See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Carbocation stability is governed by electron delocalization, hyperconjugation, inductive effects, and special structural features. Cyclopropyl rings adjacent to a carbocation provide exceptional stabilization through bent-bond (banana bond) overlap — the Walsh orbitals of cyclopropane have significant p-character that overlaps with the empty p orbital of the adjacent carbocation. This is known as cyclopropylcarbinyl stabilization. Step 1 — Analyze option (a): The diphenylmethyl (benzhydryl) cation C6H5-CH(+)-C6H5 is stabilized by resonance delocalization into two phenyl rings. This is a secondary benzylic carbocation with extensive resonance — quite stable. Step 2 — Analyze option (b): The structure shown appears to be a cycloheptadienyl cation (non-aromatic 7-membered ring carbocation). This is NOT the aromatic tropylium cation (which has 3 double bonds and 6 pi electrons). With only 2 double bonds shown and a formal negative sign at the bottom carbon, this appears to be an anti-aromatic or non-aromatic species, making it relatively unstable. Step 3 — Analyze option (c): The tert-butyl cation (CH3)3C+ is a tertiary carbocation stabilized by hyperconjugation and inductive donation from three methyl groups. This is moderately stable but lacks the extended delocalization of benzylic or cyclopropylcarbinyl systems. Step 4 — Analyze option (d): The tricyclopropylcarbocation has three cyclopropyl groups attached to the central carbocation. Each cyclopropyl group donates electron density through cyclopropylcarbinyl stabilization via overlap of the bent bonds (Walsh orbitals) with the empty p orbital. Three such groups provide three-fold cyclopropylcarbinyl stabilization. Studies and experimental evidence show that the tricyclopropylcarbocation is extraordinarily stable — more stable even than the triphenylcarbocation in many assessments — because each cyclopropyl ring donates more effectively per group than a phenyl ring due to the high p-character of cyclopropane C-C bonds. Step 5 — Comparison: Tricyclopropylcarbocation (d) > diphenylmethyl cation (a) > tert-butyl cation (c) > cycloheptadienyl cation (b). The tricyclopropylcarbocation benefits from three strong cyclopropylcarbinyl stabilizations simultaneously, making it the most stable among the given options. Therefore, the correct answer is D.