The order of reactivity of alkyl halide in the reaction`R-X+Mg to RMgX` is :

Options

A. `RI gt RBr gt RCl`
B. `RCl gt RBr gt RI`
C. `RBr gt RCl gt RI`
D. `RBr gt RI gt RCl`

Explanation

To determine the order of reactivity of alkyl halides in the reaction \( R-X + Mg \rightarrow RMgX \), we need to analyze the strength of the carbon-halogen bonds in different alkyl halides. The halides we are considering are iodine (I), bromine (Br), and chlorine (Cl).

### Step-by-Step Solution:

1. **Identify the Alkyl Halides**: We are looking at three types of alkyl halides: \( R-I \) (alkyl iodide), \( R-Br \) (alkyl bromide), and \( R-Cl \) (alkyl chloride).

2. **Understand Bond Strength**: The reactivity of these alkyl halides in the formation of Grignard reagents (\( RMgX \)) is influenced by the strength of the carbon-halogen bond. We can summarize the bond strengths as follows:
- \( R-I \) bond is the weakest.
- \( R-Br \) bond is stronger than \( R-I \) but weaker than \( R-Cl \).
- \( R-Cl \) bond is the strongest.

3. **Relate Bond Strength to Reactivity**: The weaker the carbon-halogen bond, the more reactive the alkyl halide will be in forming the Grignard reagent. This is because a weaker bond is easier to break, allowing the magnesium to attach to the alkyl group.

4. **Rank the Alkyl Halides**: Based on the bond strengths:
- \( R-I \) (weakest bond) will be the most reactive.
- \( R-Br \) will be less reactive than \( R-I \) but more reactive than \( R-Cl \).
- \( R-Cl \) (strongest bond) will be the least reactive.

5. **Final Order of Reactivity**: Therefore, the order of reactivity of alkyl halides in the reaction with magnesium is:
\[ R-I > R-Br > R-Cl \]

### Conclusion:
The correct answer for the order of reactivity of alkyl halides in the reaction \( R-X + Mg \rightarrow RMgX \) is \( R-I > R-Br > R-Cl \).