Which one gives a red precipitate with ammoniacal solution of copper(I) chloride?
(A) Ethane
(B) Ethene
(C) Ethyne
(D) Propane

Options

A. Ethane
B. Ethene
C. Ethyne
D. Propane

Answer

To solve the question, "Which one gives a red precipitate with ammoniacal solution of copper(I) chloride?" we need to analyze the given options and understand the chemical behavior of each compound with respect to copper(I) chloride.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Option A: Ethane (C2H6) - an alkane
- Option B: Ethene (C2H4) - an alkene
- Option C: Ethyne (C2H2) - a terminal alkyne
- Option D: Propane (C3H8) - an alkane

2. **Understand the Reaction**:
- The question mentions the reaction with "ammoniacal solution of copper(I) chloride". Terminal alkynes (compounds with a triple bond at the end of the carbon chain) are known to react with this solution to form a red precipitate.

3. **Identify Terminal Alkynes**:
- Among the given options, only Ethyne (C2H2) is a terminal alkyne, which has the structure C≡C-H.

4. **Reaction of Ethyne with Ammoniacal Copper(I) Chloride**:
- When Ethyne reacts with ammoniacal copper(I) chloride (CuCl in NH3 solution), it forms copper acetylide (CuC≡C) and ammonium chloride (NH4Cl), along with water (H2O).
- The reaction can be summarized as follows:
\[ \text{C}_2\text{H}_2 + \text{Cu}_2\text{Cl}_2 + 2\text{NH}_4\text{OH} \rightarrow \text{CuC} \equiv \text{C} \text{Cu} + 2\text{NH}_4\text{Cl} + 2\text{H}_2\text{O} \]
- The product, copper acetylide, is known to give a red precipitate.

5. **Conclusion**:
- Since only Ethyne (C2H2) can react to form the red precipitate with ammoniacal copper(I) chloride, the correct answer is:
- **Option C: Ethyne**.