The product formed when ethyne is passed through a hot tube containing finely divided iron is

1. Analyze the question

The question asks for the classification of the product formed when ethyne ($C_2H_2$) undergoes a specific chemical reaction: passing it through a hot tube containing finely divided iron.

2. Explain the concept

This reaction is a classic example of cyclic polymerization (or trimerization).

• The Reaction: When ethyne gas is passed through a red-hot iron tube (at about 400°C–500°C), three molecules of ethyne combine to form a single molecule of benzene ($C_6H_6$).
• The Equation: $3C_2H_2 \rightarrow C_6H_6$
• The Product: Benzene is the simplest member of the aromatic hydrocarbon family. Aromatic compounds are characterized by a stable, cyclic ring structure with delocalized pi electrons.

3. Show the solution

• Step 1: Identify the reactants and conditions. Ethyne ($HC \equiv CH$) + Hot Iron Tube $\rightarrow$ ?
• Step 2: Recognize the reaction type. The iron acts as a catalyst for the trimerization of ethyne.
• Step 3: Determine the product. The trimerization of ethyne produces benzene ($C_6H_6$).
• Step 4: Classify the product. Benzene is the fundamental building block of aromatic chemistry. It is unsaturated (due to the ring structure), but its defining chemical classification in this context is "aromatic."

4. Identify the answer

Comparing our findings to the options:

• A is incorrect because while it is a polymer, "aromatic" is a more specific and accurate chemical classification for benzene.
• B is incorrect because benzene is unsaturated.
• C is incorrect because hydration involves adding water.
• D is correct because the product, benzene, is the primary aromatic hydrocarbon.

5. Exam Tip

Whenever you see "ethyne" and "hot iron tube" in a JAMB question, immediately think "Benzene". Remember the mnemonic: "3 Ethyne = 1 Benzene". Since benzene is the parent of all aromatic compounds, the answer will almost always be "aromatic."