Question

Which of the following finely divided metals can be used as catalyst in the hydrogenation of alkenes and alkynes? (i) Pt (ii) Fe (iii) Ni (iv) Pd

• A. (i), (ii) & (iii)
• B. (ii), (iii) & (iv)
• C. (i), (iii) & (iv)
• D. (ii) & (iii)
• E. (i), (ii), (iii) \& (iv)

Hint:

Finely divided Ni (Raney Nickel) is a very common laboratory catalyst for hydrogenation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
Hydrogenation is the process of adding hydrogen (H\(_2\)) across a double or triple bond, converting an unsaturated compound into a saturated one. This reaction typically requires a catalyst to proceed at a reasonable rate. The question asks to identify the standard catalysts for this reaction from a given list.
Step 2: Detailed Explanation
Catalytic hydrogenation is a cornerstone of organic synthesis. The reaction involves adsorbing both the unsaturated organic molecule and hydrogen gas onto the surface of a metal catalyst. The most effective and commonly used catalysts are specific transition metals.
Let's analyze the given options:
- (i) Platinum (Pt): Platinum, often used as finely divided platinum metal or as platinum oxide (PtO\(_2\), known as Adams' catalyst), is a highly effective catalyst for hydrogenation, often working at room temperature and pressure. It is a standard choice.
- (ii) Iron (Fe): While iron is a widely used catalyst in industrial processes like the Haber-Bosch process for ammonia synthesis, it is not a common or effective catalyst for the hydrogenation of alkenes and alkynes under typical laboratory conditions.
- (iii) Nickel (Ni): Finely divided Nickel, particularly in the form of Raney Nickel, is a very common industrial and laboratory catalyst for hydrogenation. It is less expensive than platinum or palladium but often requires higher temperatures and pressures. It is a standard choice.
- (iv) Palladium (Pd): Palladium, usually supported on charcoal (Pd/C), is another excellent and widely used catalyst for hydrogenation, with activity similar to platinum. It is also used in a deactivated form (Lindlar's catalyst) for the selective reduction of alkynes to cis-alkenes. It is a standard choice.
Therefore, the metals commonly used for the hydrogenation of alkenes and alkynes are Platinum (Pt), Nickel (Ni), and Palladium (Pd).
Step 4: Final Answer
The correct combination is (i) Pt, (iii) Ni, and (iv) Pd.