(i) Alkyl iodides develop colouration on long standing particularly in light.
Explain.

Answer

**Step-by-Step Solution:**

1. **Understanding Alkyl Iodides**: Alkyl iodides are organic compounds that contain iodine atoms bonded to alkyl groups. The general formula can be represented as R-I, where R is the alkyl group and I is the iodine atom.

2. **Effect of Light**: When alkyl iodides are exposed to light for an extended period, a photochemical reaction occurs. Light provides the energy necessary to break the bond between the carbon atom (C) and the iodine atom (I) in the alkyl iodide.

3. **Bond Weakness**: The C-I bond in alkyl iodides is relatively weak due to the large size difference between the carbon atom and the iodine atom. This size difference contributes to the bond's instability, making it more susceptible to breaking when energy is applied.

4. **Formation of Radicals**: Upon exposure to light, the C-I bond breaks homolytically, resulting in the formation of free radicals. This can be represented as:
\[ R-I \xrightarrow{h\nu} R\cdot + I\cdot \]
Here, \(R\cdot\) is the alkyl radical, and \(I\cdot\) is the iodine radical.

5. **Iodine Molecule Formation**: The iodine radicals (\(I\cdot\)) can then combine with each other to form molecular iodine (I2):
\[ 2 I\cdot \rightarrow I_2 \]
The formation of I2 is significant because it is responsible for the coloration observed in the solution.

6. **Coloration**: The molecular iodine (I2) has a characteristic violet or brown color, which is why the alkyl iodide solution develops coloration upon prolonged exposure to light.

7. **Conclusion**: In summary, the coloration of alkyl iodides upon long-standing exposure to light is due to the photochemical cleavage of the C-I bond, leading to the formation of iodine molecules (I2), which exhibit a distinct color.

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