Topic 5: Alkenes
5.1 - 5.3: Structure and Isomerism
Alkenes and cycloalkanes are unsaturated hydrocarbons containing at least one carbon-carbon double bond, consisting of a σ (sigma) bond and a π (pi) bond.
Alkenes exhibit geometric isomerism due to restricted rotation around the C=C double bond. Isomers are named using the E-Z system, which provides a more comprehensive naming method than the traditional cis-trans system.
5.4 - 5.6: Addition Reactions & Mechanisms
Alkenes are highly reactive due to the high electron density of the π bond, making them susceptible to attack by electrophiles.
Addition Reactions Table
| Reagent | Conditions | Product |
|---|---|---|
| Hydrogen () | Nickel Catalyst | Alkane |
| Halogens () | Room Temp | Di-substituted halogenoalkane |
| Hydrogen Halides (HX) | Room Temp | Mono-substituted halogenoalkane |
| Steam () | Acid Catalyst | Alcohol |
| Acidified | Cold, Dilute | Diol |
Electrophilic Addition Mechanism
You must use curly arrow notation to show the movement of electron pairs, starting from either a bond or a lone pair.
Regioselectivity: In the addition of HX to unsymmetrical alkenes (e.g., propene), the major product is determined by the relative stability of carbocation intermediates (Tertiary > Secondary > Primary).
5.7 - 5.8: Polymerisation & Sustainability
Alkenes undergo addition polymerisation where many monomer units join to form a long chain.
Chemists aim to limit environmental damage from polymer disposal by:
- Developing biodegradable polymers.
- Removing toxic waste gases (like HCl) produced during the incineration of certain polymers.
Exam Practice
Q1: State the qualitative test for a C=C double bond and the expected observation.
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Test: Add bromine water ().
Observation: The orange/brown solution decolourises.
Q2: Predict the major product of the reaction between propene and hydrogen bromide. Explain your choice.
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Product: 2-bromopropane.
Explanation: The reaction proceeds via a secondary carbocation intermediate, which is more stable than the primary carbocation that would lead to 1-bromopropane.