See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Molecular polarity arises from an uneven distribution of electron density, resulting in a net dipole moment. Among the given structures, the one with the greatest separation of charge (or intrinsic bond dipoles that add constructively) will be most polar. Step 1: Analyze option (a) — a benzene ring fused to a cyclooctatetraene (COT) ring. COT is non-planar and non-aromatic (tub-shaped, 8 π electrons, antiaromatic if planar). When fused to benzene, there is some charge transfer but the overall molecule has relatively low polarity due to the symmetric hydrocarbon framework. Step 2: Analyze option (c) — naphthalene. This is a symmetric, fully conjugated, planar polycyclic aromatic hydrocarbon. Due to its high symmetry, bond dipoles cancel and the net dipole moment is essentially zero. Naphthalene is nonpolar. Step 3: Analyze option (d) — a bicyclic structure with explicit positive and negative charges on different carbon atoms. While this appears to have charge separation, these are formal charges drawn on what may be a zwitterionic species; however, in context this structure seems unusual and the charges may represent a specific resonance or strained system. Step 4: Analyze option (b) — azulene. Azulene is an isomer of naphthalene consisting of a fused 7-membered ring (tropylium-like, electron-poor) and a 5-membered ring (cyclopentadienyl-like, electron-rich). The 7-membered ring tends to be electron-deficient (partial positive character) and the 5-membered ring tends to be electron-rich (partial negative character), because the 7-membered ring wants 6 π electrons (aromatic tropylium cation) and the 5-membered ring wants 6 π electrons (aromatic cyclopentadienyl anion). This cross-conjugation leads to a substantial ground-state dipole moment of approximately 1.08 D, directed from the 7-membered ring toward the 5-membered ring. This is remarkably large for a pure hydrocarbon. Step 5: Compare all options. Naphthalene (c) has ~0 D dipole. Azulene (b) has ~1.08 D. The COT-benzene fusion (a) has some polarity but less than azulene. Option (d) with explicit charges might seem polar, but azulene's intrinsic ground-state polarity from its aromatic subunit charge distribution is well-established and is the classic textbook example of a polar hydrocarbon. Why other options fail: - (a): The fused COT-benzene system has some asymmetry but lacks the strong aromatic driving force for charge separation that azulene has. - (c): Naphthalene is symmetric and essentially nonpolar (dipole ≈ 0 D). - (d): While it shows formal charges, azulene's delocalized ground-state polarity is the chemically recognized answer in this context. Therefore, the correct answer is B.