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SN2 Reaction: Stereochemistry

S_N2 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.

Predicting Products: SN1 vs. SN2

Predicting Products: S_N1 vs. S_N2

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,...

SN1 Reaction: Mechanism

S_N1 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...

SN2 Reaction: Transition State

S_N2 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...

SN1 Reaction: Stereochemistry

S_N1 Reaction: Stereochemistry

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...

SN2 Reaction: Mechanism

S_N2 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...

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Updated: Jun 20, 2026

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Published on: October 30, 2018

Steric effects and solvent effects on SN2 reactions.

Yongho Kim¹, Christopher J Cramer, Donald G Truhlar

¹Department of Chemistry, Kyung Hee University, 1 Seochun-dong, Kiheung-Gu, Yongin, Gyeonggi-Do, 449-701, Korea.

The Journal of Physical Chemistry. A

|September 2, 2009

Summary

Quantum mechanical calculations reveal solvent and substituent effects on nucleophilic substitution reactions. Differential solvation effects largely cancel, leading to similar activation free energies for different substituents in aqueous solution.

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Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

Published on: November 27, 2015

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Last Updated: Jun 20, 2026

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Published on: October 30, 2018

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

Published on: November 27, 2015

Area of Science:

Physical Chemistry
Computational Chemistry
Organic Chemistry

Background:

Ionic nucleophilic substitution reactions are fundamental in organic chemistry.
Understanding the interplay of solvent and substituent effects is crucial for reaction mechanism elucidation.

Purpose of the Study:

To computationally disentangle solvent and substituent effects on SN2 reactions.
To compare the impact of different substituents (H vs. CN) and alkyl groups (CH3 vs. (CH3)3C) on reaction energetics.

Main Methods:

Quantum mechanical calculations were employed.
SN2 reactions of chloride ion with substituted chloroalkanes were studied.
Comparisons were made between gas phase and aqueous solution environments.

Main Results:

Transition state distortion and dielectric descreening were dominant solvent effects.
These effects largely canceled each other, minimizing differential solvation contributions to activation energy differences.
Solvation was less unfavorable for X=H than X=CN due to charge separation.

Conclusions:

Solvent and substituent effects exhibit complex interplay in ionic nucleophilic substitution.
Differential solvation plays a minor role in differentiating activation energies between CH3 and (CH3)3C substituents.
Aqueous activation free energies are predicted to be similar for H and CN substituents despite differing gas-phase behaviors.