Homolytic Fission and Radical Formation
What is Homolytic Fission?
Definition
Homolytic fission
Homolytic fission is the breaking of a covalent bond in such a way that each atom involved in the bond takes one electron, creating two radicals.
- A covalent bond is formed by a shared pair of electrons between two atoms.
- When this bond undergoes homolytic fission, both atoms retain one of the shared electrons, forming species with unpaired electrons—radicals.
- Consider the diatomic chlorine molecule, $Cl_2$.
- When exposed to UV light, the bond between the two chlorine atoms undergoes homolytic fission:
- $$
\text{Cl}_2 \xrightarrow{\text{UV light}} \cdot\text{Cl} + \cdot\text{Cl}
$$ - The dot (•) represents the unpaired electron on each chlorine atom.
- These chlorine radicals are highly reactive because they seek to pair their unpaired electrons.
Homolytic fission requires an input of energy, such as UV light or heat, to overcome the bond energy of the covalent bond.
Steps in Radical Formation: The Role of UV Light
Radical formation via homolytic fission occurs in three stages: initiation, propagation, and termination.
Initiation Step: Breaking the Bond
- In the initiation step, UV light provides the energy needed to break a covalent bond evenly.
- This step is often observed in halogens like chlorine or bromine, where the bond energy is relatively low.
- The movement of individual electrons during this process is represented using fish hook arrows(single-barbed arrows).
- A chlorine molecule $Cl_2$ absorbs UV light.
- The bond between the two chlorine atoms breaks homolytically.
- Each chlorine atom retains one electron, forming two chlorine radicals.
$$
\text{Cl}_2 \xrightarrow{\text{UV light}} \cdot\text{Cl} + \cdot\text{Cl}
$$
- When drawing fish hook arrows, start the arrow at the bond being broken and end it at the atom that receives the electron.
- This ensures clarity in illustrating electron movement.
Real-World Example: CFC Breakdown and Ozone Depletion
- Chlorofluorocarbons (CFCs) were once widely used in refrigeration and aerosol sprays but are now infamous for their role in depleting the ozone layer.
