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Related Concept Videos

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 the 4n +...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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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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Aromatic Hydrocarbon Anions: Structural Overview01:18

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

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

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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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Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para...
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Dihydropyrene as an Aromaticity Probe for Partially Quinoid Push-Pull Systems.

Yves Garmshausen1, Kristin Klaue1, Stefan Hecht1

  • 1Department of Chemistry & IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor Strasse 2, 12489, Berlin, Germany.

Chempluschem
|January 22, 2020
PubMed
Summary

This study shows 15,16-dimethyl-15,16-dihydropyrene (DHP) is an excellent probe for measuring aromaticity using nuclear magnetic resonance (NMR) spectroscopy. A DHP derivative revealed 12% partial quinoid character, enhancing our understanding of aromaticity.

Keywords:
aromaticitydonor-acceptor systemsfused-ring systemsquinoidssolvatochromism

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Area of Science:

  • Organic Chemistry
  • Physical Chemistry
  • Spectroscopy

Background:

  • Aromaticity is crucial for understanding organic compound structure and reactivity.
  • Experimental quantification of aromaticity remains a significant challenge in chemistry.
  • Nuclear magnetic resonance (NMR) spectroscopy offers a pathway for experimental aromaticity measurements.

Purpose of the Study:

  • To evaluate 15,16-dimethyl-15,16-dihydropyrene (DHP) as a probe for experimental aromaticity quantification.
  • To synthesize and characterize a push-pull DHP derivative to assess its quinoid character.
  • To extend the utility of DHP in studying the phenomenon of aromaticity.

Main Methods:

  • Utilized proton NMR spectroscopy to observe high-field shifts of methyl groups in DHP due to ring-current effects.
  • Synthesized a DHP derivative with electron-donating and electron-accepting groups.
  • Employed solvatochromism and ring-current measurements to quantify partial quinoid character.

Main Results:

  • 15,16-dimethyl-15,16-dihydropyrene (DHP) was confirmed as a suitable probe for aromaticity.
  • A push-pull DHP derivative exhibited approximately 12% partial quinoid character.
  • NMR spectroscopy and solvatochromism provided quantitative measures of aromaticity and quinoid character.

Conclusions:

  • DHP is a valuable tool for the experimental study of aromaticity.
  • The synthesized DHP derivative provides insights into the interplay of electronic effects and aromaticity.
  • This research advances the understanding and measurement of aromaticity in organic chemistry.