11.5 Introduction to reaction mechanisms

• Covalent bonds can be broken by homolytic fission or heterolytic fission
• In homolytic fission, a covalent bonds is broken with each atom receiving one electron from the bonded pair, forming two radicals
• A radical is a species with an unpaired electron shown by a dot
• In heterolytic fission, a covalent bond is broken with one of the bonded atoms receives both electrons from the bonded pair, forming two oppositely charged ions
• Reaction mechanisms show how a reaction takes place

  • Curly arrows are used to show the movement of a pair of electrons
  • The direction of the arrow shows the movement of the electron pair
  • A curly arrow must start from a bond, a lone pair or a negative charge
• In an addition reaction, two reactant molecules join together to form a single product
• In a substitution reaction, an atom or group of atoms exchanges with a different atom or group of atoms
• In an elimination reaction, a molecule loses a small molecule such as water