Which reagent will you use for the following reaction ?
`CH_3 CH_2 CH_2 CH_3 to CH_3 CH_2 CH_2 CH_2 Cl + CH_3CH_2 CHClCH_3`

Options

A. `Cl_2//UV` light
B. `NaCl + H_2SO_4`
C. `Cl_2` gas in dark
D. `Cl_2` gas in the presence of iron in dark.

Explanation

To determine the reagent that will facilitate the reaction of normal butane (CH₃CH₂CH₂CH₃) to produce 1-chlorobutane (CH₃CH₂CH₂CH₂Cl) and 2-chlorobutane (CH₃CH₂CHClCH₃), we can analyze the options provided.

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
The reaction involves the chlorination of butane, which is a free radical substitution reaction. This type of reaction typically requires a halogen (Cl₂) and a source of energy (like heat or light) to initiate the process.

2. **Evaluate the Reagents**:
- **Option A: Cl₂ in the presence of ultraviolet light**: This is a common method for halogenation. The UV light provides the energy needed to break the Cl-Cl bond, generating chlorine radicals that can react with butane.
- **Option B: NaCl + H₂SO₄**: This combination is not suitable for chlorination. It typically leads to the formation of alkyl halides through different mechanisms, but not through free radical substitution.
- **Option C: Cl₂ gas in dark**: Chlorination does not occur in the dark because the absence of light means there are no radicals formed to initiate the reaction.
- **Option D: Cl₂ gas in the presence of iron in dark**: While iron can catalyze some reactions, chlorination specifically requires light to generate the necessary radicals.

3. **Conclusion**:
The most appropriate reagent for the reaction of normal butane to yield 1-chlorobutane and 2-chlorobutane is **Option A: Cl₂ in the presence of ultraviolet light**.

### Final Answer:
**The reagent to be used is Cl₂ in the presence of ultraviolet light.**