Question

Determine which of these molecules has a more delocalized orbital, and justify your choice. (Hint: Both molecules contain two benzene rings. In naphthalene, the two rings are fused together. In biphenyl, the two rings are joined by a single bond, around which the two rings can rotate.)

Short answer

Naphthalene has a more delocalized orbital system due to its fused ring structure.

Step-by-step solution

Understand the Structure of Naphthalene

Naphthalene consists of two benzene rings fused together, sharing a pair of carbon atoms. This structural fusion allows for continuous overlap of p orbitals across both rings, facilitating greater electron delocalization within the entire molecule.

Understand the Structure of Biphenyl

Biphenyl consists of two benzene rings connected by a single bond. This single bond allows for rotation, which can disrupt the overlap of p orbitals between the rings, especially if the rings are not coplanar, reducing the potential for electron delocalization between them.

Analyze Delocalization in Naphthalene

Due to the fused ring structure of naphthalene, the electrons in the π orbitals can delocalize over a larger area, encompassing both rings. The shared bonds between the rings facilitate resonance and an extended conjugated system.

Analyze Delocalization in Biphenyl

In biphenyl, although each benzene ring can sustain its own resonance, the potential for delocalization across both rings as a single system is limited. The single bond connecting the rings might hinder optimal p orbital overlap, especially if the rings are not in the same plane.

Compare and Conclude

Naphthalene's fused ring structure inherently supports more extensive delocalization due to uninterrupted conjugation. This contrasts with biphenyl's potential rotational barrier, making naphthalene the molecule with more delocalized orbitals.

Key concepts

Naphthalene Structure

Naphthalene is an aromatic hydrocarbon that consists of two fused benzene rings. In its structure, these rings share two carbon atoms, creating a continuous structure without interruption. This fusion is crucial because it allows the p orbitals from the carbon atoms to overlap seamlessly across both rings. As a result, the electrons, which occupy these overlapping p orbitals, can move freely around the entire molecule.

One of the key results of this structural feature is enhanced electron delocalization. In naphthalene, all the electrons in the π (pi) orbitals can participate in creating a unified electron cloud over both rings. This type of electron movement contributes to the molecule’s stability and is a key concept in understanding its aromatic nature.

Biphenyl Structure

Biphenyl, another type of aromatic hydrocarbon, features two distinct benzene rings connected by a single bond. Unlike naphthalene, where the rings are fused, biphenyl's rings are only linked by this single bond, allowing them to rotate independently of each other. This freedom of rotation is significant because it can hinder the optimal overlap of p orbitals if the rings are not aligned.

When a biphenyl molecule's rings are coplanar, or lying flat, p orbital overlap can occur, facilitating some electron delocalization. However, if the rings are twisted, the overlap is reduced, limiting the potential for shared electron movement across both rings. Consequently, biphenyl does not exhibit the same level of electron delocalization as seen in fused-ring structures like naphthalene.

Electron Delocalization

Electron delocalization is a key concept in understanding molecular stability, particularly in aromatic compounds. This process involves electrons moving freely over a series of connected p orbitals. The more seamless the overlap of these orbitals, the greater the electron delocalization.

In naphthalene, the structural fusion of the benzene rings leads to a more extensive overlap, providing an expansive electron cloud over the entire molecule. In contrast, biphenyl’s single bond connection between the benzene rings may result in interrupted overlap, especially if the rings are not coplanar, thus limiting electron delocalization. Enhanced electron delocalization increases the molecule's aromatic stability, making it less reactive and more energetically favorable.

Conjugation

Conjugation refers to the interaction between alternating single and multiple bonds, allowing for electron delocalization across all the atoms involved. This is essential in many organic structures for providing stability and lowering the molecule's energy state.

In naphthalene, conjugation is facilitated by the direct overlap of p orbitals across the fused rings, creating a large, delocalized system. This system allows pi electrons to flow freely, preserving the aromatic character throughout the structure. Conversely, biphenyl has a potential break in conjugation if the benzene rings twist or are not perfectly aligned, as the single bond between them may not maintain optimal interaction for delocalization.

Resonance

Resonance is the way we describe the delocalization of electrons in molecules that cannot be represented by a single Lewis structure. In organic chemistry, particularly in aromatic systems, resonance serves as an indicator of stability.

Naphthalene has several resonance structures due to its continuous electron cloud sharing between the fused rings, reflecting its highly stable nature. These structures provide a visual representation of how electrons can delocalize over several atoms, reinforcing the molecule’s aromatic character. On the other hand, biphenyl, with its potential ordeal in maintaining coplanarity, may have fewer resonance structures available to depict complete delocalization, emphasizing a lesser degree of stability. Resonance structures in biphenyl demonstrate this struggle for complete p orbital overlap.