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Related Concept Videos

Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n + 2 rule.
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively.
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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 overlap of p...
Aromatic Compounds: Overview01:25

Aromatic Compounds: Overview

In general, the term ‘aromatic’ indicates a pleasant smell or fragrance from fresh flowers, freshly prepared coffee, etc. In the early history of organic chemistry, many benzene derivatives were isolated from the pleasant odor oils of the plants. For example, vanillin was isolated from the oil of vanilla, methyl salicylate from the oil of wintergreen, and cinnamaldehyde from the oil of cinnamon. They all had a pleasant odor; hence the name aromatic was given.
In 1825, Faraday isolated benzene...

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Aromatic pathways in twisted hexaphyrins.

Heike Fliegl1, Dage Sundholm, Stefan Taubert

  • 1Department of Chemistry, P.O. Box 55 (A.I. Virtanens plats 1), FIN-00014 University of Helsinki, Finland. Heike.Fliegl@helsinki.fi

The Journal of Physical Chemistry. A
|June 18, 2010
PubMed
Summary
This summary is machine-generated.

This study reveals that expanded porphyrins

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Area of Science:

  • Computational chemistry
  • Organic chemistry
  • Aromaticity studies

Background:

  • Expanded porphyrins are macrocyclic compounds with unique electronic properties.
  • Determining the aromaticity of these systems is crucial for understanding their stability and reactivity.
  • Previous studies suggested specific current pathways for aromaticity in these molecules.

Purpose of the Study:

  • To investigate the aromatic pathways and degree of aromaticity in expanded porphyrins.
  • To elucidate the magnetically induced current routes and strengths.
  • To challenge existing models of current flow in these systems.

Main Methods:

  • Gauge-including magnetically induced current (GIMIC) approach.
  • Density functional theory (DFT) calculations.
  • Analysis of pi-electron pathways and ring currents.

Main Results:

  • Hexaphyrins obeying the (4n + 2) pi-electron rule are aromatic; those obeying the 4n rule are antiaromatic.
  • [26]- and [30]hexaphyrins are aromatic, [28]hexaphyrin is antiaromatic, and [24]hexaphyrin is nonaromatic.
  • Current pathways are not always along the inner C-N-C route; they can split or use the outer C=C route, especially in [24]hexaphyrin.

Conclusions:

  • Explicit calculation of current pathways is essential for accurate aromaticity assessment.
  • The pyrrole rings in hexaphyrins do not sustain strong individual ring currents.
  • Current flow models in expanded porphyrins need refinement based on calculated pathways.