Question

Resonance energy per benzene ring decreases in the order (a) Naphthalene \(>\) Benzene \(>\) Anthracene \(>\) Phenanthrene (b) Benzene \(>\) Naphthalene \(>\) Anthracene \(\geq\) Phenanthrene (c) Benzene \(>\) Naphthalene \(>\) Phenanthrene \(>\) Anthracene (d) All have equal resonance energy

Short answer

(c) Benzene > Naphthalene > Phenanthrene > Anthracene

Step-by-step solution

Understanding Resonance Energy

Resonance energy is a measure of the stability provided by the delocalization of electrons within a molecule, particularly in aromatic compounds like benzene and polyaromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene.

Analyzing Molecular Structures

Benzene has one aromatic ring, while naphthalene, anthracene, and phenanthrene are polycyclic aromatic hydrocarbons. Naphthalene has two fused rings, anthracene has three linearly fused rings, and phenanthrene has three rings fused in an angular arrangement.

Resonance Energy Considerations

Benzene, with its single aromatic ring, has a high resonance energy due to efficient electron delocalization. Naphthalene, with two rings, still retains significant resonance stabilization, making it more stable than anthracene and phenanthrene, which face structural constraints that reduce resonance stabilization.

Determining the Order

Considering the structures and resonance effects, the resonance energy per ring is highest for benzene followed by naphthalene due to the presence of fewer rings (more efficient electron delocalization per ring). Phenanthrene's angular structure allows better resonance energy compared to the linear structure of anthracene, which is less efficient.

Key concepts

Aromatic Compounds

Aromatic compounds are a unique class of molecules known for their stability and distinctive chemical properties. The hallmark of aromatic compounds is their ring-like structure, which allows for a special type of electron arrangement. This electron arrangement, called 'delocalization,' creates a cloud of electrons above and below the plane of the molecule, contributing to the compound's overall stability.

In the simplest form, an aromatic compound is benzene, which consists of a single six-carbon ring with alternating double bonds. These double bonds aren't fixed in place but instead form a stable structure through resonance, which is a key feature of aromaticity.

• Benzene is the simplest aromatic hydrocarbon, also known as an "arene."
• Polycyclic aromatic compounds include naphthalene, anthracene, and phenanthrene, which have multiple connected aromatic rings.
• Aromaticity confers special stability to molecules, keeping them intact under various conditions.

Understanding these foundational aspects of aromatic compounds helps to appreciate their role in chemistry, particularly regarding resonance energy and molecular stability.

Molecular Stability

Molecular stability in aromatic compounds is significantly influenced by resonance energy. This energy results from the delocalization of electrons across the aromatic ring structure, which reduces the overall energy of the molecule, making it more stable.

• Resonance energy is directly related to the stability of a molecule; higher resonance energy implies greater stability.
• Benzene is a classic example, as its structure allows for maximum resonance stabilization.
• Polycyclic aromatics, like naphthalene, anthracene, and phenanthrene, also benefit from resonance but vary in stability due to structural differences.

Each ring added to a polycyclic aromatic hydrocarbon creates more complexity and can potentially reduce stability depending on the arrangement of the rings.

Naphthalene, for instance, with its two fused rings, offers a resonance energy profile that is slightly different from benzene, but still retains significant molecular stability. Anthracene and phenanthrene, on the other hand, with three rings each, display variations in stability based on their distinctive structures.

Electron Delocalization

Electron delocalization plays a critical role in understanding the concept of resonance energy in aromatic compounds. This term refers to electrons spreading out over multiple atoms rather than being localized between two specific atoms, as in typical covalent bonds. In aromatic compounds, the delocalization occurs over the entire ring system, offering unique attributes.

The delocalized electrons form a "cloud" that extends above and below the carbon rings, contributing to the overall stability of the molecule. This phenomenon is often described using resonance structures, which are different but valid representations of the same molecule that highlight how electrons can be distributed across the structure.

• This electron cloud is facilitated by the overlapping p-orbitals of adjacent carbon atoms, which house these delocalized electrons.
• Such a configuration reduces localized electron pair interactions, decreasing overall energy and increasing stability.
• Efficient delocalization results in higher resonance energy, meaning more stable aromatic systems, like benzene compared to larger polycyclic aromatics.

In polycyclic aromatic compounds, the extent and efficiency of electron delocalization can be influenced by how the rings are arranged. For instance, phenanthrene's angular arrangement allows electrons to spread more effectively compared to anthracene's linear structure, highlighting the importance of structural geometry in electron delocalization.