The properties of the alkenes

• Alkenes are unsaturated hydrocarbons. They contain at least one carbon to carbon double bond in their structure
• Aliphatic alkenes that contain one double bond hae the general formula $C_nH_2n$
• For each carbon atom of the double bond, three of the four outer electrons are used in three sigma bonds: one to the other carbon atom involved in the double bond, and the other two electrons to two other atoms (carbon or hydrogen).
• For each carbon atom or the double bond, one of the four outer carbon atoms is not involved in a sigma bond. 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 Pi bond. The Pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms
• The Pi bond in a double bond locks the two carbon atoms in position and prevents them from rotating around the double bond. This makes the geometry of the alkenes different from that of the alkanes.
• The shape around each of the carbon atoms in the double bond is trigonal planar because:

  • there are three regions of electron density around each of the carbon atoms
  • the three regions repel each other a far apart as possible so the bond angle around each each carbon atom is 120
  • all of the atoms are in the same plane