See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: NBS (N-bromosuccinimide) is a reagent used for allylic bromination via a radical mechanism. It selectively brominates the allylic position — the carbon adjacent to the carbon-carbon double bond. Step 1: Identify the double bond. The double bond is between carbons b and d in the cyclohexene ring. Step 2: Identify allylic positions. The allylic positions are carbons directly adjacent to the double bond carbons (b and d). Carbon a is adjacent to carbon b (part of the double bond), making it an allylic position. Carbon c is adjacent to carbon d (part of the double bond), also making it allylic. Step 3: Determine which allylic position is more reactive. Carbon a bears a CH3 group and an H atom — it is a tertiary allylic carbon (attached to the ring on two sides plus a methyl group), making the radical formed at a more stable (tertiary allylic radical) compared to the radical at c (secondary allylic radical). Step 4: NBS bromination proceeds through the most stable allylic radical intermediate. The tertiary allylic radical at carbon a is more stable than the secondary allylic radical at carbon c. Therefore, NBS preferentially abstracts the hydrogen from carbon a to give the tertiary allylic radical, which is then trapped by bromine to give the major product at position a. Step 5: Why other options fail: - Position b: This is a vinylic carbon (part of the double bond); NBS does not brominate vinylic positions under radical allylic conditions. - Position c: This is a secondary allylic carbon, less stable radical than at a. - Position d: This is also a vinylic carbon bearing the CH3 group; not an allylic bromination site. Therefore, the correct answer is A.