JEE Chemistry 2025 Question with Solution

Given: The molecular formula is $C_7H_{12}$ and the question states substituted benzene derivatives.

Find: The total number of structural isomers.

From the solution, first calculate the degree of unsaturation:

Substituting $C = 7$ and $H = 12$,

A benzene ring itself contributes degree of unsaturation $4$. Therefore, a true substituted benzene derivative cannot have molecular formula $C_7H_{12}$ because its degree of unsaturation is only $2$.

Thus, the solution explicitly notes that there is a mismatch between the molecular formula and the phrase substituted benzene derivatives, indicating a likely error in the question statement.

However, the provided the solution and answer key proceed with the accepted result and conclude that the number of structural isomers is $8$.

Therefore, the required numerical answer is $8$.

Given: the solution provides two approaches and both end with the accepted answer $8$.

Find: Whether the listed answer can be extracted reliably from the source.

Approach 1 on the source computes degree of unsaturation as

and then still reports the count as $8$.

Approach 2 clearly states that $C_7H_{12}$ cannot represent a substituted benzene derivative because a benzene ring requires degree of unsaturation at least $4$. It further says that, assuming the intended interpretation is the answer-key interpretation, the number shown is $8$.

So, although the chemistry statement in the question is inconsistent, the source solution is internally clear about the extracted final answer.

Therefore, based on the solution, the final numerical answer is $8$.