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Related Concept Videos

Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Qualitative Analysis03:46

Qualitative Analysis

For solutions containing mixtures of different cations, the identity of each cation can be determined by qualitative analysis. This technique involves a series of selective precipitations with different chemical reagents, each reaction producing a characteristic precipitate for a specific group of cations. Metal ions within a group are further separated by varying the pH, heating the mixture to redissolve a precipitate, or adding other reagents to form complex ions.
For instance, group IV...
Complexation Equilibria: Factors Influencing Stability of Complexes01:09

Complexation Equilibria: Factors Influencing Stability of Complexes

In complexation reactions, metal cations are the electron pair acceptors, and the ligands are the electron pair donors. The stability of the metal complexes depends primarily on the complexing ability of the central metal ion and the nature of the ligands. Generally, the complexing ability of the metal ion depends on the size and charge of the ion. As the metal ion size increases, the stability of the metal complexes decreases, provided that the valency of the metal ion and the ligands remain...
Crystal Field Theory - Octahedral Complexes02:58

Crystal Field Theory - Octahedral Complexes

Crystal Field Theory
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...
Colors and Magnetism03:02

Colors and Magnetism

Color in Coordination Complexes
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
Crystal Field Theory - Tetrahedral and Square Planar Complexes02:46

Crystal Field Theory - Tetrahedral and Square Planar Complexes

Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...

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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

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Hyperaromatic stabilization of arenium ions.

Jaya S Kudavalli1, Derek R Boyd, Dara Coyne

  • 1School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.

Organic Letters
|November 9, 2010
PubMed
Summary

The dehydration rates of benzene-cis- and trans-1,2-dihydrodiols differ significantly due to distinct intermediate conformations. A specific conformation is stabilized by hyperconjugation and aromatic no-bond resonance, explaining the observed kinetics.

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Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals
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Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

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Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals
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Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

Published on: April 14, 2020

Area of Science:

  • Organic Chemistry
  • Physical Chemistry
  • Reaction Kinetics

Background:

  • Acid-catalyzed dehydration is a fundamental reaction in organic chemistry.
  • Understanding reaction mechanisms requires detailed analysis of intermediates and transition states.
  • Stereochemistry plays a crucial role in determining reaction pathways and rates.

Purpose of the Study:

  • To investigate the significant rate difference in acid-catalyzed dehydration between benzene-cis- and trans-1,2-dihydrodiols.
  • To elucidate the structural and electronic factors governing the observed stereoselectivity.
  • To provide a mechanistic explanation supported by computational and experimental data.

Main Methods:

  • Kinetic studies to determine the relative dehydration rates (k(cis)/k(trans)).
  • Computational chemistry (MP2 calculations) to model intermediate structures and energies.
  • Analysis of resonance structures and hyperconjugation effects.
  • Comparison with pK(R) measurements for related aromatic carbocations.

Main Results:

  • A dramatic rate difference of k(cis)/k(trans) = 4500 was observed.
  • The cis isomer dehydrates significantly faster than the trans isomer.
  • A β-hydroxycarbocation intermediate's conformation, stabilized by hyperconjugation and aromatic no-bond resonance, was identified as key.
  • Benzoannelation was found to destabilize benzenium ions, as evidenced by pK(R) values.

Conclusions:

  • The pronounced difference in dehydration rates is attributed to the conformational preferences of the β-hydroxycarbocation intermediate.
  • Aromatic no-bond resonance and hyperconjugation significantly stabilize the reactive intermediate derived from the cis-diol.
  • Computational and experimental data collectively support the proposed mechanism involving conformational stabilization.