Related Experiment Video
Updated: Apr 22, 2026
09:35
Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
Published on: September 18, 2016
9.7K
A quantitative metric for conjugation in polyene hydrocarbons having a single classical structure.
1Department of Chemistry University of Missouri Kansas City, Missouri 64110-2499, United States.
The Journal of Physical Chemistry. A
|October 14, 2014
Summary
A new quantitative measure, the number of Dewar resonance structures (DS), precisely quantifies conjugation in hydrocarbons. Higher DS numbers indicate greater conjugation and enhanced stability for these molecules.
Area of Science:
- Organic Chemistry
- Physical Chemistry
Background:
- Conjugation is a key concept in understanding hydrocarbon stability and reactivity.
- Existing measures of conjugation lack quantitative precision.
- Valence-bond theory provides a framework for analyzing electronic interactions in conjugated systems.
Purpose of the Study:
- To introduce a quantitative measure for the extent of conjugation in hydrocarbons.
- To establish a relationship between conjugation and molecular stability.
- To explore different levels of conjugation in various hydrocarbon systems.
Main Methods:
- Introduction of Dewar resonance structures (DS) as a quantitative metric.
- Analysis of purely cross-conjugated hydrocarbons and polyenes.
- Application of a valence-bond (VB) approach to conjugated hydrocarbons with a single classical structure (K=1).
Main Results:
- The number of Dewar resonance structures (DS) directly quantifies conjugation interaction.
- Greater DS numbers correlate with increased hydrocarbon stability.
- Three distinct levels of conjugation (cross-conjugated, linear extended two-way, ring-enhanced) were identified for systems with a single classical structure.
Conclusions:
- Dewar resonance structures offer a novel and quantitative perspective on conjugation.
- This approach provides a new understanding of the stability of conjugated hydrocarbons.
- The derived analytical expressions offer predictive power for conjugation extent.
Related Concept Videos
01:16Structure of Conjugated Dienes
5.4K
Introduction
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the...
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the...
5.4K
01:28Stability of Conjugated Dienes
3.3K
Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
3.3K
01:17Thermal Electrocyclic Reactions: Stereochemistry
1.6K
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
1.6K
01:24π Molecular Orbitals of 1,3-Butadiene
8.5K
Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the...
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the...
8.5K
01:18Aromatic Hydrocarbon Cations: Structural Overview
3.4K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
Removing one hydrogen from the intervening CH2 group...
3.4K
01:18Aromatic Hydrocarbon Anions: Structural Overview
3.4K
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
Due to the absence of continuous...
3.4K