From which of the following sequences does the reactivity of alkyl halides in the nucleophilic substitution reaction follows :

Options

A. `R - I gt R - Br gt R - Cl gt R - F`
B. `R - Cl gt R - F gt R - Br gt R - I`
C. `R - F gt R - Cl gt R - Br gt R - I`
D. `R - I gt R - F gt R - Cl gt R - Br`

Answer

To determine the reactivity of alkyl halides in nucleophilic substitution reactions, we need to analyze the leaving group ability of the halides involved. The leaving group is the part of the molecule that departs during the reaction, and its ability to leave is crucial for the reaction to proceed efficiently.

### Step-by-Step Solution:

1. **Identify the Halides**: The halides we are considering are fluoride (F), chloride (Cl), bromide (Br), and iodide (I).

2. **Understand Leaving Group Ability**: The reactivity of alkyl halides in nucleophilic substitution reactions is influenced by the ability of the halide to act as a leaving group. A better leaving group will enhance the reactivity of the alkyl halide.

3. **Evaluate Basicity of Halides**: The strength of a leaving group is inversely related to its basicity. Weaker bases are better leaving groups. The order of basicity for the halides is:
- F⁻ (strong base)
- Cl⁻ (weaker base than F⁻)
- Br⁻ (weaker base than Cl⁻)
- I⁻ (weakest base)

4. **Determine the Order of Reactivity**: Since I⁻ is the weakest base, it is the best leaving group, followed by Br⁻, Cl⁻, and F⁻. Therefore, the order of reactivity of alkyl halides in nucleophilic substitution reactions is:
- Alkyl iodide (RI) > Alkyl bromide (RBr) > Alkyl chloride (RCl) > Alkyl fluoride (RF)

5. **Conclusion**: Based on the analysis, the correct sequence that describes the reactivity of alkyl halides in nucleophilic substitution reactions is:
- RI > RBr > RCl > RF

### Final Answer:
The reactivity of alkyl halides in nucleophilic substitution reactions follows the sequence: **RI > RBr > RCl > RF**.