See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Dipole moment arises from an uneven distribution of electron density in a molecule. Symmetric molecules have zero or very low dipole moments, while asymmetric molecules with charge separation have higher dipole moments. Step 1 - Evaluate naphthalene (a): Naphthalene is a symmetric fused bicyclic aromatic hydrocarbon (two benzene rings fused linearly). It has a center of symmetry, so its dipole moment is essentially zero (μ ≈ 0 D). Step 2 - Evaluate phenanthrene (b): Phenanthrene is a tricyclic aromatic hydrocarbon with an angular fusion. While it is slightly less symmetric than naphthalene or anthracene, it is still a pure hydrocarbon with nearly zero dipole moment (μ ≈ 0 D) due to the nearly symmetric distribution of the π system. Step 3 - Evaluate anthracene (c): Anthracene is a tricyclic aromatic hydrocarbon with three linearly fused benzene rings. It is highly symmetric (D2h symmetry), and its dipole moment is zero (μ = 0 D). Step 4 - Evaluate azulene (d): Azulene is a non-benzenoid aromatic hydrocarbon composed of a fused five-membered ring (cyclopentadienyl) and a seven-membered ring (cycloheptatrienyl), both sharing one bond. The five-membered ring tends to bear a partial negative charge (analogous to cyclopentadienyl anion character) and the seven-membered ring tends to bear a partial positive charge (analogous to tropylium cation character). This charge separation results in a significant dipole moment of approximately 1.08 D, which is far greater than the essentially zero dipole moments of the purely symmetric benzenoid hydrocarbons naphthalene, anthracene, and phenanthrene. Step 5 - Conclusion: Among the four options, azulene is the only compound with a substantial dipole moment due to its inherent structural asymmetry and charge polarization between the fused 5- and 7-membered rings. Therefore, the correct answer is D.