See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Trimethylbenzene refers to benzene with three methyl (CH3) groups attached to the ring. The molecular formula is C9H12. We need to count the number of distinct structural isomers by considering all unique positions for three methyl substituents on a benzene ring. Step 1: Label the positions on the benzene ring as 1 through 6. We need to place three methyl groups on three of these six positions, accounting for the symmetry of the benzene ring. Step 2: Enumerate the possible arrangements: - 1,2,3-trimethylbenzene (hemimellitene): three methyl groups on consecutive carbons. - 1,2,4-trimethylbenzene (pseudocumene): two adjacent methyls and one separated by one carbon. - 1,3,5-trimethylbenzene (mesitylene): three methyl groups in alternating positions (symmetric). Step 3: Check for duplicates using ring symmetry: - 1,2,3- is distinct from 1,2,4- and 1,3,5-. - 1,2,4- cannot be reduced to 1,2,3- or 1,3,5- by rotation or reflection. - 1,3,5- is fully symmetric and unique. - Any other combination (e.g., 1,2,5- or 1,3,4-) reduces to one of the three above by ring symmetry. Step 4: Total distinct isomers = 3 (1,2,3-trimethylbenzene; 1,2,4-trimethylbenzene; 1,3,5-trimethylbenzene). Why other options fail: - (a) 2: Undercounts; misses one of the three distinct arrangements. - (c) 4 and (d) 6: Overcount; these arise from not applying ring symmetry correctly. Therefore, the correct answer is B.