If benzene were 1,3,5-cyclohexatriene, the carbon-carbon...

Name: Problem 3, Chapter 14: Organic Chemistry
Uploaded: 2020-06-19T16:26:13-08:00
Duration: 5 min 8 s
Channel: Mercedes Mazza
Description: If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain. (Check your book to see figure)

If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain. (Check your book to see figure)

Key Concepts

Resonance Theory

Resonance theory describes how the true electronic structure of a molecule is best represented as a hybrid of several contributing structures rather than any single Lewis structure. The theory emphasizes that these contributors are merely a convenient way to depict electron delocalization and are not distinct, isolable forms of the molecule.

Resonance Contributors vs. Resonance Hybrid

The resonance contributors are hypothetical depictions used to illustrate different possible electron arrangements in a molecule. However, the actual structure is a resonance hybrid—a single, stable entity that is a weighted average of the contributors. Viewing the contributors as interconverting structures with distinct bond lengths is incorrect, as they are simply representations rather than real, dynamically shifting forms.

Bond Length Uniformity and Electron Delocalization

In molecules with delocalized electrons, such as aromatic compounds, the electron density is spread evenly over the structure, leading to uniform bond lengths that are intermediate between single and double bonds. This contrasts with a localized bonding model that would predict alternating bond lengths. The resonance hybrid concept explains the observed equalization of bond lengths.

Aromatic Stabilization

Aromatic stabilization arises from the delocalization of electrons in a cyclic, planar structure, which leads to enhanced stability compared to non-aromatic analogs. This delocalization cannot be properly represented by drawing alternating single and double bonds, as true aromatic compounds exhibit equal bond metrics due to the electron cloud being evenly distributed around the ring.