What are Alkenes? (Edexcel IGCSE Science (Double Award) (Modular)): Revision Note

Alkenes

What are alkenes?

• Alkenes are unsaturated hydrocarbons

• The general formula of an alkene is C_nH_2n

• 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