The correct order of reactivity towards nucleophilic substitution reaction is

Options

A. `CH_3F gt CH_3Cl gt CH_3Br gt CH_3I`
B. `CH_3I gt CH_3Br gt CH_3Cl gt CH_3F`
C. `CH_3I gt CH_3Cl gt CH_3Br gt CH_3F`
D. `CH_3I gt CH_3Br gt CH_3F gt CH_3Cl`

Explanation

To determine the correct order of reactivity towards nucleophilic substitution reactions for haloalkanes, we need to consider the nature of the leaving groups involved in the reactions. The leaving group is crucial because it influences how easily the bond can break, allowing the nucleophile to attack.

### Step-by-Step Solution:

1. **Identify the Haloalkanes**: We are comparing four haloalkanes: methyl iodide (CH3I), methyl bromide (CH3Br), methyl chloride (CH3Cl), and methyl fluoride (CH3F).

2. **Understand the Leaving Group Ability**: The ability of a leaving group to depart from the molecule is a key factor in nucleophilic substitution reactions. The order of leaving group ability from best to worst is generally:
- Iodine (I) > Bromine (Br) > Chlorine (Cl) > Fluorine (F)

3. **Analyze the Bond Strength**: The bond strength between carbon and the halogen affects the reactivity. As we move from fluorine to iodine, the bond length increases, and the bond strength decreases:
- C-F bond (strongest, shortest)
- C-Cl bond
- C-Br bond
- C-I bond (weakest, longest)

4. **Determine Reactivity Order**: Since the weaker the bond, the easier it is for the leaving group to leave, we can establish the reactivity order:
- CH3I (most reactive) > CH3Br > CH3Cl > CH3F (least reactive)

5. **Conclusion**: Therefore, the correct order of reactivity towards nucleophilic substitution reactions for the given haloalkanes is:
- **CH3I > CH3Br > CH3Cl > CH3F**

### Final Answer:
The correct order of reactivity towards nucleophilic substitution reaction is:
**CH3I > CH3Br > CH3Cl > CH3F**

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