Alkenes

    OCR
    GCSE

    Alkenes are a homologous series of unsaturated hydrocarbons characterized by the presence of a carbon-carbon double bond (C=C) functional group. This high electron density region renders alkenes significantly more reactive than alkanes, facilitating electrophilic addition reactions with halogens, hydrogen, and steam. They serve as critical feedstocks in the petrochemical industry, primarily produced via the cracking of long-chain alkanes, and are the fundamental monomers for addition polymerization. Mastery requires understanding the general formula CnH2n, structural isomerism, and the diagnostic bromine water test for unsaturation.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating the general formula CnH2n and identifying the C=C double bond as the functional group
    • Award 1 mark for describing the test for unsaturation: add bromine water, observing a colour change from orange to colourless
    • Credit responses that show the double bond becoming a single bond in addition polymerisation diagrams, with extension bonds passing through brackets
    • Award 1 mark for explaining that alkenes are unsaturated because they contain at least one carbon-carbon double bond

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the alkene, but your description of the bromine water test used 'clear'—switch this to 'colourless' for the mark"
    • "Good drawing of the polymer chain, but you left the double bond inside the brackets. Remember, the double bond opens up to form the chain"
    • "You've stated the general formula correctly. Now apply this to predict the formula for an alkene with 15 carbon atoms"
    • "Strong definition of unsaturation. To improve, explicitly link the reactivity of the alkene to the presence of the C=C bond"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating the general formula CnH2n and identifying the C=C double bond as the functional group
    • Award 1 mark for describing the test for unsaturation: add bromine water, observing a colour change from orange to colourless
    • Credit responses that show the double bond becoming a single bond in addition polymerisation diagrams, with extension bonds passing through brackets
    • Award 1 mark for explaining that alkenes are unsaturated because they contain at least one carbon-carbon double bond

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Memorise the specific phrase 'orange to colourless' for the bromine water test; 'discoloured' or 'clear' will not earn the mark
    • 💡When drawing repeat units, ensure the bonds extending through the brackets are drawn horizontally and clearly connect to the carbon atoms
    • 💡For combustion questions, note that alkenes burn with a smokier/sootier flame than alkanes due to a higher carbon-to-hydrogen ratio

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Describing the result of the bromine water test as turning 'clear' instead of 'colourless' (examiners strictly reject 'clear')
    • Retaining the double bond inside the repeat unit when drawing addition polymers
    • Confusing the general formula of alkenes (CnH2n) with alkanes (CnH2n+2) when calculating molecular formulae
    • Failing to place the 'n' subscript outside the brackets in polymer notation

    Key Terminology

    Essential terms to know

    Functional group chemistry (C=C double bond)
    Addition reactions (halogenation, hydrogenation, hydration)
    Addition polymerization and repeat units
    Structural isomerism and nomenclature
    Qualitative analysis (Test for unsaturation)

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Draw
    Predict
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG 7","title":"Tests for organic functional groups","relevance":"Distinguishing between alkanes and alkenes using bromine water"}

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