Alkanes and alkenes

There are two different sub-sets of hydrocarbons: alkanes and alkenes.

Alkanes

The alkanes are a subset of hydrocarbons whose names all end in –ane.

A table showing the molecular formulae and full structural formulae of alkanes: methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, hexane C6H14, heptane C7H16 and octane C8H18

The names, molecular formulae and the structural formulae of the first eight alkanes must be learned. Using a mnemonic can make learning the names easier.

NameMolecular formulaFull structural formula
MethaneCH4Monsters
EthaneC2H6Eat
PropaneC3H8Pupils
ButaneC4H10But
PentaneC5H12Prefer
HexaneC6H14Hairy
HeptaneC7H16Haggis
OctaneC8H18Occasionally

Alkanes contain more than double the number of hydrogen atoms than carbon atoms. To calculate how many hydrogen atoms an alkane has, double the number of carbons and add two. The general formula of the alkanes is \({C_n}{H_{2n + 2}}\).

Alkenes

The second subset of hydrocarbons is called the alkenes. Their names all end in –ene for example ethene.

Alkenes all contain a carbon to carbon double bond which makes them more reactive than the alkanes.

A table showing the molecular formulae and full structural formulae of alkenes: ethene C2H4, propene C3H6, butene C4H8, pentene C5H10, hexene C6H12, heptene C7H14, octene C8H16.

Alkenes contain twice as many hydrogen atoms as carbon atoms. The alkenes have the general formula \({C_n}{H_{2n}}\).

Alkenes are more reactive than alkanes because they have a double bond.

Step one

1 of 3: Two stoppered test tubes contain bromine water, which is an orange-brown colour. An alkane is added to one, and an alkene added to the other.Two test tubes of bromine water. An alkane is added to one, an alkene to the other.

Step two

2 of 3: The test tubes are shaken.2 of 3: The test tubes are shaken.

Step three

3 of 3: The alkane liquid remains orange-brown. The alkene liquid has turned colourless.3 of 3: The alkane liquid remains orange-brown. The alkene liquid has turned colourless.