See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Identify compound B from the oxidation reaction. (A) C8H10 is oxidized by KMnO4 to give (B) C8H6O4. KMnO4 oxidizes alkyl substituents on a benzene ring to carboxylic acid groups (-COOH). C8H6O4 has molecular formula consistent with a benzene ring bearing two -COOH groups (benzenedicarboxylic acid), since C6H4(COOH)2 = C8H6O4. This means (A) must be a dimethylbenzene (xylene), with two methyl groups being oxidized to two carboxyl groups. Step 2 - Eliminate option (a). Ethylbenzene (C8H10) upon KMnO4 oxidation gives benzoic acid (C7H6O2), not C8H6O4. So (a) is eliminated. Step 3 - Determine which xylene isomer gives only one product upon bromination with Br2/Fe. Br2/Fe is electrophilic aromatic bromination. The number of unique monobromination products depends on the symmetry of the dicarboxylic acid (B). - Para-xylene (b) oxidizes to terephthalic acid (benzene-1,4-dicarboxylic acid). The ring has only one unique ring position available for substitution (positions 2,3,5,6 are all equivalent in pairs; actually positions 2,3 are equivalent and so are 5,6, but by symmetry positions 2 and 3 are equivalent to each other, and position 2 is equivalent to position 3 by the mirror plane). In terephthalic acid, all four remaining H positions (2, 3, 5, 6) are equivalent due to the high symmetry (D2h), so bromination gives only ONE product (bromo at position 2 = bromo at position 3 = bromo at 5 = bromo at 6). Thus only one monobromo product is formed. - Meta-xylene (c) oxidizes to isophthalic acid (benzene-1,3-dicarboxylic acid). The remaining positions are 2, 4, 5, and 6. Position 2 (between two COOH groups) is unique, position 4 and 6 are equivalent (flanked by one COOH), position 5 is unique. This gives multiple distinct products, not one. - Ortho-xylene (d) oxidizes to phthalic acid (benzene-1,2-dicarboxylic acid). Remaining positions are 3, 4, 5, 6. Positions 3 and 6 are equivalent, positions 4 and 5 are equivalent, giving two distinct monobromo products, not one. Step 4 - Confirm answer. Only para-xylene (option b, 1,4-dimethylbenzene) satisfies all conditions: oxidation gives C8H6O4 (terephthalic acid) and bromination of terephthalic acid gives only one monobromo product (C8H5BrO4) due to the symmetry of the para-substituted ring. Therefore, the correct answer is B.