Question

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

Short answer

Azulene's dipole moment arises from resonance structures showing charge separation across its rings.

Step-by-step solution

Understanding Dipole Moment

A dipole moment occurs when there is an uneven distribution of electron density across a molecule, leading to partial charges on different parts of the molecule. Azulene has a dipole moment of 1.0 D, indicating a significant difference in electron density within its structure.

Analyzing Azulene's Structure

Azulene is composed of a seven-membered ring fused to a five-membered ring. This unique structure suggests that there can be a delocalization of electrons to balance electron distribution more towards one side.

Drawing Resonance Structures

Draw alternative resonance structures of azulene. One of these involves moving a pair of pi electrons from the seven-membered ring towards the five-membered ring, and vice versa, creating partial positive and negative charges. This shifting reflects a resonance state emphasizing the uneven distribution of electron density.

Comparing Resonance Structures

Among these structures, some may show charge separation (one side of the molecule with a partial positive charge and the other with a negative one). This charge separation in resonance structures indicates a permanent dipole moment.

Conclusion Based on Resonance Examination

Integrating the resonance structures, the polarization within the molecule (due to distinct electron delocalization pathways) adequately explains the observed dipole moment of 1.0 D in azulene, indicating substantial electron displacement from one ring to the other.

Key concepts

Resonance Structures

Resonance structures are a captivating concept in chemistry. They represent different legitimate ways to draw the electron distribution in a molecule, showing that no single structure can entirely describe a molecule's electron configuration.
For molecules like azulene, resonance structures reveal the shifting of electrons. These structures are not real configurations the molecule flips between, but rather an average or hybrid that more accurately reflects the electron cloud's position.

• Each resonance structure represents a possible configuration of electrons across the bonds.
• Structures with pi electrons moving between rings can indicate areas of higher and lower electron density.
• Highlights areas where positive and negative charges could momentarily reside.

By exploring the resonance structures, we can better understand why certain molecules, like azulene, have large dipole moments. The dipole moment in azulene arises because some resonance forms show a separation of charges, enhancing the molecular dipole.

Azulene

Azulene is a fascinating organic compound known for its unusual properties. It is an isomer of naphthalene, meaning it shares the same molecular formula but differs in structure. Unlike the usual aromatic hydrocarbons, azulene has a unique structure that splits the aromatic system into two fused rings: a heptagon and a pentagon.
This distinct structural setup contributes significantly to its chemical properties.

• The larger, seven-membered ring provides a path for delocalized electrons to migrate efficiently to the smaller, five-membered ring.
• The unequal electron distribution is key to understanding why azulene possesses a pronounced dipole moment.
• This split-ring structure is also why azulene appears blue, which is unusual because most polycyclic aromatic hydrocarbons are colorless.

In a nutshell, azulene is more than just a peculiar isomer of naphthalene; its structure holds the key to its intriguing electric and optical behavior.

Electron Delocalization

Electron delocalization is crucial for understanding many properties in organic molecules, including the dipole moment of azulene. Delocalization refers to electrons spread over several atoms, rather than localized between a specific pair of atoms in a covalent bond.
In azulene, this delocalization occurs prominently between its two rings.

• The delocalization aids in stabilizing the molecule due to the distribution of electrons over a more significant area.
• It allows electrons to constantly shift, creating temporary positive and negative poles in the molecular structure.
• This shifting and balancing act is essential in understanding both the chemical reactivity and polarization of azulene.

The movement of electrons between the rings explains the dipole moment because as electrons move, they create regions with partial charges, leading to an imbalance in electron density. This electron movement between the rings accounts for the dipole observed in azulene.