Question

Identify the product X obtained in the following reaction:
\[ \text{CH}_3\!-\!(\text{CH}_2)_4\!-\!\text{CH}_3 \;\xrightarrow[\;773\,\text{K},\,10\text{–}20\,\text{atm}\;]{\text{Cr}_2\text{O}_3}\; X \]

• A. \(\text{CH}_3\!-\!\text{CH}=\text{CH}\!-\!\text{CH}_3 + \text{CH}_2=\text{CH}_2 + \text{H}_2\)
• B. \(2\,\text{CH}_3\!-\!\text{CH}=\text{CH}_2 + \text{H}_2\)
• C. \(\text{C}_6\text{H}_6 + 4\,\text{H}_2\)
• D. \(\text{CH}_3\!-\!(\text{CH}_2)_3\!-\!\text{CH}=\text{CH}_2 + \text{H}_2\)

Hint:

n-Alkanes with six or more carbon atoms undergo \textbf{aromatization} in the presence of metal oxide catalysts like \(\text{Cr}_2\text{O}_3\) at high temperature, forming benzene derivatives.

Answer 1

The Correct Option is C

Step 1: Identify the reactant and conditions.
The given compound \(\text{CH}_3-(\text{CH}_2)_4-\text{CH}_3\) is n-hexane. The reaction is carried out at high temperature (773 K) and high pressure (10–20 atm) in the presence of chromium oxide (\(\text{Cr}_2\text{O}_3\)). These conditions are characteristic of dehydrocyclization (aromatization) reactions.

Step 2: Apply the concept of dehydrocyclization.
Straight-chain alkanes with six carbon atoms undergo cyclization followed by dehydrogenation in the presence of catalysts like \(\text{Cr}_2\text{O}_3\). Thus, n-hexane converts into benzene with the release of hydrogen gas.

Step 3: Write the balanced reaction.
\[ \text{C}_6\text{H}_{14} \;\xrightarrow{\text{Cr}_2\text{O}_3,\,773\,\text{K}}\; \text{C}_6\text{H}_6 + 4\,\text{H}_2 \]

Step 4: Analyze the options.
(A) and (B) represent cracking reactions, not aromatization.
(D) represents dehydrogenation to an alkene, which is incomplete under these conditions.
(C) correctly represents the formation of benzene via dehydrocyclization.

Step 5: Conclusion.
Therefore, the correct product \(X\) is benzene along with four molecules of hydrogen.