Question

When a mixture of vapours of phenol and hydrogen is passed over nickel catalyst at $433\ \mathrm{K}$, the product obtained is

• A. Benzene
• B. Benzaldehyde
• C. Cyclohexane
• D. Cyclohexanol

Hint:

Distinguish this clearly from heating phenol with Zinc dust ($\mathrm{Zn}$): reaction with Zinc dust causes reduction by deoxygenation to produce benzene, whereas reaction with $\mathrm{H_2/Ni}$ causes ring hydrogenation to yield cyclohexanol.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The prompt describes a chemical reaction where phenol vapours are mixed with hydrogen gas ($\mathrm{H_2}$) and passed over a heated active nickel ($\mathrm{Ni}$) catalyst bed maintained at an elevated temperature ($433\ \mathrm{K}$). We need to deduce the primary chemical product of this pathway.

Step 2: Key Formula or Approach:
Passing hydrogen gas over an unsaturated cyclic system in the presence of an active transition metal catalyst like Nickel, Platinum, or Palladium at elevated temperatures prompts a catalytic hydrogenation. This reaction breaks down carbon-carbon double bonds ($\pi$ bonds) by adding hydrogen atoms across them.

Step 3: Detailed Explanation:
Phenol consists of a hydroxyl group ($\mathrm{-OH}$) bound to an aromatic benzene ring: $\mathrm{C_6H_5OH}$.
The aromatic core contains three conjugated $\pi$ double bonds distributed around its six-membered carbon ring.
Under rigorous conditions—specifically heating up to $433\ \mathrm{K}$ over a nickel catalyst—the system forces complete catalytic hydrogenation of the aromatic ring system.
Three moles of molecular hydrogen ($\mathrm{3H_2}$) add across the three internal double bonds of the ring: $$\mathrm{C_6H_5OH + 3H_2 \xrightarrow[433\ K]{Ni} C_6H_{11}OH}$$ The aromatic ring system converts entirely into a fully saturated six-membered ring (cyclohexane ring), while preserving the attached hydroxyl ($\mathrm{-OH}$) functional group. The resulting product molecule is cyclohexanol.

Step 4: Final Answer:
The chemical product obtained is cyclohexanol, which corresponds to option (D).