JEE Main 2025 Jan 29 Shift 2, Chemistry Q22: Aromatic Hydrocarbons: Benzene

A negative Baeyer test rules out alkenes and alkynes, so the $C_9{H}_{12}$ isomers must be aromatic (degree of unsaturation $4$ corresponds to a benzene ring with saturated side chains).

Aromatic $C_9{H}_{12}$ isomers: $n$-propylbenzene, isopropylbenzene (cumene), three ethyl-methylbenzenes (ortho, meta, para), and three trimethylbenzenes (1,2,3-; 1,2,4-; 1,3,5-).

Count the unique ring positions available for electrophilic aromatic substitution in each:

$n$-Propyl- and isopropylbenzene: 3 distinct sites (o, m, p).

1-Ethyl-2-methylbenzene: four ring carbons (3, 4, 5, 6) and each is in a different environment $\Rightarrow$ 4 sites.

1-Ethyl-3-methylbenzene: positions 2, 4, 5, 6 are all in different environments $\Rightarrow$ 4 sites.

1-Ethyl-4-methylbenzene: only 2 distinct sites by symmetry.

1,2,3-Trimethylbenzene: 2 sites; 1,2,4-trimethylbenzene: 3 sites; 1,3,5-trimethylbenzene: 1 site.

$\therefore$ Exactly two isomers (the ortho- and meta-ethyl methyl benzenes) have four distinct aromatic substitution positions.