Section 1 of 5Structure of Alkenes
Unsaturated
- Alkenes are unsaturated hydrocarbons because they contain at least one carbon–carbon double bond.
General formula
- The general formula of alkenes is CₙH₂ₙ, so ethene, C₂H₄, and propene, C₃H₆, both fit this pattern.
Functional group
- The C=C double bond is the functional group of alkenes, and it is what makes them reactive.
Electron density
- The C=C bond contains an area of high electron density, which makes it more readily attacked than a C–C single bond.
Naming
- Alkene names replace the -ane ending with -ene, and a number shows where the double bond begins in longer chains.
Section 2 of 5The First Three Alkenes
1
The double bond can open up, so atoms can add to each carbon atom of the C=C group.
2
Each carbon atom of the double bond then forms a new single bond, so the molecule gains two new bonds.
3
Because of this extra reactivity, alkenes undergo addition reactions that alkanes cannot undergo.
4
Alkenes are made industrially by cracking long-chain hydrocarbons, so they are valuable starting materials.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₂=CHCH₃ |
| But-1-ene | C₄H₈ | CH₂=CHCH₂CH₃ |
Section 3 of 5Why Alkenes Are Reactive
Why Alkenes Are Reactive
Double bond
- The C=C double bond has a region of high electron density that attracts reagents, and one of its bonds opens up during addition.
Comparison
- Alkanes have only single bonds, so they are much less reactive than alkenes towards reagents such as bromine.
Combustion
- Alkenes burn in air to form carbon dioxide and water when oxygen is sufficient.
Polymers
- The reactivity of the C=C bond allows alkenes to join together to form addition polymers.
Section 4 of 5Worked Example
Question Find the molecular formula of an alkene with 6 carbon atoms.
- The alkene general formula is CₙH₂ₙ
- With n = 6, there are 2 × 6 = 12 hydrogen atoms
- The molecular formula is C₆H₁₂
Answer C₆H₁₂
Section 5 of 5Exam Checklist & Tips
4.4.1 – Key Points for the Exam
- Alkene An unsaturated hydrocarbon with the general formula CₙH₂ₙ and at least one C=C double bond.
- Unsaturated Containing at least one carbon–carbon double bond.
- Functional group The atom or group of atoms that gives a homologous series its characteristic chemistry.
- Addition reaction A reaction in which two molecules combine to form one larger molecule.
- Addition polymer A polymer formed by joining many alkene monomers through their C=C bonds.
Common Mistakes
- Calling alkenes saturated; they have a double bond, so they are unsaturated.
- Using the general formula CₙH₂ₙ₊₂ for alkenes, which is the formula for alkanes.
- Naming but-2-ene with the double bond between carbons 1 and 2, which is but-1-ene.
- Stating that both parts of the C=C bond break in an addition reaction; only one of them opens up.