Benzene reacts with hydrogen in the presence of nickel catalyst at 180ºC to give?

1. Analyze the question

The question asks for the product formed when benzene ($C_6H_6$) undergoes a hydrogenation reaction (reaction with hydrogen) in the presence of a nickel catalyst at a specific temperature (180ºC).

2. Explain the concept

This is a classic addition reaction in organic chemistry. Benzene is an unsaturated aromatic hydrocarbon. Because it contains three double bonds within its ring structure, it can undergo addition reactions under specific conditions.

When benzene is reacted with hydrogen gas ($H_2$) in the presence of a finely divided nickel catalyst at an elevated temperature (typically 150ºC – 200ºC), the hydrogen atoms add across the double bonds, breaking them and saturating the ring. This process converts the aromatic benzene into a saturated cyclic alkane.

3. Show the solution

The chemical equation for this reaction is:

• Reactants: Benzene ($C_6H_6$) + 3 moles of Hydrogen ($3H_2$).
• Conditions: Nickel catalyst ($Ni$) at 180ºC.
• Product: The hydrogen atoms saturate the benzene ring, resulting in cyclohexane ($C_6H_{12}$), which is a cyclic alkane.

4. Identify the answer

Comparing our result ($C_6H_{12}$) with the given options:

• A. xylene (a dimethylbenzene)
• B. toluene (methylbenzene)
• C. cyclopentane (a 5-carbon ring)
• D. cyclohexane (a 6-carbon saturated ring)

The correct option is D.

5. Tip

Remember that benzene is an aromatic ring, but when you "saturate" it completely with hydrogen, it loses its aromaticity and becomes a simple cycloalkane. A quick way to remember this is that the prefix "cyclo-" indicates a ring, and "-hexane" indicates six carbons, matching the six carbons in the original benzene ring.