Alkenes (Oxford AQA IGCSE Combined Science Double Award)

What are Alkenes?

• All alkenes contain a carbon- carbon double bond, which is shown as two lines between two of the carbon atoms i.e. C=C

  • They are therefore called unsaturated hydrocarbons 

• Alkenes have the general formula C_nH_2n 

Alkenes Table

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6

• The presence of the carbon-carbon double bond, which is the functional group  is what allows alkenes to react in ways that alkanes cannot

• An example of this is their ability to react with bromine water, turning it from orange to colourless

  • This occurs because the bromine adds across the double bond in the alkene to form a saturated compound 

  • The bromine water is no longer in solution so the orange colour is lost 

  • Alkanes are unable to react with bromine water because they do not contain a double bond, only single bonds

Using bromine water to test for saturation

In alkenes, bromine water turns colourless, and in alkanes it remains orange