R3.3.2 Homolytic fission

Homolytic Fission and Radical Formation

What is Homolytic Fission?

1. A covalent bond is formed by a shared pair of electrons between two atoms.
2. When this bond undergoes homolytic fission, both atoms retain one of the shared electrons, forming species with unpaired electrons—radicals.

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

1. In the initiation step, UV light provides the energy needed to break a covalent bond evenly.
2. This step is often observed in halogens like chlorine or bromine, where the bond energy is relatively low.
3. The movement of individual electrons during this process is represented using fish hook arrows(single-barbed arrows).

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 $CC{l}_{3}F$

When UV light interacts with trichlorofluoromethane $CC{l}_{3}F$, a chlorine-carbon bond undergoes homolytic fission, producing a chlorine radical:

$${\text{CCl}}_3\text{F}\stackrel{\text{UV light}}{\to }\cdot {\text{CCl}}_2\text{F}+\cdot \text{Cl}$$

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.

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:

1. Atmospheric Chemistry: Radicals play a central role in the breakdown of pollutants, such as the destruction of ozone by CFCs.
2. Polymerization: In industry, radicals initiate chain reactions to produce polymers like polyethylene.
3. Biological Processes: Radicals are formed in metabolic reactions and are implicated in ageing and diseases like cancer.

Key Takeaways:

• Homolytic Fission: The even breaking of a covalent bond, producing two radicals.
• Radicals: Reactive species with an unpaired electron, formed during homolytic fission.
• Initiation Step: UV light often provides the energy required to form radicals.
• Applications: Radicals are involved in processes like ozone depletion, polymerization, and biological reactions.