Question

In the following reaction the catalyst used is-

A. Al₂O₃
B. Cr₂O₃
C. Al₂O₃ & Cr₂O₃
D. Zn dust

Answer 1

Hint: In most of the oxidation reactions in organic chemistry Cr₂O₃ is used to oxidize the compounds and also for cyclization. Cr₂O₃ is a strong oxidizing agent. Al₂O₃ is also a good oxidizing agent and also a dehydrogenation catalyst. It removes H₂ molecules from the compounds. Together they act as a good oxidizing agent and as well as used as a catalyst for aromatization. The conversion of aliphatic organic compounds into aromatic compounds is called aromatization. Aromatic compounds are more stable than aliphatic compounds.

Complete Step by Step Answer:
Given reaction in the question is-


This chemical reaction is practically, chemically, or thermodynamically not possible because the reactant in this reaction contains 6 radical carbon which is a highly unstable compound so it cannot be formed even as an intermediate in the reaction.

But we can convert n-Hexane into benzene using Al₂O₃ & Cr₂O₃ as catalyst because Cr₂O₃ & Al₂O₃ acts as a good oxidizing agent and good dehydrogenating agent respectively. So together in combination with each other, they act as a good catalyst for cyclization & aromatization in organic chemistry. It is also called a reforming process.

The given reaction will be:

And the mechanism of the reaction is



In the reaction, the terminal carbons oxidized in presence of Cr₂O₃ and form a covalent bond converting n-hexane into cyclohexane. Then in presence of Al₂O₃, it aromatizes and forms benzene.
Thus, Option (C) is correct

Note: Vanadium oxide is also used for the aromatization of benzene. To produce toluene from n-hexane we can use Friedel Craft Alkylation. This aromatization of n-hexane is also called dehydroaromatization.