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Related Concept Videos

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...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
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...
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.
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.
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.

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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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Double aromaticity and ring currents in all-carbon rings.

Patrick W Fowler1, Noriyuki Mizoguchi, David E Bean

  • 1Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK. p.w.fowler@sheffield.ac.uk

Chemistry (Weinheim an Der Bergstrasse, Germany)
|June 9, 2009
PubMed
Summary

Neutral carbon rings exhibit double aromaticity, confirmed by visualizing ring currents. This aromaticity arises from both in-plane and out-of-plane pi systems, following Hückel

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Area of Science:

  • Theoretical Chemistry
  • Computational Chemistry
  • Materials Science

Background:

  • Aromaticity is a fundamental concept in chemistry, typically associated with delocalized pi electrons in cyclic systems.
  • Understanding aromaticity in novel carbon allotropes is crucial for predicting their electronic and chemical properties.

Purpose of the Study:

  • To demonstrate and visualize the phenomenon of double aromaticity in neutral, planar carbon rings.
  • To investigate the behavior of both in-plane and out-of-plane pi electron systems.

Main Methods:

  • Density Functional Theory (DFT) calculations using the B3LYP/6-31G(d)//B3LYP/6-31G(d) level of theory.
  • Visualization of induced ring currents at the ipsocentric level for carbon rings C(6) to C(30).
  • Analysis of electron distribution in relation to Hückel's rule and Frost-Musulin diagrams.

Main Results:

  • Onset of delocalized current in the in-plane pi system observed at C(10)/C(11).
  • Both in-plane and out-of-plane pi systems exhibit diatropic/paratropic currents, consistent with Hückel's rule.
  • Out-of-plane pi system with 4m+2 electron occupation takes precedence, as predicted by Frost-Musulin diagrams.
  • Current-density maps reveal distinct double-doughnut and double-track topographies for out-of-plane and in-plane currents, respectively.

Conclusions:

  • Neutral planar carbon rings display double aromaticity, involving both in-plane and out-of-plane pi systems.
  • The observed aromaticity is governed by a unified framework of angular momentum rules.
  • These findings expand the understanding of aromaticity beyond traditional models.