Question

Naphthalene and azulene are constitutional isomers of molecular formula \(\mathrm{C}_{10} \mathrm{H}_{8}\). Naphthalene is a colorless solid with a dipole moment of zero. Azulene is a solid with an intense blue color and a dipole moment of \(1.0 \mathrm{D}\). Account for the difference in dipole moments of these constitutional isomers.

Short answer

Question: Explain the difference in dipole moments between naphthalene and azulene, and how this difference is related to their molecular structures. Answer: The difference in dipole moments between naphthalene and azulene can be attributed to their distinct molecular structures. In naphthalene, the planar structure with two benzene rings leads to a symmetric electron density distribution, resulting in a net dipole moment of zero, which indicates a nonpolar molecule. In contrast, azulene has a non-symmetric structure with a seven-member and a five-member ring, causing an uneven electron density distribution, resulting in a non-zero dipole moment, indicating a polar molecule.

Step-by-step solution

Understanding dipole moment

A dipole moment is a measure of the molecular polarity of a compound, which depends on the electronegativity of the atoms and the molecular structure. A non-zero dipole moment indicates that a molecule is polar and has unequal distribution of electron charge, whereas a zero dipole moment means the molecule is nonpolar with symmetric distribution of electron charge.

Drawing the molecular structures of naphthalene and azulene

Naphthalene has two benzene rings fused together, with each ring containing six carbon atoms (\mathrm{C}) and alternating single and double bonds. The hydrogen atoms (\mathrm{H}) are attached to each carbon atom on the outer perimeter of the rings. In an ideal naphthalene molecule, the two benzene rings are planar and at the same level. Azulene, on the other hand, has a structure with a seven-member ring (heptagonal) and a five-member ring (pentagonal) fused together. The seven-member ring has alternating single and double bonds, while the five-member ring is an aromatic system similar to cyclopentadiene. The hydrogen atoms are attached to the carbon atoms on the outer perimeter of the rings.

Determining the electron distribution and possible dipole moments

For naphthalene, since the two benzene rings are planar and at the same level, the electron density distribution is symmetric, leading to a net dipole moment of zero (\(˜0 \mathrm{D}\)). This means that naphthalene is a nonpolar molecule. In azulene, the structure is not symmetric, and the electron density is not evenly distributed between the seven-member and five-member rings due to the different ring sizes and the spacial orientation of the carbon atoms involved in the alternating single and double bonds. This uneven electron density distribution results in a non-zero dipole moment of \(1.0 \mathrm{D}\), indicating that azulene is a polar molecule.

Explaining the color difference

The color difference between naphthalene (colorless) and azulene (intense blue) can be attributed to their different molecular structures which create distinct energy gaps between their highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO). The energy gaps determine the wavelengths of light that can be absorbed by the molecules, which in turn give rise to their observed colors. Naphthalene's energy gap corresponds to the absorption of ultraviolet light, which is not visible to the human eye, while azulene's energy gap allows it to absorb light in the visible range, giving it the blue color. In conclusion, the difference in dipole moments between naphthalene and azulene can be attributed to their distinct molecular structures, which lead to different electron density distributions and molecular polarities.

Key concepts

Molecular Polarity

Understanding molecular polarity is crucial when analyzing compounds like naphthalene and azulene. At its core, molecular polarity refers to the distribution of electrical charges across a molecule. This is judged by the arrangement of atoms and the electronegativity—the tendency to attract shared electrons—of each atom.

When a molecule like azulene has an uneven distribution of electrons due to an asymmetrical structure, it exhibits a non-zero dipole moment, an indicator of polarity. Polarity influences a molecule's chemical properties, including its solubility, boiling point, and reactivity. It's also pivotal in intermolecular interactions, affecting how molecules attract or repel each other.

Aromatic Compounds

Aromatic compounds, such as naphthalene and azulene, are not just scented molecules but rather a class of compounds characterized by their stable ring structures and delocalized π electrons over the ring system. This unique feature allows them to absorb certain wavelengths of light – naphthalene absorbs in the ultraviolet range, while azulene absorbs in the visible range, hence its vivid blue color.

Aromatic Stability and Its Effects

Aromatic stability arises from electron delocalization in these rings, which imparts special electronic and structural properties. This concept is essential in organic chemistry as it includes famous compounds like benzene and plays a significant role in understanding the chemical behavior and reactivity of various organic molecules.

Constitutional Isomers

Constitutional isomers are molecules with the same molecular formula but differ in the connectivity of their atoms. A clear example is seen when comparing naphthalene and azulene, both of which have the formula \(\textrm{C}_{10}\textrm{H}_{8}\) but differ vastly in structure and, consequently, in properties like dipole moment and color.

Constitutional isomers can illustrate the incredible variety of organic molecules, as they might have little in common beyond their elemental composition. Their different connectivities lead to unique physical and chemical properties, including state of matter, reactivity, and interaction with polar or non-polar solvents.

HOMO-LUMO Energy Gap

The HOMO-LUMO energy gap is a fascinating concept illustrating the difference in energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This energy difference correlates with the color we observe; for naphthalene, the gap corresponds to ultraviolet light absorption, while for azulene it's within the visible spectrum, giving it a distinctive blue hue.

Molecular color is therefore directly related to these energy gaps, and modulation of this gap is one way chemists can design compounds with specific optical properties. Such manipulations have practical applications in dyes, pigments, and organic electronic devices, among others.