Structure & Formula
- All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C
- All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot
- The names and structure of the first four alkenes are shown below:
Table of alkenes
| Displayed formula | Name | Molecular formula |
 |
ethene | C2H4 |
 |
propene | C3H6 |
 |
but-1-ene | C4H8 |
 |
pent-1-ene | C5H10 |
The first four members of the alkene homologous series
- Compounds that have a C=C double bond are also called unsaturated compounds
- That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group
- Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds
- This makes them much more reactive than alkanes
A carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms
Exam Tip
The numbers in butene, pentene and hexene refer to the carbon atom in which the C=C begins, counting from the left. E.g. pent-2-ene, C5H10 has the C=C between the 2nd and 3rd carbon atoms. In pent-3-ene the C=C bond is between the 3rd and 4th carbon atoms from the left.
