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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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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

4.1K
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

4.0K
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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Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

14.1K
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 +...
14.1K
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

7.8K
All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
7.8K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

2.0K
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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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Cyclopropyl Group: An Excited-State Aromaticity Indicator?

Rabia Ayub1,2, Raffaello Papadakis1,2, Kjell Jorner1,2

  • 1Department of Chemistry-BMC, Uppsala University, Box 576, 751 23, Uppsala, Sweden.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 7, 2017
PubMed
Summary

The cyclopropyl group can indicate excited-state aromaticity. It remains closed on aromatic rings in excited states, unlike antiaromatic or nonaromatic rings, aiding in their distinction.

Keywords:
aromaticitydensity functional calculationselectronic structuresphotochemistrysmall-ring compounds

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Area of Science:

  • Photochemistry
  • Theoretical Chemistry
  • Organic Chemistry

Background:

  • Baird's rule predicts reversed aromaticity in excited states compared to the ground state.
  • Cyclopropyl groups are known probes for radical character.

Purpose of the Study:

  • To evaluate the cyclopropyl group as an indicator for excited-state aromaticity.
  • To distinguish between excited-state aromatic, antiaromatic, and nonaromatic systems.

Main Methods:

  • Quantum chemical calculations
  • Photoreactivity experiments

Main Results:

  • Cyclopropyl groups remain closed on excited-state aromatic systems.
  • Opening of cyclopropyl substituents on [4n]annulenes disrupts excited-state aromaticity.
  • Exceptions include polycyclic compounds like biphenylene and multi-heteroatom systems.

Conclusions:

  • The cyclopropyl group serves as a reliable indicator for excited-state aromaticity.
  • This method can differentiate excited-state aromatic from antiaromatic and nonaromatic compounds.