Question

Explain why the concept of delocalized molecular orbitals is essential to an understanding of bonding in the benzene molecule, \(\mathrm{C}_{6} \mathrm{H}_{6}\)

Short answer

The concept of delocalized molecular orbitals, which allows electrons to move freely across multiple atoms, is essential to understanding bonding in the benzene molecule. In benzene, six p electrons form delocalized molecular orbitals across all six carbon atoms, creating a ring of pi electron density that stabilizes the molecule and contributes to its unique chemical behavior.

Step-by-step solution

Describe Benzene Structure

Benzene, \( C_{6} H_{6} \), is an organic compound composed of six carbon atoms arranged in a hexagonal planar ring. Each carbon atom is bonded to its two neighbors and one hydrogen atom through sigma bonds. This leaves one electron in a p orbital perpendicular to the plane of the ring on each carbon atom.

Explain Concept of Delocalized Molecular Orbitals

In many molecules, electrons are not confined between two adjacent atoms, but rather can be spread over three or more atoms, creating delocalized molecular orbitals. These orbitals extend over several adjacent atoms, allowing electrons, particularly π electrons, to move freely across multiple atoms.

Link Concept of Delocalized Molecular Orbitals to Benzene

The six p electrons in the benzene molecule, which were not used in the sigma bonding, form delocalized molecular orbitals across all six carbon atoms. This delocalization creates a ring of pi electron density above and below the plane of the carbon atoms. It stabilizes the benzene molecule and contributes to its unique chemical behavior.

Key concepts

Benzene Structure

Benzene, with the formula \( C_6H_6 \), is a fundamental molecule in organic chemistry, cherished for its stability and symmetrical structure. The benzene molecule is made of six carbon atoms that form a perfect hexagon. Each carbon atom in benzene shares electrons with its two adjacent carbon neighbors and one hydrogen atom, forming strong sigma (\( \sigma \)) bonds. These sigma bonds make up the simple framework or skeleton of benzene, ensuring the planar, hexagonal shape remains intact.

Every carbon atom in benzene also has an additional electron in a p orbital that stands perpendicular to the plane formed by the sigma bonds. These extra electrons are crucial, as they are not fixed between two specific atoms, leading to an interesting phenomenon known as electron delocalization, a key feature in benzene's renowned stability.

Sigma and Pi Bonds

In benzene and many other molecules, two types of bonds set the stage for electron interaction: sigma and pi (\( \pi \)) bonds. Sigma bonds are the result of head-on overlapping between atomic orbitals. They primarily contribute to the molecule's structural framework, forming a stable connection between atoms in the benzene ring. These bonds are quite strong due to the direct overlap of orbitals, which allows them to hold the atoms in fixed positions relative to one another.

In addition to sigma bonds, benzene has pi bonds, which arise from the lateral overlap of p orbitals. Each carbon atom's additional p orbital electron can engage in this side-by-side overlap. However, what's remarkable about pi bonds in benzene is their delocalization. Instead of being confined between just two carbon atoms, pi electrons form a continuous ring of electron density that encompasses the entire benzene ring. This pi electron cloud above and below the plane of the carbon atoms offers benzene its unique traits, such as exceptional stability and less reactive behavior compared to many other alkenes.

Electron Delocalization

Electron delocalization is a powerful concept in understanding the molecular structure and stability of benzene. In many molecules, electrons are typically localized, moving between two directly bonded atoms. Delocalization in benzene, however, allows the electrons to be shared among several atoms at once. Specifically, in benzene, the six carbon atoms share their p orbital electrons across the entire molecule.

This widespread sharing forms a delocalized pi electron cloud or molecular orbital that stretches above and below the benzene ring.

• It results in a significant increase in the molecule's stability, known as resonance stabilization.
• This electron cloud is uniform, meaning that every carbon-carbon bond in benzene is of equal length and has partial double-bond character.
• The delocalized orbital nature also imparts unique chemical properties to benzene, such as its unpredictably high resistance against addition reactions typically seen in alkenes.

By understanding electron delocalization, one can better grasp why benzene is less reactive and more stable than other unsaturated hydrocarbons.