JEE Main 2025 Apr 3 Shift 2, Chemistry Q25: Aromatic Hydrocarbons: Benzene

C${}_9$H${}_{12}$ has a degree of unsaturation of 4, exactly consistent with one benzene ring plus three saturated carbons attached as substituents. Enumerate the distinct ways of placing C${}_3$H${}_7$ worth of alkyl groups on the ring:

1. One propyl branch: $n$-propylbenzene.

2. One isopropyl branch: isopropylbenzene (cumene).

3. Methyl + ethyl on the ring, three positional isomers: ortho-, meta-, and para-ethyltoluene (3 isomers).

4. Three methyls on the ring (trimethylbenzenes), three positional isomers: 1,2,3- (hemimellitene), 1,2,4- (pseudocumene) and 1,3,5- (mesitylene) trimethylbenzene (3 isomers).

Total $=1+1+3+3=8$ structural isomers.

$\therefore$ The answer is 8.