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Related Concept Videos

SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

9.8K
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...
9.8K
Predicting Products: SN1 vs. SN202:27

Predicting Products: SN1 vs. SN2

13.1K
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,...
13.1K
SN1 Reaction: Stereochemistry02:15

SN1 Reaction: Stereochemistry

8.7K
This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
8.7K
SN2 Reaction: Transition State02:26

SN2 Reaction: Transition State

10.0K
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...
10.0K
SN2 Reaction: Mechanism02:27

SN2 Reaction: Mechanism

13.6K
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...
13.6K
SN1 Reaction: Mechanism02:25

SN1 Reaction: Mechanism

11.8K
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...
11.8K

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Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides
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Do π-conjugative effects facilitate SN2 reactions?

Chia-Hua Wu1, Boris Galabov, Judy I-Chia Wu

  • 1Center for Computational Chemistry and Department of Chemistry, University of Georgia , Athens, Georgia 30602, United States.

Journal of the American Chemical Society
|January 24, 2014
PubMed
Summary
This summary is machine-generated.

Substrate-nucleophile electrostatic interactions, not pi-conjugation, control SN2 reaction rates. Attractive forces lower activation barriers, accelerating reactions, while repulsive forces increase barriers, slowing them down.

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry
  • Physical Chemistry

Background:

  • The traditional view attributes SN2 reaction acceleration to pi-conjugation in the transition state.
  • This view is challenged for substrates with multiple bonds at the Cβ position.

Purpose of the Study:

  • To investigate the factors governing SN2 identity exchange reaction rates.
  • To determine the relative importance of electrostatic interactions versus pi-conjugation in SN2 transition states.

Main Methods:

  • Rigorous quantum chemical investigations.
  • Block-localized wave function (BLW) computations.

Main Results:

  • Substrate-nucleophile electrostatic interactions are the primary drivers of SN2 reaction rate trends.
  • Attractive Cβ(δ(+))···X(δ(-)) interactions lower activation barriers and enhance rates.
  • Repulsive Cβ(δ(-))···X(δ(-)) interactions increase activation barriers and retard rates.
  • While pi-conjugation does lower activation barriers, its effect is similar across various substrates and cannot explain the observed range of barrier heights.

Conclusions:

  • Electrostatic interactions are more significant than pi-conjugation in determining SN2 reaction rates.
  • The acceleration of SN2 reactions with Cβ multiple bonds is mainly governed by electrostatic effects, not solely pi-conjugation.