JEE Chemistry 2025 Question with Solution

Given: Hydrocarbons have molecular formula $C_9H_{12}$ and give a negative Baeyer's test.

Find: The total number of isomers with four different non-aliphatic substitution sites.

A negative Baeyer's test means the compounds do not contain ordinary carbon-carbon double or triple bonds that decolourise the reagent. For formula $C_9H_{12}$, the relevant hydrocarbons are aromatic isomers.

Possible aromatic isomers here are the three trimethylbenzene isomers:

1. $1,2,3$-trimethylbenzene
2. $1,2,4$-trimethylbenzene
3. $1,3,5$-trimethylbenzene

Now check the number of distinct non-aliphatic substitution sites on the ring.

• In $1,2,3$-trimethylbenzene, the remaining ring positions are all non-equivalent, so there are four different non-aliphatic substitution sites.
• In $1,2,4$-trimethylbenzene, there are also four different non-aliphatic substitution sites.
• In $1,3,5$-trimethylbenzene, the ring has high symmetry, so the available positions are equivalent and do not give four different substitution sites.

Therefore, only two isomers satisfy the condition.

Thus, the required number of isomers is $2$.

Given: Hydrocarbons with molecular formula $C_9H_{12}$ give a negative Baeyer's test.

Find: How many isomers have four different non-aliphatic substitution sites.

Step 1: Understand the implication of Baeyer's test. A negative Baeyer's test indicates absence of reactive unsaturation of the alkene or alkyne type. Hence the compounds are taken as aromatic hydrocarbons for this formula.

Step 2: Identify possible aromatic structures. For $C_9H_{12}$, benzene derivatives with three methyl substituents fit the formula. The possible trimethylbenzene isomers are:

1. $1,2,3$-trimethylbenzene
2. $1,2,4$-trimethylbenzene
3. $1,3,5$-trimethylbenzene

Step 3: Check symmetry and substitution sites. The ring positions available for further substitution must be non-equivalent.

• $1,2,3$-trimethylbenzene satisfies this condition.
• $1,2,4$-trimethylbenzene also satisfies this condition.
• $1,3,5$-trimethylbenzene does not satisfy it because of symmetry.

Step 4: Count valid isomers. Only two of the three isomers meet the requirement.

Therefore, the final answer is $2$.