3.4Organic Chemistry
Alkenes — AQA A Level Chemistry Revision Notes & Topical Questions
5 min read 10 parts Lesson 4 of 8
Alkenes: Bonding, Isomerism, Reactions and Addition Polymers — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Describe the sigma and pi bonding in alkenes, E/Z isomerism, and the main addition reactions of alkenes.Alkenes are unsaturated hydrocarbons with the general formula CnH2n…
- sigma (σ) bond
- pi (π) bond
- electrophilic addition
- Worked example 1
- Worked example 2
- Worked example 3
Part 1 of 4Structure and bonding
Alkenes are unsaturated hydrocarbons with the general formula CnH2n and a C=C double bond.
Part 2 of 4Isomerism in alkenes
Alkenes show chain and positional isomerism (but-1-ene, but-2-ene) and E/Z isomerism when each carbon of the double bond has two different groups attached. The Z isomer has the higher-priority groups on the same side, the E isomer on opposite sides. The isomers have different physical properties, such as boiling points (the Z isomer is usually polar and boils higher).
Part 3 of 4Reactions of alkenes
Alkenes undergo electrophilic addition:
| Reagent | Conditions | Product from ethene |
|---|---|---|
| H2 | Ni catalyst, about 150 °C | Ethane |
| Br2 (or Cl2) | Room temperature | 1,2-dibromoethane |
| HBr (or HCl) | Room temperature | Bromoethane |
| H2O (steam) | H3PO4 catalyst, 300 °C, 60 atm | Ethanol |
| Conc. H2SO4 then H2O | Cold, then warm | Ethanol |
With an unsymmetrical alkene such as propene and HBr, two products are possible: the major product comes via the more stable carbocation. Secondary carbocations are more stable than primary (more alkyl groups push electron density towards the positive carbon), so 2-bromopropane is the major product.
Part 4 of 4Worked examples
Explain why alkenes are more reactive than alkanes. [2]2 marks
Alkenes contain a C=C with a pi bond [1], a region of high electron density that attracts electrophiles; the pi bond is weaker than the sigma bond [1].
Name the major product of propene and HBr and explain. [3]3 marks
2-bromopropane [1]; a secondary carbocation forms [1] and is more stable than the primary because of the positive inductive effect of two alkyl groups [1].
Give the conditions to convert ethene to ethane. [2]2 marks
Hydrogen gas [1]; nickel catalyst at about 150 °C [1].
Key terms
| Term | Meaning |
|---|---|
| Alkene | A hydrocarbon containing a carbon-carbon double bond. |
| Sigma bond | A covalent bond formed by head-on overlap of orbitals. |
| Pi bond | A covalent bond formed by sideways overlap of p orbitals. |
| Electrophile | An electron-pair acceptor. |
| Unsaturated | Containing C=C or C≡C multiple bonds. |
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