Question

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

Short answer

Correct answer: (c) Benzene > Naphthalene > Phenanthrene > Anthracene.

Step-by-step solution

Understand Resonance Energy

Resonance energy is the extra stability of a conjugated system due to resonance, typically measured per ring in aromatic compounds. More resonance energy per ring implies greater stability.

Review Structures of Compounds

Compare the structures: Benzene is a single aromatic ring; Naphthalene has two fused rings; Anthracene and Phenanthrene have three fused aromatic rings, but with different arrangements.

Evaluate Resonance Energy

Naphthalene has two aromatic rings sharing resonance which reduces per ring resonance energy compared to benzene. For Anthracene and Phenanthrene, having additional rings decreases per ring resonance stability.

Compare Stability Per Ring

Benzene has the highest per ring stability because it has the most delocalized pi electrons per ring. Phenanthrene is more stable than Anthracene per ring due to its more compact structure allowing better delocalization.

Order the Compounds

Based on stability, the order of resonance energy per ring is Benzene > Naphthalene > Phenanthrene > Anthracene.

Key concepts

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules, known for their unique stability and distinctive aromas. They contain a set of conjugated π-electrons, which are crucial for their chemical properties. These compounds must satisfy Hückel's rule, which states that they possess \(4n + 2\) π-electrons, where \(n\) is a non-negative integer. The simplest and most well-known example of an aromatic compound is benzene.

Aromatic compounds are characterized by their planar ring structures that allow for delocalization of electrons, which provides exceptional stability.

• The aromaticity contributes to its non-reactivity compared to other non-aromatic compounds.
• It results in lower reactivity, specifically making them less prone to addition reactions and favor substitution instead.

The stability these compounds possess is largely due to what is known as "resonance energy," which is the energy difference between its actual structure and the most stable Lewis Dot structure.

Benzene Stability

Benzene represents the paragon of stability among aromatic compounds. It's a hexagonal planar ring consisting of six carbon atoms, each bonded to a hydrogen atom. The six π-electrons in benzene are delocalized over the entire ring structure.

This delocalization is key to its stability and resistance to react with other substances. As a result, benzene will typically undergo substitution reactions rather than addition reactions.

• The resonance energy of benzene is particularly high, indicating significant stability.
• It manifests as a lower overall energy state for the molecule, making it less reactive.

Benzene's stability is an excellent reference point in the study of resonance energy, as it sets a baseline for comparing other aromatic compounds such as naphthalene and anthracene.

Conjugated Systems

Conjugated systems feature alternating single and double bonds that allow π-electrons to flow freely across atoms. This free movement of electrons is crucial for the concept of resonance and contributes significantly to the stability of these molecules.

In aromatic compounds, these conjugated systems play a vital role in their structure and reactivity.

• Such systems contribute to lower energy configurations due to electron delocalization.
• In naphthalene and anthracene, the conjugation is extended over two or more rings, impacting the resonance energy per ring.

This arrangement allows the system to absorb energy efficiently and release it slowly, further adding to its stability. The effectiveness of conjugated systems is best observed in suppressed reactivity and enhanced resonance energies.

Delocalized Electrons

Delocalized electrons are the driving force behind the exceptional stability of aromatic compounds. These electrons are not associated with a single atom or a single bond; instead, they are spread across several atoms in the molecule.

This feature allows for greater flexibility and resonance within the molecular structure. When electrons are delocalized, the molecule reaches a state of lower energy, thus becoming more stable.

• Delocalization results in the resonance stability found in aromatic compounds.
• It ensures that molecules like benzene have equal bond lengths, despite the presence of double bonds.

In summary, delocalization is essential for understanding why aromatic compounds possess such unique and desirable properties, especially their reduced reactivity and remarkable resonance energies.