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Related Concept Videos

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.
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...
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...
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 Compounds: Overview01:25

Aromatic Compounds: Overview

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 isolated benzene...
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 Videos

26pi aromatic core-modified hexaphyrins: syntheses, characterization, and structural diversities.

Rajneesh Misra1, Rajeev Kumar, Tavarekere K Chandrashekar

  • 1Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India.

The Journal of Organic Chemistry
|February 10, 2007
PubMed
Summary

Researchers synthesized novel core-modified hexaphyrins using MacDonald condensation. Catalyst choice and concentration influenced product yield and structure, with varying degrees of heterocyclic ring inversion observed in different hexaphyrin derivatives.

Related Experiment Videos

Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Hexaphyrins are macrocyclic compounds with unique electronic and structural properties.
  • Core modification offers a route to tune hexaphyrin characteristics.
  • MacDonald condensation is a key reaction for synthesizing porphyrinoids.

Purpose of the Study:

  • To synthesize and characterize novel 26pi core-modified hexaphyrins.
  • To investigate the influence of acid catalysts on synthesis yield and product formation.
  • To elucidate the solution structure and conformational mobility of these hexaphyrins.

Main Methods:

  • Acid-catalyzed MacDonald-type condensation of tripyrrane with pentafluorobenzaldehyde.
  • Varying acid catalysts (TFA, PTSA) and concentrations.
  • Structural characterization using 1H, 2-D COSY, and HSQC NMR spectroscopy.
  • Electronic absorption spectroscopy.

Main Results:

  • Successful synthesis of dioxa-, dithia-, and diselenahexaphyrins.
  • Product yield and structure were dependent on catalyst type and concentration.
  • NMR studies revealed distinct solution structures and conformational mobility, including varying degrees of heterocyclic ring inversion.
  • Electronic absorption spectra supported observed conformational changes upon protonation.

Conclusions:

  • The synthetic strategy allows for the preparation of diverse core-modified hexaphyrins.
  • Catalyst control is crucial for directing the synthesis of specific hexaphyrin derivatives.
  • Core modification significantly impacts hexaphyrin conformation and electronic properties.