Alkenes

• Alkenes formula:

  CnH2n
• Alkenes are hydrocarbons with the functional group C=C.
• More reactive than Alkanes.
• Alkenes are unsaturated hydrocarbons, due to their double bond C.
• 4 smallest Alkenes:

  Ethene (C2H4)

  Propene (C3H6)

  Butene (C4H8)

  Pentene (C5H10)
• Addition reactions are common in Alkenes. This is when the C=C opens up to allow C to bond elsewhere.
• When a phosphoric acid catalyst is present, alcohols can be formed when reacting alkenes with steam.

  • Ethene (C2H4) + Steam (H20) -> Ethanol (C2H5OH)
  • Butene (C4H8) + Steam (H20) -> Butanol (C4H9OH)
• Alkenes are involved in combustion reactions with Oxygen. The outcome is similar to when other hydrocarbons react with Oxygen.

  • Ethene (C2H4) + Oxygen (O2) -> Carbon monoxide (CO) + Carbon (C) + Water (H2O)

  Because their combustion is incomplete, they burn with smoky flames.
• When a nickel catalyst is present, Alkanes can be formed by combining Hydrogen with Alkenes.

  • Ethene (C2H4) + Hydrogen (H2) -> Ethane (C2H6)
  • Propene (C3H6) + Hydrogen (H2) -> Propane (C3H8)

  This is called Hydrogenation.
• Alkenes will react with Bromine water when shaken. This causes the solution to go from orange-brown to colourless.

  • Ethene (C2H4) (colourless) + Bromine (Br2) (orange-brown solution) -> Dibromoethane (CH2BrCH2Br) (colourless)

  This is an effective test when distinguishing between Alkenes and Alkanes.
• Homologous series

  A group of compounds with the same functional group. Eg. Alkenes