Introduction to Alkenes (OCR A Level Chemistry)

• Each carbon atom has four electrons in its outer shell (electronic configuration: 1s²2s²2p²)
• Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a full outer shell configuration
• These electrons are found in orbitals within the respective atoms
• When forming a covalent bond, the orbitals overlap in such a way to form two types of bonds
  • Sigma bonds (σ)
  • Pi bonds (π)

• When carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains one spare electron
• When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)
• The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap

Each carbon atom in ethene forms two sigma bonds with hydrogen atoms and one σ bond with another carbon atom. The fourth electron is used to form a π bond between the two carbon atoms

• The carbon of an alkene double bond has three σ bonding pairs of electrons
  • Two pairs forming σ bonds with other atoms
  • One pair forming σ bonds with the other carbon atom of the double bond
• The three bonding pair of electrons are in the plane of the molecule and repel each other
• This means that the molecule adopts a planar arrangement with bond angles of 120^o 
• The fourth π bonding pair forms the double bond in combination with the carbon-carbon σ bond 

Ethene

• • Each carbon atom uses three of its four electrons to form σ bonds
  • Two σ bonds are formed with the hydrogen atoms
  • One σ bond is formed with the other carbon atom
  • The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
  • This means that the C-C is a double bond: one σ and one π bond

The overlap of the two p orbitals results in the formation of a π bond in ethene in which the bonding pair of electrons repel each other to force the molecule into a planar configuration with bond angles of 120 ^o