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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.
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...
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

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...
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
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Una nueva mirada a las dianiones aromáticas.

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
Resumen

Los dianiones de hidrocarburos aromáticos son inestables y pierden rápidamente electrones. Este estudio encuentra que sus vidas son sorprendentemente cortas, del orden de los femtosegundos, lo que desafía las suposiciones anteriores.

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Área de la Ciencia:

  • La química cuántica es una química cuántica.
  • Química teórica es la química teórica.
  • Química física es la química física de las cosas.

Sus antecedentes:

  • La aromaticidad típicamente implica estabilidad, pero las dianiones son excepciones.
  • Se sabe que los dianiones de ciclooctatetraeno y ciclobutadieno son inestables.
  • Comprender la estabilidad de las dianiones "aromáticas" es crucial.

Objetivo del estudio:

  • Para investigar los tiempos de vida de desprendimiento automático de dianiones de hidrocarburos aromáticos.
  • Para determinar si los sistemas pi aromáticos mejoran la estabilidad contra la pérdida de electrones.
  • Para comparar las vidas de los dianiones conjugados cruzados (Y-aromáticos) frente a los dianiones conjugados lineales.

Principales métodos:

  • Primer estudio ab initio de las vidas de separación automática de dianion.
  • Comparación con los cálculos del estado ligado para dianiones metastables.
  • Análisis de los efectos de la repulsión electrón-electrón en la estabilidad.

Principales resultados:

  • Los dianiones de hidrocarburos exhiben vidas de desprendimiento automático muy cortas (femtosegundos).
  • La aromaticidad no confiere mayor estabilidad; se correlaciona con energías más altas y vidas más cortas.
  • Los dianiones conjugados cruzados no necesariamente poseen vidas más largas que los isómeros lineales.

Conclusiones:

  • Los dianiones de fase gaseosa sólo son estables si el exceso de electrones se localiza en subgrupos de alta afinidad.
  • La deslocalización de electrones en un solo sistema pi conduce a una inestabilidad extrema.
  • Las técnicas de dispersión de electrones son probablemente el único método para observar estas especies transitorias.