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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...
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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.
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.

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Related Experiment Video

Updated: Jun 29, 2026

Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
09:35

Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

Published on: September 18, 2016

Push-pull vs captodative aromaticity.

Bagrat A Shainyan1, Anja Fettke, Erich Kleinpeter

  • 1A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of Russian Academy of Science, 1 Favorsky Street, 664033 Irkutsk, Russia. bagrat@irioch.irk.ru

The Journal of Physical Chemistry. A
|October 7, 2008
PubMed
Summary

This study theoretically examines vinylogous fulvalenes, revealing unique "push-pull" and "captodative" aromaticity. These findings quantify aromatic stabilization and magnetic properties, offering insights into novel molecular structures.

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Published on: October 18, 2018

Area of Science:

  • Computational Chemistry
  • Organic Chemistry
  • Theoretical Chemistry

Background:

  • Vinylogous fulvalenes, featuring cyclopropenyl and cyclopentadienyl moieties, represent a unique class of organic compounds.
  • Understanding the aromaticity and electronic properties of these molecules is crucial for developing new materials and understanding chemical bonding.

Purpose of the Study:

  • To theoretically investigate the aromaticity of vinylogous fulvalenes with cyclopropenyl and cyclopentadienyl groups.
  • To characterize novel 'push-pull' and 'captodative' aromaticity in these systems.
  • To quantify aromatic stabilization energy and magnetic properties using advanced computational methods.

Main Methods:

  • Density Functional Theory (DFT) calculations using the B3LYP/6-311G(d,p) level of theory.
  • Assessment of Aromatic Stabilization Energy (ASE) via homodesmotic reactions and heats of hydrogenation.
  • Calculation of Through-Space NMR Shieldings (TSNMRS) using the GIAO perturbation method and Nucleus Independent Chemical Shift (NICS) concept.

Main Results:

  • Both types of vinylogous fulvalenes exhibit distinct 'push-pull' and 'captodative' aromaticity.
  • The presence of aromatic moieties and a central double bond is essential for the stability of captodative mesoionic structures.
  • TSNMRS successfully visualizes and quantifies the partial aromaticity of the ring systems, correlating with electronic properties.

Conclusions:

  • Vinylogous fulvalenes represent a new class of aromatic compounds with tunable electronic properties.
  • The study provides a theoretical framework for understanding and designing molecules with push-pull and captodative electronic effects.
  • Computational methods like TSNMRS are valuable tools for characterizing aromaticity in complex molecular systems.