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Related Concept Videos

Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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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...
10.6K
Aromatic Compounds: Overview01:25

Aromatic Compounds: Overview

11.8K
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...
11.8K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

3.5K
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...
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Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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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.
3.3K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

4.4K
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,...
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Aromaticity in transition structures.

Paul von Ragué Schleyer1, Judy I Wu, Fernando P Cossío

  • 1Department of Chemistry, Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, USA.

Chemical Society Reviews
|March 19, 2014
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Aromaticity, a key chemical concept, applies to transition structures, influencing reaction pathways. This review contrasts aromatic and non-aromatic transition structures in various reactions.

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Area of Science:

  • Organic Chemistry
  • Physical Chemistry
  • Computational Chemistry

Background:

  • Aromaticity explains stability and reactivity in unsaturated organic compounds.
  • The concept extends to inorganic molecules, saturated systems, and transition structures.
  • Not all delocalized transition structures exhibit aromaticity.

Purpose of the Study:

  • To review the application of aromaticity to chemical reaction transition structures.
  • To contrast aromaticity in pericyclic versus pseudo-pericyclic reaction transition structures.
  • To briefly describe computational methods for evaluating aromaticity in transition structures.

Main Methods:

  • Review of existing literature on aromaticity and transition structures.
  • Comparative analysis of pericyclic and pseudo-pericyclic reaction transition structures.
  • Brief overview of computational chemistry techniques.

Main Results:

  • Aromaticity plays a significant role in pericyclic reaction transition structures.
  • Aromaticity is less important or absent in pseudo-pericyclic reaction transition structures.
  • Non-pericyclic reactions can also feature aromatic transition structures.

Conclusions:

  • Aromaticity is a relevant, though not universal, property of chemical transition structures.
  • Understanding aromaticity in transition states aids in predicting reaction outcomes.
  • Computational methods are crucial for assessing aromaticity in these transient species.