See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The acidity of a C-H bond in hydrocarbons depends on the stability of the conjugate base (carbanion) formed after deprotonation. Greater stability of the carbanion means stronger acid. Step 1 - Identify the acidic proton in each molecule. (a) Benzene: sp2 C-H bonds; deprotonation gives a phenyl anion, which is not specially stabilized by aromaticity of the ring. (b) 1,2-Dihydronaphthalene: contains sp3 CH2 groups at C1 and C2; deprotonation gives an allylic/benzylic carbanion stabilized by conjugation with the aromatic ring and the adjacent double bond. (c) Indene: contains one sp3 CH2 group at C1 (the methylene bridging the five-membered ring); deprotonation gives the indenyl anion. The five-membered ring then becomes a cyclopentadienyl-type anion fused to a benzene ring. This anion is aromatic (6 pi electrons in the five-membered ring: the lone pair on the carbanion carbon + 2 double bonds = 6 pi electrons satisfying Hückel's rule, 4n+2 with n=1). Aromatization of the anion provides exceptional stabilization. (d) Benzosuberene (benzene fused to a seven-membered ring with one sp3 CH2): deprotonation gives a carbanion in the seven-membered ring; if it became aromatic it would need 8 pi electrons (anti-aromatic for the seven-membered ring), so no aromatic stabilization is gained. Step 2 - Compare stabilities. - Option (c) indene: upon deprotonation at the sp3 CH2, the resulting anion is isoelectronic with the cyclopentadienyl anion (a classic 6 pi-electron aromatic system), fused to a benzene ring. This gives exceptional aromatic stabilization to the conjugate base, making indene by far the most acidic of the four hydrocarbons (pKa ~20 vs ~43 for typical sp3 C-H). - Options (a), (b), (d) do not produce an aromatic anion upon deprotonation, so their conjugate bases are far less stable. Step 3 - Why other options fail. (a) Benzene anion: no special aromatic stabilization of the carbanion. (b) Allylic/benzylic anion: resonance-stabilized but not aromatic; much less stable than the indenyl anion. (d) Seven-membered ring carbanion: would be antiaromatic or non-aromatic; destabilized or not specially stabilized. Therefore, the correct answer is C.