Alkenes (Cambridge (CIE) IGCSE Chemistry)

Catalytic cracking

• Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds (C=C)

• They have covalent bonds

• Their general formula is C_nH_2n

• The presence of the double bond, C=C, 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

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

Manufacture of alkenes

• Although there is use for each fraction obtained from the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed

• These long chain hydrocarbon molecules are further processed to produce other products

• A process called catalytic cracking is used to convert longer-chain molecules into short-chain and more useful hydrocarbons

• Shorter chain alkanes, alkenes and hydrogen are produced from the cracking of longer chain alkanes

• Alkenes can be used to make polymers and the hydrogen used to make ammonia

• Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

• Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them

• The vapours then pass over a hot powdered catalyst of alumina or silica

• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

• The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes

• Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

The cracking of decane 

Decane is catalytically cracked to produce octane for petrol and ethene for ethanol

Distinguishing from alkanes

• The presence of the C=C double bond in an alkene but not an alkane allows us to distinguish between them

• To do this, bromine water is used

• Bromine water is an orange coloured solution

• When bromine water is shaken with an alkane the solution remains orange

• When bromine water is shaken with an alkene, the solution will go colourless, as the bromine can add across the double bond meaning it is no longer in solution

How to distinguish between alkanes and alkenes 

Alkenes will decolourise bromine water