Q17E - Organic Chemistry (Mcmurry)

Question

Azulene, an isomer of naphthalene, has a remarkably large dipole moment for a hydrocarbon ($\mathbf{\mu }\mathbf{=}$5 1.0 D). Explain, using resonance structures.

Step-by-Step Solution

Answer

The molecule shows a dipole moment from a large seven-membered ring to a five-membered ring.

The resonating structure is shown below.

Resonating structure

Step-by-Step Solution Step 1: Dipole moment

It is defined as the measurement of two opposite charge particles.

It is a vector quantity that depends upon the direction of the dipole.

Polar molecules show dipole moments from positive to negative.

Step 2: Dipole moment of azulene

In azulene, the electron density passes from the seven-membered ring to the five-membered ring which satisfies Huckel’s rule for both the rings—seven as well as the five-membered ring.

Five membered rings resemble the cyclopentadienyl anion and have 6π electrons; and the seven-membered ring resembles the cycloheptatrienyl cation.

The electro potential map shows that the five-membered ring is more electron-rich than the seven-membered ring, and the dipole moment occurs from the seven-membered ring to the five-membered ring as shown below.

Resonance structure