Homolytic Fission (HL IB Chemistry)

• In a reaction mechanism, curly arrows show the movement of electrons
• A single-headed curly arrow shows the movement of a single electron
  • These arrows can be called single barbed arrows, fish-hook arrows or half-curly arrows

What is homolytic fission?

• Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals
  • Remember: A radical is a chemical species that contains an unpaired electron
• The homolytic fission of halogens is the initiation step (first step) in a sequence of steps that form a chain reaction

Homolytic fission of a chlorine-chlorine bond

The covalent bond breaks evenly and each chlorine atom receives one electron resulting in the formation of two chlorine radicals, Cl•

• The mechanism of homolytic fission can also be represented using Lewis formulas:

Mechanism of homolytic fission using Lewis formulas

The mechanism of homolytic fission using Lewis formulas shows the specific movement of electrons

• When drawing mechanisms, ensure:
  • That the curly arrows start at an electron-rich region
    • In this case, the curly arrows should start from the middle of the covalent bond
  • That the curly arrows finish at their correct destination
    • In this case, each curly arrow should finish at a chlorine atom

Types of homolytic fission

• Since bond breaking is an endothermic process, energy is required for homolytic fission to occur
• The amount of energy required depends on the strength of the covalent bond being broken
  • Thermolytic fission: For weaker bonds, simply heating the compound could provide sufficient energy

XX X• + X• 

• • Photolytic fission: For stronger bonds such as halogen bonds, exposing the compound to high-energy UV light provides the required energy

XX X• + X•