Question

Which of the following reaction is NOT correctly represented? 

• A. Methylbenzene + Br\(_2\) \(\xrightarrow{hv}\) Benzyl bromide
• B. Benzene diazonium chloride + Cu\(_2\)Br\(_2\)/HBr \(\to\) Bromobenzene
• C. Methylbenzene + Br\(_2\)/Fe (dark) \(\to\) Ortho- and para-bromotoluenes
• D. Methylbenzene + Br\(_2\) \(\xrightarrow{hv}\) Benzyl bromide

Hint:

For aromatic halogenation: - Use Lewis acids (Fe/FeCl\(_3\)) → ring substitution. - Use light (\(hv\)) → benzylic radical substitution. - Diazonium salts undergo Sandmeyer reactions to form aryl halides.

Answer 1

The Correct Option is D

To determine which reaction is incorrectly represented, let's analyze each option:

1. \(Methylbenzene + Br_2 \xrightarrow{hv} Benzyl bromide\): 
   This reaction is a free-radical bromination, which occurs at the benzylic position (side chain) of methylbenzene, forming benzyl bromide. This reaction is correctly represented.
2. \(Benzene diazonium chloride + Cu_2Br_2/HBr \to Bromobenzene\): 
   This reaction is an example of the Sandmeyer reaction, where diazonium salts are converted to bromobenzene using copper(I) bromide. This reaction is correctly represented.
3. \(Methylbenzene + Br_2/Fe (dark) \to Ortho- and para-bromotoluenes\): 
   This is an example of electrophilic aromatic substitution, where bromine substitutes hydrogen atoms on the aromatic ring, favoring ortho- and para-positions due to the ortho-para directing nature of the methyl group. This reaction is correctly represented.
4. \(Methylbenzene + Br_2 \xrightarrow{hv} Benzyl bromide\): 
   This is identical to Option 1 and is correctly represented. However, repeating it twice among the options suggests identifying the "incorrectly represented" reaction. Note: Keep in mind that when asked for an incorrect representation within a context of error (like duplicate option), technically, none of these reactions are actually incorrect.

Conclusion:

The reaction that stands out due to duplication is not technically incorrect in representation, but within the context of the typical exam pattern, Options 1 and 4 should not both be present as correct answers. Hence, presenting the same correct reaction twice suggests an error in examination design rather than chemical representation.