Question

Explain homolysis and heterolysis in reaction mechanism with appropriate example

Answer 1

Homolysis and heterolysis are two different mechanisms by which a chemical bond can break in the context of reaction mechanisms. They differ primarily in the way that the electrons are distributed between the atoms involved in the bond cleavage.

### Homolysis

Homolysis refers to the cleavage of a bond in such a way that each of the bonded atoms retains one of the shared electrons, resulting in the formation of two free radicals. This type of bond breaking typically occurs in non-polar covalent bonds.

**Example of Homolysis:**
Consider the homolysis of a chlorine molecule (Cl₂):

\[ \text{Cl-Cl} \xrightarrow{\text{heat or light}} 2 \cdot \text{Cl}^\bullet \]

In this reaction, when Cl₂ undergoes homolysis due to the application of heat or light, the Cl-Cl bond breaks evenly, producing two chlorine free radicals (Cl·), each with an unpaired electron.

### Heterolysis

Heterolysis, on the other hand, is the process of bond cleavage where one atom retains both of the shared electrons, resulting in the formation of a positively charged ion (cation) and a negatively charged ion (anion). Heterolysis typically occurs in polar covalent bonds.

**Example of Heterolysis:**
Consider the heterolysis of hydrogen chloride (HCl):

\[ \text{H-Cl} \rightarrow \text{H}^+ + \text{Cl}^- \]

In this example, when the H-Cl bond undergoes heterolysis, the chlorine atom retains both electrons from the bond while the hydrogen atom loses its electron, resulting in the formation of a chloride ion (Cl⁻) and a hydrogen ion (H⁺).

### Summary of Key Differences

1. **Electron Distribution:**
- **Homolysis:** Each atom retains one electron, forming two radicals.
- **Heterolysis:** One atom retains both electrons, forming a cation and an anion.

2. **Bond Type:**
- **Homolysis:** Typically occurs in non-polar covalent bonds.
- **Heterolysis:** Typically occurs in polar covalent bonds.

3. **Products:**
- **Homolysis:** Generates radicals (e.g., Cl·).
- **Heterolysis:** Generates ions (e.g., H⁺ and Cl⁻).

These two mechanisms play significant roles in various chemical processes, including radical reactions, acid-base reactions, and the formation of ionic compounds.