Long alkanes are less useful than short ones, so they are broken down by cracking: heating the vapour over a hot catalyst (catalytic cracking) or mixing with steam at very high temperature (steam cracking). Cracking makes shorter alkanes for fuels and alkenes. Alkenes, such as ethene C₂H₄, have a carbon–carbon double bond, general formula CₙH₂ₙ. They are unsaturated: bromine water turns from orange to colourless when shaken with an alkene, but stays orange with an alkane.
Alkenes undergo addition reactions: hydrogen adds to make alkanes, and steam adds (with a catalyst) to make alcohols. Alcohols contain the –OH group: methanol, ethanol, propanol. Ethanol is made by fermentation of sugar with yeast or from ethene and steam. Alcohols burn as fuels and can be oxidised to carboxylic acids, such as ethanoic acid in vinegar.
Worked example
How could you tell hexane from hexene?
- Add bromine water to each and shake.
- Hexene is an alkene, so it decolourises bromine water.
Answer:Hexane is an alkane, so the bromine water stays orange.
Key words
- cracking
- breaking long hydrocarbons into shorter ones
- alkene
- hydrocarbon with a C=C double bond, CₙH₂ₙ
- unsaturated
- contains a C=C double bond
- functional group
- the group that gives a family its reactions, e.g. –OH
New questions every time.
NGSS-referenced: HS-PS1-2