Which of the following statements is correct ?

Options

A. Replacement of halogen by `NH_(2)` (using `NH_(3)`) in alkyl halide is a nucleophilic substitution reaction
B. Aryl halides show more reactivity as compared to alkyl halides in the replacement of halogen by the `NH_(2)` group
C. During the replacement of halogen by `-NH_(2)` group (using `NH_(3)`), ammonia is taken in large excess so as to avoid the formation of `2^(@)~ and 3^(@)~` amines.
D. None of these

Answer

To determine which of the given statements is correct, we will analyze each statement regarding nucleophilic substitution reactions involving alkyl halides and aryl halides.

### Step-by-Step Solution:

1. **Understanding Nucleophilic Substitution**:
- The first statement claims that the replacement of halogen by NH2 in alkyl halides is a nucleophilic substitution reaction.
- In this reaction, NH3 (ammonia) acts as a nucleophile due to the presence of a lone pair of electrons on nitrogen. It attacks the carbon atom bonded to the halogen (e.g., chlorine in CH3Cl), leading to the formation of an amine (RNH2) and the release of the halogen.

**Conclusion**: This statement is correct.

2. **Reactivity of Aryl Halides vs. Alkyl Halides**:
- The second statement suggests that aryl halides show more reactivity compared to alkyl halides in the context of nucleophilic substitution reactions.
- However, aryl halides (like chlorobenzene) have a carbon-chlorine bond with partial double bond character due to resonance. This makes the bond stronger and less reactive towards nucleophiles compared to alkyl halides (like CH3Cl), which do not have this resonance stabilization.

**Conclusion**: This statement is incorrect.

3. **Formation of Amines**:
- The third statement discusses the use of excess ammonia to avoid the formation of secondary and tertiary amines during the substitution reaction.
- When ammonia is used in excess, it helps to ensure that the reaction primarily produces primary amines (RNH2) rather than allowing the formation of secondary (R2NH) or tertiary amines (R3N). This is because the primary amine formed can further react with alkyl halides if ammonia is not in excess.

**Conclusion**: This statement is correct.

4. **Final Evaluation**:
- The first statement is correct.
- The second statement is incorrect.
- The third statement is correct.

### Final Answer:
The correct statements are:
- Statement 1: Replacement of halogen by NH2 in alkyl halides is a nucleophilic substitution reaction (Correct).
- Statement 3: During the replacement of halogen, ammonia is taken in large quantities to avoid the formation of secondary and tertiary amines (Correct).