See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Free radical bromination (Br2/hv) replaces one hydrogen atom with bromine. We must count the number of structurally distinct (constitutional isomer) positions where bromine can be substituted, ignoring stereoisomers. Step 1: Draw methylcyclohexane and identify unique carbon positions. Methylcyclohexane has the following distinct carbon environments: 1. The methyl group carbon (–CH3): substitution here gives bromomethyl-cyclohexane (i.e., the CH2Br group on the ring). 2. C-1 (the ring carbon bearing the methyl group): substitution here gives 1-bromo-1-methylcyclohexane. 3. C-2 (adjacent to C-1, equivalent positions on both sides of the ring): substitution here gives 1-bromo-2-methylcyclohexane. 4. C-3 (next carbon): substitution here gives 1-bromo-3-methylcyclohexane. 5. C-4 (opposite end of ring): substitution here gives 1-bromo-4-methylcyclohexane. Step 2: Count the distinct structural positions. - Methyl carbon (exocyclic CH3) → 1 product - Ring C-1 → 1 product - Ring C-2 → 1 product (C-2 on either side is equivalent by symmetry) - Ring C-3 → 1 product (C-3 on either side is equivalent by symmetry) - Ring C-4 → 1 product Total = 5 distinct constitutional isomers (excluding stereoisomers). Step 3: Why other options fail. - Option (a) 4: undercounts; the methyl group itself can be brominated as a distinct position. - Option (c) 8 and (d) 10: these counts include stereoisomers (cis/trans pairs), which the question explicitly excludes. Therefore, the correct answer is B.