Reactivity 3.3.2 — Radical Formation by Homolytic Fission 

Homolytic fission is the breaking of a covalent bond in such a way that each atom involved retains one of the two electrons from the shared pair. This results in the formation of two radicals — highly reactive species with unpaired electrons.

Key Features of Homolytic Fission:

• Each atom takes one electron from the bond — represented using a single-barbed arrow (also known as a fish hook: ↷).
• Requires energy — typically supplied by UV light or heat.
• Commonly observed in reactions involving halogens such as \( \text{Cl}_2 \), \( \text{Br}_2 \), etc.

The Initiation Step in Chain Reactions

In chain reactions such as the halogenation of alkanes, the initiation step involves the homolytic fission of a halogen molecule. This generates the first pair of radicals and sets the reaction in motion.

Example: Chlorine Molecule under UV Light

Under ultraviolet light, a chlorine molecule absorbs energy, and the Cl–Cl bond undergoes homolytic fission:

\( \text{Cl}_2 \xrightarrow{\text{UV}} \cdot\text{Cl} + \cdot\text{Cl} \)

Each chlorine atom takes one of the bonding electrons, producing two chlorine radicals. These radicals are extremely reactive and are responsible for initiating further chemical reactions (e.g., substitution reactions with alkanes).

Fish Hook Notation: Single-Electron Movement

In diagrams, the movement of a single electron during homolytic fission is shown using a single-barbed arrow (↷). This distinguishes it from normal curly arrows (used for electron-pair movement in ionic reactions).

Diagrammatic Representation:

Energy Considerations

The Cl–Cl bond requires an input of energy (~243 kJ/mol) to break. UV light provides sufficient energy to cleave this bond, which is why halogenation of alkanes typically occurs in the presence of UV light or at high temperatures.