In the nucleophilic substitution reactions `( S_(N) 2` or `S_(N)1` ) , the reactivity of alkyl halids follows the sequence

Options

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

Answer

To determine the reactivity of alkyl halides in nucleophilic substitution reactions (SN2 or SN1), we need to analyze the bond strengths and the nature of the halogens involved. The order of reactivity is influenced by the ability of the leaving group to depart from the alkyl group.

### Step-by-Step Solution:

1. **Understanding Nucleophilic Substitution**:
- Nucleophilic substitution reactions involve a nucleophile attacking an electrophilic carbon atom bonded to a leaving group (halide). The leaving group is replaced by the nucleophile.

2. **Identifying the Halogens**:
- The halogens in question are: Iodine (I), Bromine (Br), Chlorine (Cl), and Fluorine (F). Their reactivity as leaving groups varies due to their bond strengths with carbon.

3. **Bond Strengths**:
- The bond dissociation energies (BDE) for the C-X bonds (where X is the halogen) decrease in the order:
- C-F > C-Cl > C-Br > C-I
- This means that the C-I bond is the weakest, making it the easiest to break, while the C-F bond is the strongest and the hardest to break.

4. **Reactivity Order**:
- Since the reactivity of alkyl halides in nucleophilic substitution reactions is inversely related to the strength of the C-X bond, the order of reactivity will be:
- R-I > R-Br > R-Cl > R-F
- Therefore, alkyl iodides (R-I) are the most reactive, followed by alkyl bromides (R-Br), then alkyl chlorides (R-Cl), and finally alkyl fluorides (R-F).

5. **Conclusion**:
- The correct order of reactivity for alkyl halides in nucleophilic substitution reactions is:
- R-I > R-Br > R-Cl > R-F

### Final Answer:
The correct option is: **R-I > R-Br > R-Cl > R-F**

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