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.
Example
- 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.
Note
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).
Example
- 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.
$$
\text{Cl}_2 \xrightarrow{\text{UV light}} \cdot\text{Cl} + \cdot\text{Cl}
$$
Tip
- 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.
- The process begins when CFCs are exposed to UV light, leading to the formation of radicals.
Example
Trichlorofluoromethane $CCl_3F$
- When UV light interacts with trichlorofluoromethane $CCl_3F$, a chlorine-carbon bond undergoes homolytic fission, producing a chlorine radical: $$
\text{CCl}_3\text{F} \xrightarrow{\text{UV light}} \cdot\text{CCl}_2\text{F} + \cdot\text{Cl}
$$ - The chlorine radical $\cdot\text{Cl} $ is highly reactive and can attack ozone $O_3$ molecules in the atmosphere, initiating a chain reaction that depletes the ozone layer.
Note
- Radical reactions, such as those involving CFCs, are chain reactions.
- Once initiated, they can continue as long as radicals are present.
Why Are Radicals So Reactive?
- Radicals are reactive because they contain an unpaired electron.
- Atoms and molecules generally prefer stability, which occurs when all electrons are paired.
- Radicals seek to pair their unpaired electron, often reacting with other species and forming new radicals in the process.
A Closer Look at Fish Hook Arrows
- In radical reactions, fish hook arrows are used to represent the movement of single electrons.
- This is different from the double-barbed arrows used to show the movement of electron pairs.
Applications of Radical Chemistry
Radicals are involved in many natural and industrial processes. Here are a few examples:
- Atmospheric Chemistry: Radicals play a central role in the breakdown of pollutants, such as the destruction of ozone by CFCs.
- Polymerization: In industry, radicals initiate chain reactions to produce polymers like polyethylene.
- Biological Processes: Radicals are formed in metabolic reactions and are implicated in ageing and diseases like cancer.
Self review
Can you describe how radicals are formed through homolytic fission and why they are so reactive?
