Q15-35E - Organic Chemistry (Mcmurry)

Question

Calicene, like azulene (Problem 15-17), has an unusually large dipole moment for a hydrocarbon. Explain, using resonance structures.

Calicene

Step-by-Step Solution

Answer

Dipole moment in calicene

Resonance contributors calicene

Step 1: Aromatic compound

An aromatic compound is defined as cyclic, planar, and completely conjugated compounds, which contain $(\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2})\mathbf{\pi }$ electrons. This is known as Huckel’s rule. Dipole means that a polar bond has positive and negative ends. 

Step 2: Calicene

When the charges are separated in calicene. The cyclopentadienyl anion and cyclopropenyl cation become aromatic as they obey Huckel’s rule ($(4\mathrm{n}+2)\mathrm{\pi }$electrons). Hence the species which is aromatic will be more stable. So, calicene has an unusually large dipole moment. So, the direction is as shown:

Dipole moment in calicene

Step 3: Dipole moment

The dipole moment of calicene can be explained by its resonance structures, here, the five-membered ring gets a negative charge and the three-membered ring gets a positive charge. Hence, the species which is aromatic will be more stable. The resonance contributors are as shown:

Resonance contributor’s calicene