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Aromatic Compounds: Overview01:25

Aromatic Compounds: Overview

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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...
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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

Five-Membered Heterocyclic Aromatic Compounds: Overview

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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,...
4.3K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

3.4K
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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NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

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Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
4.7K
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...
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A Direct, Regioselective and Atom-Economical Synthesis of 3-Aroyl-N-hydroxy-5-nitroindoles by Cycloaddition of 4-Nitronitrosobenzene with Alkynones
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Novel aromatic and antiaromatic systems.

Ronald Breslow1

  • 1Department of Chemistry, Columbia University, New York, NY, 10027, USA. rb33@columbia.edu.

Chemical Record (New York, N.Y.)
|October 23, 2014
PubMed
Summary

Researchers synthesized the cyclopropenyl cation, the first non-six pi electron aromatic system. This work explored aromaticity in systems with 4n+2 and 4n pi electrons, finding evidence for antiaromaticity in the latter, impacting conductivity studies.

Area of Science:

  • Organic Chemistry
  • Aromaticity Studies
  • Materials Science

Background:

  • Pioneering work in non-benzenoid aromatic compounds, including tropolone chemistry.
  • Exploration of aromaticity beyond the traditional six pi electron systems.
  • Established Hückel's rule (4n+2 pi electrons) for aromaticity.

Purpose of the Study:

  • To synthesize and characterize novel non-benzenoid aromatic compounds.
  • To investigate the properties of systems with 4n cyclically conjugated pi electrons.
  • To examine the electrical conductivity of aromatic and antiaromatic systems.

Main Methods:

  • Synthesis of the cyclopropenyl cation and its derivatives.
  • Investigation of monocyclic systems with 4n pi electrons.
Keywords:
cyclopropeneelectrical conductivitymolecular wiresnanosciencetriplet state

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  • Study of fused systems combining aromatic and antiaromatic components.
  • Measurement of electrical conductivities in various organic molecules.

    • Main Results:

    • Successful creation of the cyclopropenyl cation, the simplest aromatic system with non-six pi electrons.
    • Evidence supporting antiaromaticity in 4n pi electron systems like cyclopropenyl anion and cyclobutadiene.
    • Observation that aromaticity increases electrical resistance in molecules like thiophene.

    Conclusions:

    • Non-benzenoid aromatic compounds expand the understanding of aromaticity.
    • Antiaromatic compounds exhibit unique electronic properties, potentially leading to enhanced conductivity.
    • Antiaromatic molecules are predicted to be superior electrical conductors compared to aromatic or non-aromatic counterparts.