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Related Concept Videos

SN2 Reaction: Mechanism02:27

SN2 Reaction: Mechanism

The kinetic studies of SN2 reactions suggest an essential feature of its mechanism: it is a single-step process without intermediates. Here, both the nucleophile and the substrate participate in the rate-determining step.
The presence of the more electronegative halogen in the substrate creates a polarized carbon-halide bond. The halide pulls the electron cloud generating an electrophilic center at the carbon atom. Thus, the carbon atom carries a partial positive charge while the halide has a...
SN2 Reaction: Kinetics02:14

SN2 Reaction: Kinetics

Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a reaction.
SN2 Reaction: Transition State02:26

SN2 Reaction: Transition State

An SN2 reaction of an alkyl halide is a single-step process in which bond formation between the nucleophile and the substrate and bond breaking between the substrate and the halide occurs simultaneously through a transition state without forming an intermediate.
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...
Predicting Products: SN1 vs. SN202:27

Predicting Products: SN1 vs. SN2

Nucleophilic substitution reactions of alkyl halides can proceed via an SN1 or an SN2 mechanism. While in SN2 reactions, the nucleophile attacks the substrate simultaneously as the leaving group departs, in SN1 reactions, the substrate first dissociates to give the carbocation intermediate. Various factors such as the structure of the substrate, the strength of the nucleophile, and the nature of the solvent promote one mechanism over the other.
With increased substitution on the alkyl halide,...
SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
SN1 Reaction: Mechanism02:25

SN1 Reaction: Mechanism

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 polar...

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Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses
08:55

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Published on: June 7, 2018

Can one predict changes from S(N)1 to S(N)2 mechanisms?

Thanh Binh Phan1, Christoph Nolte, Shinjiro Kobayashi

  • 1Department Chemie und Biochemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München, Germany.

Journal of the American Chemical Society
|July 29, 2009
PubMed
Summary

This study reveals that benzhydryl bromide reactions with amines proceed via concurrent S(N)1 and S(N)2 pathways. The S(N)2 mechanism is favored only when the benzhydrylium ion lifetime necessitates it, demonstrating a shift based on intermediate stability.

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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses
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Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
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Area of Science:

  • Organic Chemistry
  • Reaction Kinetics
  • Physical Organic Chemistry

Background:

  • Substituted benzhydryl bromides react with amines in DMSO, producing amines, benzophenones, and benzhydrols.
  • Kinetic analysis reveals a rate law indicating parallel amine-dependent (S(N)2) and amine-independent (S(N)1) pathways.

Purpose of the Study:

  • To elucidate the mechanisms of reactions between substituted benzhydryl bromides and amines.
  • To investigate the interplay between S(N)1 and S(N)2 processes based on reaction conditions and intermediate lifetimes.
  • To quantify nucleophile and electrophile parameters using a generalized rate equation.

Main Methods:

  • Kinetic investigations at 20°C to determine the rate law.
  • Correlation of rate constants with Hammett's sigma(+) constants for S(N)1 reactions.
  • Application of the log k = s(E + N) equation to analyze S(N)1/S(N)2 transition.
  • Generation of benzhydrylium ions using laser flash photolysis.

Main Results:

  • Evidence for simultaneous S(N)1 and S(N)2 reaction mechanisms was observed.
  • S(N)1 rates correlated with substituent effects (rho = -3.22), while S(N)2 rates did not, indicating distinct transition states.
  • The shift to the S(N)2 mechanism was linked to the lifetime of the benzhydrylium ion intermediate reaching the vibrational limit.
  • DMSO was identified as a stronger O-nucleophile than water and alcohols.

Conclusions:

  • The reaction mechanism transitions from S(N)1 to S(N)2 when the benzhydrylium ion lifetime is limited by the rate of reaction with the nucleophile.
  • The study provides a quantitative framework for understanding nucleophilic substitution reactions involving carbocation intermediates.
  • The nucleophilicity of DMSO was characterized relative to other O-nucleophiles.