Which one of the following reagents distinguish ethylene from acetylene?

Options

A. Aqueous alkaline `KMnO_4`
B. `Cl_2` dissolved in `C Cl_4`
C. Ammoniacal `Cu_2Cl_2`
D. Conc. `H_2SO_4`

Explanation

To distinguish between ethylene (C2H4) and acetylene (C2H2), we can use ammoniacal copper(II) chloride (Cu2Cl2). Here's a step-by-step solution:

### Step 1: Understand the Reactivity of the Compounds
- Ethylene (C2H4) is an alkene with a double bond.
- Acetylene (C2H2) is an alkyne with a triple bond.
- Alkynes generally react differently than alkenes due to the presence of the triple bond.

### Step 2: Identify the Reagent
- The reagent in question is ammoniacal copper(II) chloride (Cu2Cl2). This reagent is known to react specifically with alkynes.

### Step 3: Reaction with Acetylene
- When acetylene (C2H2) reacts with ammoniacal copper(II) chloride, it forms a complex known as copper acetylide (CuC2H). The reaction can be represented as:
\[ \text{C2H2} + \text{Cu2Cl2} \rightarrow \text{CuC2} + \text{NH4Cl} + 2\text{H2O} \]
- This reaction indicates that acetylene can be distinguished because it forms a precipitate of copper acetylide.

### Step 4: Reaction with Ethylene
- Ethylene (C2H4), on the other hand, does not react with ammoniacal copper(II) chloride. Therefore, no product is formed when ethylene is treated with this reagent.

### Step 5: Conclusion
- Since acetylene reacts to form a copper complex while ethylene does not react, ammoniacal copper(II) chloride can be used to distinguish between the two compounds.

### Final Answer
- The reagent that can distinguish ethylene from acetylene is **Ammoniacal Cu2Cl2**.

---