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E1 Reaction: Kinetics and Mechanism02:46

E1 Reaction: Kinetics and Mechanism

15.9K
Here, in contrast to the E2 reaction mechanism, we delve into the aspects of the E1 reaction mechanism, which has two steps: rate-limiting loss of the leaving group and abstraction of the beta hydrogen by a weak base. Typically, the experimental proof for the E1 mechanism is via kinetic studies or isotope studies. While the former demonstrates the first-order kinetics—the dependence of the reaction solely on substrate concentration—the latter proves the abstraction of hydrogen only...
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SN1 Reaction: Mechanism02:25

SN1 Reaction: Mechanism

12.4K
Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism. 
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a...
12.4K
Alkyl Halides02:45

Alkyl Halides

17.5K
Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp3-hybridized and exhibits a tetrahedral shape.
Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp2 -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl...
17.5K
E2 Reaction: Kinetics and Mechanism02:45

E2 Reaction: Kinetics and Mechanism

10.7K
SN2 substitutions and E2 eliminations of alkyl halides proceed via a concerted pathway. While the nucleophile attacks the alpha carbon in SN2 reactions, it functions as a strong base and abstracts a beta hydrogen in the E2 mechanism. The rate-limiting transition state in E2 elimination reactions is characterized by partially broken carbon–hydrogen and carbon–halogen bonds and a partially formed pi bond between the alpha and beta carbons. The beta hydrogen and halide are eliminated...
10.7K
Elimination Reactions02:25

Elimination Reactions

14.3K
A nucleophile can react with an alkyl halide to give the substitution product by displacing the halogen. Or it can function as a base to give the elimination product by deprotonation of the neighboring carbon to form an alkene. In an elimination reaction, the substrate loses two groups from adjacent carbons forming at least one π bond. The carbon attached to the halogen is called the α carbon, while the adjacent carbon is called the β carbon; hence, these reactions are called...
14.3K
Predicting Products: Substitution vs. Elimination02:52

Predicting Products: Substitution vs. Elimination

12.4K
When a nucleophile and an alkyl halide react, nucleophilic substitution and β-elimination reactions compete to generate products.
The following factors can influence the mechanisms competing against each other:
12.4K

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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding
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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

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Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers.

Alexander M Deetz1, Gerald J Meyer1

  • 1Department of Chemistry, University of North Carolina at Chapel Hill, Murray Hall 2202B, Chapel Hill, North Carolina 27599-3290, United States.

JACS Au
|May 13, 2022
PubMed
Summary

This study quantifies ion stabilization using a competitive kinetic approach, revealing shifts in reduction potentials for bromide and iodide ions. Findings challenge assumptions about stabilization mechanisms, particularly for polarizable halogens.

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Ion Mobility-Mass Spectrometry Techniques for Determining the Structure and Mechanisms of Metal Ion Recognition and Redox Activity of Metal Binding Oligopeptides
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Ion Mobility-Mass Spectrometry Techniques for Determining the Structure and Mechanisms of Metal Ion Recognition and Redox Activity of Metal Binding Oligopeptides
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Area of Science:

  • Photochemistry
  • Physical Chemistry
  • Supramolecular Chemistry

Background:

  • Experimental determination of ion and radical stabilization is challenging, especially for short-lived species.
  • Stabilization significantly influences reaction kinetics and thermodynamics.
  • Quantifying stabilization magnitude requires advanced methodologies.

Purpose of the Study:

  • To develop and apply a competitive kinetic approach for quantifying halide ion stabilization.
  • To determine the impact of specific stabilizing groups on halide ion reduction potentials.
  • To investigate the role of noncovalent interactions in ion stabilization.

Main Methods:

  • Utilized a [Ir(dF-(CF3)-ppy)2(tmam)]3+ photocatalyst with a dicationic tmam ligand for halide ion association.
  • Employed competitive kinetic quenching assays in acetonitrile solutions.
  • Applied Marcus theory to correlate electron transfer rates with reduction potential shifts.
  • Verified halide ion association using 1H NMR spectroscopy.

Main Results:

  • Quantified stabilization-induced shifts in formal reduction potentials: ΔE°'(Br•/–) = 150 ± 24 meV and ΔE°'(I•/–) = 67 ± 13 meV.
  • Demonstrated that equilibrium constants (Keq) are poor indicators of reduction potential shifts.
  • Showed that noncovalent interactions, not just Coulombic forces, contribute to stabilization of polarizable halogens.

Conclusions:

  • The competitive kinetic method effectively quantifies halide ion stabilization.
  • Stabilization significantly alters halide ion redox properties.
  • Reevaluation of stabilization mechanisms, beyond simple electrostatics, is necessary for polarizable species.