The alkenes are a homologous series of compounds. They have a double carbon bond, meaning they are unsaturated. The alkenes have a formula CnH2n They are made when large alkanes from the fractional distillation of crude oil are cracked. Alkenes are much more reactive than alkanes as they have a double bond which reacts easily with other substances.
The first fewEdit
- Ethene: CH2--CH2
- Propene: CH2--CH-CH3
- Butene: CH2--CH-CH2-CH3
Reactions of AlkenesEdit
The flame is usually yellow and sooty. There is either complete combustion, where the products are only carbon dioxide and water or incomplete combustion, where carbon monoxide and water are produced.
Reaction with hydrogenEdit
Example: C2H4 + H2 = C2H6
Ethene + Hydrogen = Ethane
Reaction with bromineEdit
This is an addition reaction also, forming dibromoalkanes. This is also the test to distinguish between alkanes and alkenes (alkanes do not react quickly, and the orange bromine stays orange, while alkenes turn it colourless).
Reaction with acidified potassium manganate(VIII)Edit
This is another addition reaction, where the purple KMnO4 loses its colour. A 'diol' is formed.
Reaction with water vapour Edit
This addition reaction needs a catalyst and extreme conditions: 300oC, 60atm; catalyst: phosphoric acid. An alcohol is produced.
Addition polymerisation Edit
See main article Polymerisation