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Alkenes
This topic develops unsaturated hydrocarbon chemistry through double-bond reactivity.
12
Objectives
65
Flashcards
60
Questions
90 min
Study time
AQAA LevelChemistryOrganic chemistry
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What you need to know
12 objective pages available
Structure, bonding and reactivity4 objectives
- Describe alkenes as unsaturated hydrocarbons.
- Explain bonding in alkenes using sigma and pi bonds.
- Explain why the C=C bond is attacked by electrophiles.
- Use bromine water to test for unsaturation.
Electrophilic addition4 objectives
- Outline electrophilic addition to alkenes with hydrogen bromide.
- Explain carbocation formation and stability where appropriate.
- Predict major products from addition to unsymmetrical alkenes.
- Outline hydration of alkenes to alcohols.
Addition polymers4 objectives
- Draw repeat units from alkene monomers.
- Identify monomers from addition polymer repeat units.
- Explain addition polymerisation as repeated addition across C=C bonds.
- Distinguish addition polymers from condensation polymers.
Key terms
alkeneunsaturated hydrocarbonpi bondC=C bondelectrophilebromine waterunsaturation testElectrophilic additionHydrogen bromidecarbocationelectrophilic additionhydration
Exam tips
- Understanding Alkenes: Explain clearly by remember that alkenes are unsaturated hydrocarbons due to the presence of a C=C double bond. Link the point to Structure, bonding and reactivity, then state the evidence, calculation, mechanism, observation, or conclusion required by the question.
- Understanding Alkene Bonding: Explain clearly by remember that alkenes contain both sigma (σ) and pi (π) bonds. The σ bond is formed by the head-on overlap of orbitals, while the π bond is formed by the side-on overlap of p orbitals. This distinction is crucial for understanding their reactivity. Link the point to Structure, bonding and reactivity, then state the evidence, calculation, mechanism, observation, or conclusion required by the question.
Common mistakes
- Misunderstanding Unsaturation: Remember that alkenes contain at least one double bond (C=C), which makes them unsaturated. The formula for alkenes is CnH2n, indicating they have fewer hydrogen atoms than saturated hydrocarbons.
- Misunderstanding Bonding in Alkenes: To clarify, remember that a sigma bond is formed by the head-on overlap of orbitals, while a pi bond is formed by the side-to-side overlap of p orbitals. For example, in an alkene like ethene (C2H4), the C=C bond consists of one sigma bond and one pi bond. The sigma bond is formed first, followed by the formation of the pi bond. This distinction is crucial for understanding reactivity.
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