Question

The conversion of Propyne to Benzene can be brought out in 4 steps.
Choose the reagents to be used, in the proper sequential order, to bring out the conversion.

• A. \text{Na/ Ether, Cr}_2\text{O}_3\text{ (high T and P), HBr, H}_2\text{/Pd}
• B. \text{HBr, al(C) KOH, HBr/(C}_6\text{H}_5\text{CO})_2\text{O}_2, Cr}_2\text{O}_3\text{ (high T and P)}
• C. \text{Cr}_2\text{O}_3\text{ (high T and P), Na/ dry ether, HBr, Con(C) H}_2\text{SO}_4
• D. \text{H}_2\text{/Pd, HBr/(C}_6\text{H}_5\text{CO})_2\text{O}_2, Na/ dry ether, Cr}_2\text{O}_3\text{ (high T and P)}

Hint:

To convert propyne to benzene, start with catalytic hydrogenation to form an alkene, followed by bromination, dehydrohalogenation to form acetylene, and cyclization to benzene.

Answer 1

The Correct Option is D

To convert propyne to benzene, the following steps are needed: 1. Reduction of propyne to alkene (propene): The first step is to reduce propyne to propene using H\(_2\)/Pd (a catalytic hydrogenation). This process will hydrogenate the triple bond to a double bond, forming propene. 2. Bromination of propene: The second step is to brominate the alkene using HBr/(C\(_6\)H\(_5\)CO)₂O₂. This step will add a bromine atom to the carbon-carbon double bond in propene, resulting in a bromoalkene intermediate. 3. Dehydrohalogenation to form acetylene: The third step involves using Na/ dry ether to induce a dehydrohalogenation reaction. The bromoalkene undergoes elimination to form acetylene. 4. Cyclization to benzene: The final step involves passing the acetylene over Cr\(_2\)O\(_3\) (high T and P), which catalyzes the cyclization of acetylene to form benzene. Thus, the correct sequence of reagents is H\(_2\)/Pd, HBr/(C\(_6\)H\(_5\)CO)₂O₂, Na/ dry ether, Cr\(_2\)O\(_3\) (high T and P).