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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...

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...

E2 Reaction: Kinetics and Mechanism

E2 Reaction: Kinetics and Mechanism

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...

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...

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: 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...

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Related Experiment Video

Updated: Sep 19, 2025

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Published on: February 20, 2020

Understanding the S_N2 Versus E2 Competition.

Pascal Vermeeren¹, Thomas Hansen¹, Trevor A Hamlin¹

¹Department of Chemistry and Pharmaceutical Sciences, AIMMS, Vrije Universiteit Amsterdam, De Boelelaan 1108, Amsterdam, 1081 HZ, The Netherlands.

Chemistry (Weinheim an Der Bergstrasse, Germany)

|June 17, 2025

Summary

This summary is machine-generated.

Control over competing bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2) reactions is crucial in organic synthesis. This study uses molecular orbital theory and the activation strain model to provide guiding principles for tuning these reactions.

Keywords:

Lewis base elimination reactions reactivity solvation substitution reactions

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Area of Science:

Organic Chemistry
Computational Chemistry

Background:

Bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2) reactions often compete.
Controlling this competition is vital for effective synthetic organic chemistry.

Purpose of the Study:

To establish guiding principles for understanding and controlling the competition between SN2 and E2 reactions.
To leverage quantitative molecular orbital (MO) theory and the activation strain model for reaction design.

Main Methods:

Application of quantitative molecular orbital (MO) theory.
Utilizing the activation strain model.
Analysis of key reaction factors: Lewis base, leaving group, substrate, and solvent.

Main Results:

Introduction of novel concepts: characteristic distortivity, transition state acidity, intrinsic nucleophilicity, and apparent nucleophilicity.
Demonstration of how these factors influence the SN2/E2 competition.
Development of a theoretical framework for predicting and controlling reaction outcomes.

Conclusions:

The developed conceptual tools enable chemists to better understand and design synthetic routes.
Quantitative MO theory and the activation strain model provide a robust basis for tuning SN2 vs. E2 reactivity.
Predictive insights into reaction pathways facilitate efficient organic synthesis.