compound its characteristic chemical properties [1]. Identifying it first determines the compound class (alkene, alcohol, alkane, etc.) and therefore which reactions are possible [1]. Example: a compound with C=C is an alkene and will undergo addition reactions (halogenation, hydrogenation, hydration, hydrohalogenation) — these reactions are only possible because the pi bond of C=C is reactive and accessible [1].
Q2 (4 marks): n(CH3CH=CH2) → (-CH(CH3)-CH2-)n [1]. Repeat unit: -CH(CH3)-CH2- in brackets with subscript n [1]. Polypropylene is insoluble in water because the long carbon chain is nonpolar — there are no -OH or other hydrogen-bond donor/acceptor groups [1]. Water molecules have strong hydrogen bonds between them; since the polymer cannot participate in hydrogen bonding, the energy required to disrupt water's hydrogen bond network is not recovered by polymer-water interactions, so the two remain separate [1].
Q3 (3 marks): The student's approach is incorrect — changing the products from CO₂ to CO changes the identity of the products, not just the coefficients [1]. CO is the product of incomplete combustion (limited O₂), not complete combustion. Complete combustion of any hydrocarbon in excess O₂ produces only CO₂ and H₂O [1]. Correct balanced equation: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. Check: C: 2=2 ✓; H: 4=4 ✓; O: 6=6 ✓ [1].
Back at the start you were asked why plastic wrap clings to itself but doesn't dissolve in water — even though both are made of simple atoms. Now you know the answer.
Plastic wrap (LDPE or PVC) and a rigid container (HDPE) are both hydrocarbon-based polymers, but they differ in chain structure. LDPE has branched chains, which reduces how tightly the chains pack together, lowering crystallinity and making the material flexible. HDPE has long, linear chains that pack densely, giving high crystallinity, stronger dispersion forces between chains, and a rigid, hard material. The more branches present, the lower the melting point, the lower the density, and the more flexible the polymer.
Neither type dissolves in water because the long, nonpolar C–C chains cannot participate in hydrogen bonding with water molecules. The structural difference — linear vs branched chains — determines packing efficiency, crystallinity, and mechanical properties. The same intermolecular force reasoning that explains boiling point trends in small hydrocarbons explains rigidity in polymers.
What is a hydrocarbon? Give two examples.
What is the key difference between an addition and a substitution reaction? How many products does each produce?
What is isomerism? Give an example of two isomers sharing the molecular formula C₄H₈.
Why do addition polymers like polyethylene resist water and most chemical reagents?
State the products of complete combustion vs incomplete combustion of a hydrocarbon.
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