Related Experiment Video
Updated: Jul 21, 2026
14:01
Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition
Published on: May 22, 2015
The alpha-effect in gas-phase SN2 reactions revisited
1College of Chemistry, Sichuan University, Chengdu, PR China. yiren57@hotmail.com
Organic Letters
|December 31, 2005
Summary
This study investigates gas-phase S(N)2 reactions, finding the alpha-effect is present at saturated carbon. This challenges the long-held belief that the alpha-effect is absent in gas-phase reactions.
Area of Science:
- Physical Chemistry
- Computational Chemistry
- Organic Chemistry
Background:
- The alpha-effect describes an enhanced nucleophilicity for atoms adjacent to heteroatoms.
- Its presence in gas-phase S(N)2 reactions at sp(3) carbon is debated.
- Previous understanding suggested its absence in the gas phase.
Purpose of the Study:
- To re-examine gas-phase S(N)2 reactions at saturated carbon.
- To investigate the existence of the alpha-effect in these reactions.
- To challenge the prevailing notion regarding the alpha-effect in gas-phase chemistry.
Main Methods:
- Utilized the G2(+) theory for theoretical calculations.
- Modeled S(N)2 reactions with various nucleophiles (Nu(-)) and methyl chloride (CH(3)Cl).
- Analyzed reaction pathways for Nu(-) + CH(3)Cl --> CH(3)Nu + Cl(-).
Main Results:
- The alpha-effect was observed in gas-phase S(N)2 reactions at sp(3) carbon.
- Calculated results demonstrate the presence of the alpha-effect.
- This finding contradicts the established view of its absence in the gas phase.
Conclusions:
- The alpha-effect is confirmed to exist in gas-phase S(N)2 reactions at saturated carbon.
- This study provides computational evidence against the prior assumption.
- The findings necessitate a re-evaluation of gas-phase reaction mechanisms involving the alpha-effect.
Related Concept Videos
02:14SN2 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.
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.
02:27SN2 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...
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...
02:26SN2 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...
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...
02:23SN2 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.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
02:25SN1 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...
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...
02:27Predicting 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,...
With increased substitution on the alkyl halide,...