Alkanes
- saturated (an organic molecule containing the greatest possible number of hydrogen atoms, without carbon-carbon double or triple bonds)
- simplest alkane: methane (CH4), the main constituent of natural gas
- ethane (C2H6) gas, used to make plastics
- propane (C3H8), gas at RT, cooking, heating and vehicles
- butane (C4H10)
- Pentane (C5H12)
- Hexane (C6H14)
- FORMULA: CnH2n+2
- the larger the alkanes are, the more densely their molecules can pack together, stronger intermolecular forces, harder to break, therefore tend to be liquid or solid at RT
- always named based on the longest possible continuous chain in their structure
Alkane Reactions
- Combustion
- CH4 + 2O2 —> CO2 + 2H2O + heat energy
- Halogenation
- chlorine substituted for hydrogen
- CH4 + 3Cl2 —> CHCl3 + 3HCl
Alkenes (double carbon bonds)
- unsaturated (hydrocarbons not bonded to the maximum number of hydrogen atoms that carbons can accommodate)
Addition Reactions
Bromine water
- alkenes turn from orange to colorless when it is skin with bromine water
- alkanes no reaction
- The bromine is decolorized because a colorless dibromo compound forms
- ethene + bromine —> dibromoethane
- C2H4 + Br2 —> C2H4Br2
Reaction with steam/water
- To make alcohol: add steam
- C2H4 + H2O —> C2H5OH
Alkenes are heated with hydrogen in the presence of a nickel catalyst, alkanes are formed as the carbon double bonds break and a hydrogen atom joins to each of the carbon atoms.
Reaction with Hydrogen
- Hydrogen can be added to alkenes to ‘saturate’/make alkanes
- C2H4 + H2 —> C2H6
- phosphoric acid as catalyst
IGCSE REVISION 