See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: In free radical bromination, Br• is a highly selective radical that preferentially abstracts the hydrogen that forms the most stable carbon radical. The stability order of carbon radicals is: tertiary > secondary > primary > methyl > vinyl/aryl. Step 1: Identify the types of hydrogens present. - (a) The CH3 of the ethyl group (CH3CH2-): these are primary hydrogens on a methyl group. - (b) The CH2 of the ethyl group (CH3CH2-): these are benzylic secondary hydrogens, as the CH2 is directly attached to the aromatic ring. - (c) The aromatic ring hydrogens: these are vinylic/aryl hydrogens, very difficult to abstract by radical processes. - (d) The CH3 directly attached to the benzene ring on the right side: these are benzylic primary hydrogens. Step 2: Compare radical stability after abstraction. - Abstracting (a) gives a primary alkyl radical (non-benzylic): least stable among non-aromatic options. - Abstracting (b) gives a secondary benzylic radical: the radical is stabilized by both the secondary carbon nature and resonance delocalization into the aromatic ring. This is a benzylic secondary radical. - Abstracting (c) gives an aryl radical: very high energy, not favorable. - Abstracting (d) gives a primary benzylic radical: stabilized by resonance with the ring but only primary. Step 3: Rank the radicals. Secondary benzylic radical (from b) > primary benzylic radical (from d) > primary alkyl radical (from a) > aryl radical (from c). Step 4: Since Br• is highly selective, it will abstract the hydrogen that gives the most stable radical, which is the benzylic secondary hydrogen at position (b), the CH2 of the ethyl group. Why other options fail: - (a) gives a non-benzylic primary radical: less stable than (b) and (d). - (c) gives an aryl radical: extremely unstable, essentially not formed under these conditions. - (d) gives a primary benzylic radical: more stable than (a) and (c) but less stable than the secondary benzylic radical from (b). Therefore, the correct answer is B.