Out of monochloro, monobromo and monoiodo derivatives of ethane, the least reactive compound towards nucleophilic substitutions will be :

Options

A. `C_(2)H_(5)Br`
B. `C_(2)H_(5)Cl`
C. `C_(2)H_(5)I`
D. All are equally reactive

Explanation

To determine which of the monochloro, monobromo, and monoiodo derivatives of ethane is the least reactive towards nucleophilic substitution, we can analyze the reactivity based on the strength of the carbon-halogen bonds and the ability of the halogen to act as a leaving group.

### Step-by-Step Solution:

1. **Understanding Nucleophilic Substitution**:
- In a nucleophilic substitution reaction, a nucleophile replaces a leaving group attached to a carbon atom. The reactivity of the compound in this reaction depends on how easily the leaving group can depart.

2. **Identifying the Halogens**:
- The halogens in question are chlorine (Cl), bromine (Br), and iodine (I). We need to compare their ability to act as leaving groups.

3. **Bond Strength Analysis**:
- The strength of the carbon-halogen bond is crucial in determining the reactivity:
- Carbon-Chlorine (C-Cl) bond
- Carbon-Bromine (C-Br) bond
- Carbon-Iodine (C-I) bond
- Generally, the bond strength decreases in the order: C-Cl > C-Br > C-I. This means that the C-Cl bond is the strongest, while the C-I bond is the weakest.

4. **Leaving Group Ability**:
- A better leaving group is one that can stabilize the negative charge after it leaves. The weaker the bond, the better the leaving group:
- Iodine (I) is the best leaving group because it forms I⁻, which is stable due to its larger size and ability to disperse charge.
- Bromine (Br) is a moderate leaving group.
- Chlorine (Cl) is the poorest leaving group among the three because the C-Cl bond is the strongest.

5. **Conclusion**:
- Since the reactivity of the compound in nucleophilic substitution is inversely related to the strength of the carbon-halogen bond, chloroethane (the monochloro derivative) will be the least reactive towards nucleophilic substitution due to the strong C-Cl bond.

### Final Answer:
The least reactive compound towards nucleophilic substitutions among monochloro, monobromo, and monoiodo derivatives of ethane is **chloroethane**.

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