See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The pKa of a C-H bond reflects the stability of the carbanion formed upon deprotonation. A lower pKa means greater acidity, i.e., the carbanion is more stable. Step 1: Identify the acidic C-H in each compound. (a) Triphenylmethane (Ph3C-H): The sp3 C-H is stabilized by resonance delocalization into three phenyl rings. pKa ≈ 31. (b) Diphenylmethane (Ph2CH2): The sp3 C-H is stabilized by delocalization into two phenyl rings. pKa ≈ 43. (c) 9-Phenylfluorene: The C9-H of fluorene is already part of a five-membered ring flanked by two benzene rings; upon deprotonation the carbanion achieves 14 π-electron cyclopentadienyl-like aromatic stabilization across the fluorenyl system plus an additional phenyl group. pKa ≈ 18. (d) The compound in option (d) is a fluorene core fused with two additional benzene rings (a tricyclic aromatic system with a cyclopentadienyl-type sp3 CH), making it structurally analogous to indeno[1,2,3-cd]fluorene or a related system. The sp3 C-H sits at the junction of multiple fused aromatic rings. Upon deprotonation, the resulting carbanion is delocalized over an extended polycyclic aromatic system with near-aromatic or fully aromatic stabilization. The fusion of additional rings compared to fluorene provides even greater delocalization and stabilization of the carbanion than in 9-phenylfluorene. This extended conjugation lowers the pKa further than any of the other options. Step 2: Compare stabilization. - (b) Ph2CH2: least stabilized, highest pKa (~43) - (a) Ph3CH: three phenyl groups, pKa ~31 - (c) 9-Phenylfluorene: fluorenyl aromatic stabilization + extra phenyl, pKa ~18 - (d) Polycyclic fused system: maximum delocalization over the extended aromatic framework gives the most stable carbanion and thus the lowest pKa among the four. Step 3: Why other options fail. (a) Only three phenyl groups with no cyclic aromatic carbanion stabilization — less acidic than (c) or (d). (b) Only two phenyl groups — least acidic. (c) 9-Phenylfluorene is very acidic, but the additional ring fusion in (d) provides even greater carbanion delocalization. Therefore, the correct answer is D.