Alkenes (Edexcel GCSE Chemistry)

Alkenes

• 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

• The position of the double bond in butene can be in one of two positions

• This is shown by using the number of the carbon atom along the chain where the double bond starts

Table Showing the Formulae and Structures of the Alkenes

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6
	but-1-ene	C4H8
	pent-1-ene	C5H10

• 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

Bromine & Alkenes

• Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond

• The reaction between bromine and ethene is an example of an addition reaction

• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Distinguishing between an alkane and an alkene

• Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)

• Br₂(aq) is an orange-yellow solution, called bromine water

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The bromine water test for alkenes