Alkenes

• Alkenes are unsaturated hydrocarbons; alkene molecules contain at least one C=C in the structure
• Aliphatic alkenes that contain one double bond have the general formula: CnH2n
• Alkenes can be branched, contain more than one double bond, or be cyclic
• Whilst branched alkenes obey the general formula, cyclic alkenes with more than one double bond do not
• The nature of the double bond: each carbon atom have 4 electrons in its outer shell and can use these to form bonds. For each carbon atom of the double bond, three of the 4 electrons are used in three sigma bonds, one to the other carbon of the double bond and 2 electrons to 2 other atoms (C/H)
• This leaves one electron on each carbon atom of the double bond not involved in sigma bonds
• This electron is in a p-orbital
• A Pi bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond. Each carbon atom contributes one electron to the electron pair in the Pi bond
• The Pi electron density is concentrated above and below the the line joining the nuclei of the bonding atoms
• The Pi bonds locks the two carbon atoms in position and prevents them from rotating around the double bond - makes the geometry of alkenes very different from that of alkanes, where rotation is possible around every atom
• The shape around a double bond: the shape around each of the carbon atoms in a double bond is trigonal plana

  1. there are three regions of electron density around each of the carbon atoms
  2. the three regions repel each other as far as possible, so the bond angle around each carbon atom is 120 degrees
  3. all of the atoms are in the same plane
• Alkenes

  Responsible for making flamingos pink and the smell of oranges and lemons
• Flamingos

  • Eat algae and crustaceans
  • Contain pigments called carotenoids
  • Enzymes in the liver convert them into pink and orange pigments deposited in the feathers
• Beta-carotene


  • A related chemical that makes carrots orange
  • Important as it is a building block that bodies use to manufacture vitamin A
• Limonene

  • A cyclic alkene, C10H16
  • Found naturally in the rinds of fruits
  • Largely responsible for smell and flavour of oranges and lemons
  • Also found in perfumes and household cleaning products
  • Fragrance
• Hydrogenation of alkenes:

  • when an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K
  • addition reaction takes place to form an alkane
  • this addition reaction in which hydrogen is added across the double bond is known as hydrogenation
• all c=c bonds react with hydrogen in this way