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Related Concept Videos

Newman Projections02:06

Newman Projections

Different notations are used to represent the three-dimensional structure of molecules on two-dimensional surfaces. One of the most commonly used representations is the dash-wedge formula. The dashed wedges, solid wedges, and the plane lines indicate the groups situated behind the plane, coming out of the plane, and in the plane, respectively.
The organic molecules rotate across the single bonds leading to numerous temporary three-dimensional structures of varying energy known as conformers.
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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...
Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN101:14

Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1

Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano...
1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Overview01:26

1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Overview

Nitrous acid and nitric acids are two types of acids containing nitrogen, among which nitrous acid is weaker than nitric acid. Nitrous acid with a pKa value of 3.37 ionizes in water to give a nitrite ion and the hydronium ion.
The nitrous acid is unstable. Hence, it is formed in situ from a solution of sodium nitrite and cold aqueous acids such as hydrochloric or sulfuric acid. In an acidic solution, the –OH group of nitrous acid undergoes protonation to give oxonium ion, followed by water loss...
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 with both...
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

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. Consider...

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Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes
12:07

Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes

Published on: April 1, 2013

A fresh look at aromatic dianions.

Thomas Sommerfeld1

  • 1Theoretische Chemie, Universität Heidelberg Im Neuenheimer Feld 229, 69120 Heidelberg, Germany. Thomas.Sommerfield@urz.uni-heidelberg.de

Journal of the American Chemical Society
|February 7, 2002
PubMed
Summary
This summary is machine-generated.

Aromatic hydrocarbon dianions are unstable and rapidly lose electrons. This study finds their lifetimes are surprisingly short, on the order of femtoseconds, challenging previous assumptions.

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Area of Science:

  • Quantum chemistry
  • Theoretical chemistry
  • Physical chemistry

Background:

  • Aromaticity typically implies stability, but dianions are exceptions.
  • Cyclooctatetraene and cyclobutadiene dianions are known to be unstable.
  • Understanding the stability of "aromatic" dianions is crucial.

Purpose of the Study:

  • To investigate the autodetachment lifetimes of aromatic hydrocarbon dianions.
  • To determine if aromatic pi systems enhance stability against electron loss.
  • To compare lifetimes of cross-conjugated (Y-aromatic) versus linear-conjugated dianions.

Main Methods:

  • First ab initio study of dianion autodetachment lifetimes.
  • Comparison with bound state calculations for metastable dianions.
  • Analysis of electron-electron repulsion effects on stability.

Main Results:

  • Hydrocarbon dianions exhibit very short autodetachment lifetimes (femtoseconds).
  • Aromaticity does not confer enhanced stability; it correlates with higher energies and shorter lifetimes.
  • Cross-conjugated dianions do not necessarily possess longer lifetimes than linear isomers.

Conclusions:

  • Gas-phase dianions are only stable if excess electrons localize in high-affinity subgroups.
  • Delocalizing electrons in a single pi system leads to extreme instability.
  • Electron scattering techniques are likely the only method to observe these transient species.