1. General information of alkenes
General formula: CnH2n
Unsaturated hydrocarbons
Alkenes have lower boiling points than their corresponding alkanes
2. Chemical reactions of alkenes
Alkenes are more reactive than alkanes due to double bond, and bonded to another two atoms (or atomic groups)
So, this typical type of chemical reaction of alkenes is called addition.
1) Addition of bromine: Bromination
CH2=CH2 + Br2 → CH2BrCH2Br 1,2-dibromoethane
Observation: orange to colourless (decolorise)
Application: distinguish between alkane and alkene experimentally
2) Addition of hydrogen: Hydrogenation
CH2=CH2 + H2 → CH3CH3 ethane
Conditions: Ni (catalyst), 150 – 300 oC
Application of hydrogenation: to produce margarine
vegetable oil (unsaturated fatty acid) + H2 → margarine (saturated fatty acid)
3) Addition of water: Hydration
CH2=CH2 + H2O → CH3CH2OH ethanol
Conditions: H3PO4 (catalyst), 300 oC, 6000 kPa
Application: to manufacture ethanol for industrial use
4) Polymerisation
3. Resource of alkenes
Catalytic cracking: a larger alkane molecule was broken down into smaller alkane and alkene using a higher temperature and a catalyst.
Such as:
C8H18 → C6H14 + C2H4
Conditions: Temperature: around 500 oC
Catalyst: powdered minerals such as silica, alumina and zeolite
Uses of alkanes: gasoline
Uses of alkenes: raw materials for industrial applications
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