Question

There are five resonance structures of phenanthrene, one of which is shown. Draw the other four.

Short answer

Phenanthrene has five resonance structures due to electron delocalization across its aromatic rings.

Step-by-step solution

Understand Resonance

Resonance structures are different ways of representing the same molecule where electrons are distributed differently. In aromatic compounds like phenanthrene, the resonance structures depict the delocalization of \( \pi \) electrons in the ring.

Recognize the Structure

Phenanthrene consists of three fused benzene rings. Imagine it as three hexagonal rings sharing sides, creating a curved structure rather than linear.

Assign Electron Movement

The \( \pi \) electrons, found in the double bonds, can move around the rings. Identify each double bond in phenanthrene and think about how the electrons could move to form new bonds.

Draw Resonance Structure 1

One possible structure involves shifting electrons from one bond to the neighboring position within the rings, altering the positions of double bonds in each of the three rings.

Draw Resonance Structure 2

In the next structure, shift another set of electrons around one of the connections between the rings, thereby changing the positions of the double bonds again.

Draw Resonance Structure 3

Consider yet another electron movement, this time focusing on the other side of the molecule. This movement should create a different pattern of alternating single and double bonds in place of the original.

Draw Resonance Structure 4

Finally, shift a different grouping of electrons, potentially restoring some double bond positions from earlier structures, but altering others, achieving another unique resonance form.

Key concepts

Resonance Structures

Resonance structures are like snapshots of different possible arrangements of electrons within a molecule. In the case of phenanthrene, an aromatic hydrocarbon, these structures help illustrate how electrons can be shared across multiple atoms.
This concept is crucial, as it provides a more comprehensive understanding of how molecules behave.

• Multiple Representations: Resonance structures don't represent real, distinct molecules. They are different ways to depict electron distribution.
• Electron Mobility: By showing electrons in various positions, resonance structures capture the possible states electrons can occupy.
• Overall Stability: The true structure of the molecule is a hybrid, with electron density being a balance of all possible resonance forms.

This idea is especially useful with compounds like phenanthrene to show the flexibility and sharing capability of electrons in its structure.

Aromatic Compounds

Aromatic compounds are a special class of molecules known for their ring structures and stability. Phenanthrene is an example, composed of three benzene rings fused together.
These intriguing molecules are marked by specific features that distinguish them from other compounds.

• Planar Structure: Aromatic compounds are typically flat, allowing for better overlap of \( \pi \) electrons.
• Aromaticity Rule: They follow Huckel’s rule, possessing \( 4n + 2 \) \( \pi \) electrons (where n is an integer), contributing to their stability.
• Unique Stability: This stability arises from electron delocalization over the entire ring structure.

Aromatic compounds embrace resonance and delocalization, adding to their unique chemical characteristics.

Delocalization of Electrons

Delocalization is a phenomenon where electrons are spread over several atoms, rather than localized between two.
In phenanthrene, this delocalization is depicted through its multiple resonance structures.

• Spread Across Bonds: \( \pi \) electrons in phenanthrene aren’t confined to a single bond but move across the entire ring system.
• Increases Stability: The distribution of electrons lowers the overall energy, making phenanthrene more stable than if those electrons were locked in single positions.
• Contributing to Aromaticity: This electron spread is crucial for defining phenanthrene's aromatic properties, as it satisfies the need for \( 4n + 2 \) \( \pi \) electrons.

Delocalization is a key player in the stability and characteristics of aromatic compounds.

Fused Benzene Rings

Phenanthrene consists of fused benzene rings, where two rings share a pair of carbons. This structure is essential in determining its chemical properties.
Fusing affects both the shape and electronic characteristics of the molecule.

• Shared Carbons: The junction in fused rings allows for seamless electron flow between the rings.
• Complex Aromatic System: While each benzene ring individually is aromatic, their fusion leads to a larger, unified aromatic system.
• Hybrid Resonance Forms: Due to fusion, the potential resonance structures multiply, reflecting complex electron delocalization pathways.

Fused benzene rings create a more intricate aromatic landscape, critical for phenanthrene’s stability and reactivity.