Alkenes

• General formula = $C_nH_{2n}$
• Functional group: C=C bond
• Ethene (n=2)

  
• Propene (n=3)

  
• But-1-ene (n=4)

  
• Cracking: Thermal decomposition reaction, in which an alkene (and sometimes hydrogen) are produced from an alkane.
• Cracking always produces short chain compound with a C=C bond. e.g. Cracking of ethane will give ethene and hydrogen
• Butane → Ethane + Ethene: $C_4H_{10\ ->}\ C_2H_{6\ }+$$\ C_2H_4$
• Hydrocarbons:

  • Have NO double bonds
  • Do not react with aqueous bromine, so the mixture stays orange.
• Unsaturated Hydrocarbons:

  • Have double bonds
  • React with aqueous bromine, turning the mixture from orange to colourless