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Facile S(N)2 reaction in protic solvent: quantum chemical analysis.
Young-Ho Oh1, Doo-Sik Ahn, Sang-Yoon Chung
1College of Environmental Science and Applied Chemistry (BK 21), Kyunghee University, Kyungki 449-701, Korea.
The Journal of Physical Chemistry. A
|September 21, 2007
Summary
Cations and protic solvents significantly accelerate SN2 reactions, particularly fluorination. This study reveals a novel mechanism where solvent molecules, shielded by cations, act as Lewis bases, lowering activation barriers through combined effects.
Area of Science:
- Computational Chemistry
- Physical Organic Chemistry
- Reaction Mechanisms
Background:
- SN2 reactions are fundamental in organic synthesis.
- Protic solvents and cations are traditionally considered to hinder nucleophilic reactivity.
- Understanding solvent and cation effects is crucial for optimizing reaction rates.
Purpose of the Study:
- To investigate the influence of protic solvents and various cations on unsymmetrical SN2 reactions.
- To elucidate the detailed reaction mechanism, including transition states and activation barriers.
- To propose and validate an alternative SN2 mechanism involving cooperative cation-solvent effects.
Main Methods:
- Series of computational calculations.
- Analysis of pre- and postreaction complexes.
- Determination of transition state structures and activation barrier magnitudes.
Main Results:
- Identified a novel mechanism where solvent molecules, shielded by cations, act as Lewis bases.
- Demonstrated significant lowering of the SN2 reaction barrier due to cooperative cation-solvent effects.
- Showed that combined cation and protic solvent interactions dramatically accelerate reactions, especially with fluoride ions.
Conclusions:
- The study proposes an alternative, highly efficient SN2 reaction pathway.
- Cooperative effects of cations and protic solvents can overcome traditional retarding influences.
- Findings explain the exceptional efficiency observed in fluorination reactions within tert-alcohol media.
Related Concept Videos
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...
02:27SN2 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...
02:14SN2 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.
02:23SN2 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.
02:26SN2 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...
02:27Predicting 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,...