See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Electrophilic aromatic sulfonation is reversible, and in deuterated acidic media, H/D exchange occurs on the benzene ring. Step 1: When benzene is treated with H2SO4, it undergoes electrophilic aromatic sulfonation to give benzenesulfonic acid (C6H5SO3H). This reaction is reversible — upon heating with dilute acid (or water), the sulfonic acid group is removed (desulfonation). Step 2: In this problem, the reagent is 80% D2SO4 in D2O, which provides D+ (deuterium ions) as the electrophile instead of H+. Step 3: The mechanism proceeds as follows: - D2SO4/D2O generates D+ in solution. - Benzene undergoes electrophilic aromatic substitution where D+ acts as the electrophile, replacing each ring H with D. - Because the sulfonic acid acts as a leaving group in the reverse reaction, the sulfonate group temporarily activates the ring for H/D exchange, but net result is replacement of all six H atoms on the benzene ring with D. - With a large excess of D2SO4 in D2O and heating, all six aromatic hydrogens undergo H→D exchange, yielding C6D6 (benzene-d6). Step 4: Why other options fail: - (a) C6H5SO3D: This would be the sulfonation product, but the large excess of deuterated solvent and heating drives complete H/D exchange rather than stopping at sulfonation. - (b) C6H5OD: There is no pathway for hydroxylation of benzene under these conditions. - (c) C6H5D: This would represent only one H replaced by D, but under large excess D2SO4/D2O with heating, all six hydrogens are exchanged. Step 5: The use of a large excess of D2SO4 in D2O and heating ensures complete isotopic exchange of all six ring hydrogens, giving fully deuterated benzene, C6D6. Therefore, the correct answer is D.