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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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

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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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Frost Circles for Different Conjugated Systems01:18

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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.
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Prochirality

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The concept of prochirality leads to the nomenclature of the individual faces of a molecule and plays a crucial role in the enantioselective reaction. It is a concept where two or more achiral molecules react to produce chiral products. A typical process is the reaction of an achiral ketone to generate a chiral alcohol. Here, the achiral reactant reacts with an achiral reducing agent, sodium borohydride, to generate an equimolar mixture of the chiral enantiomers of the product. For example, an...
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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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Diprotonated [28]hexaphyrins(1.1.1.1.1.1): triangular antiaromatic macrocycles.

Shin-ichiro Ishida1, Tomohiro Higashino, Shigeki Mori

  • 1Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502 (Japan).

Angewandte Chemie (International Ed. in English)
|March 12, 2014
PubMed
Summary

Protonation of [28]hexaphyrins induces conformational changes. Different acids yield distinct aromatic or antiaromatic species, with a designed hexaphyrin favoring antiaromaticity.

Keywords:
Möbius aromaticityantiaromaticityhexaphyrinprotonationtriangular shape

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Area of Science:

  • Supramolecular Chemistry
  • Organic Chemistry
  • Aromaticity Studies

Background:

  • Macrocyclic compounds like [28]hexaphyrins exhibit complex electronic properties.
  • Protonation is a key method for tuning the structure and aromaticity of macrocycles.
  • Understanding structure-property relationships in these systems is crucial for materials science.

Purpose of the Study:

  • To investigate the impact of protonation on the conformational and electronic properties of [28]hexaphyrins.
  • To explore the formation of different aromatic and antiaromatic species based on protonation conditions.
  • To design and synthesize a [28]hexaphyrin derivative that selectively forms an antiaromatic structure upon diprotonation.

Main Methods:

  • Synthesis of a peripherally hexaphenylated [28]hexaphyrin.
  • Protonation studies using different acids, specifically trifluoroacetic acid and methanesulfonic acid.
  • Spectroscopic and structural analysis to characterize the resulting species.

Main Results:

  • Protonation of [28]hexaphyrins induced significant conformational changes.
  • Trifluoroacetic acid led to monoprotonated species with Möbius aromatic character.
  • Methanesulfonic acid resulted in diprotonated, triangular, antiaromatic species.
  • The designed hexaphenylated [28]hexaphyrin favorably formed a triangular antiaromatic structure upon diprotonation.

Conclusions:

  • The choice of acid critically influences the outcome of [28]hexaphyrin protonation, dictating aromaticity and conformation.
  • Rational design of macrocycles can control the formation of specific antiaromatic structures.
  • These findings offer insights into the fundamental principles governing aromaticity in complex macrocyclic systems.