在气相和溶液中SN2反应的固体减缓
Grigoriy Vayner1, K N Houk, William L Jorgensen
1Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
Journal of the American Chemical Society
|July 22, 2004
概括
计算化学揭示,来自三-丁基的固体阻碍在S(N) 2反应中显著增加了激活能量. 溶剂也会增加激活能量,但对这些基板的影响差异最小.
科学领域:
- 计算化学是一种计算化学.
- 物理有机化学 有机化学
- 反应机制的反应机制
背景情况:
- 在有机化学中,S(N) 2反应机制是基本的.
- 了解固体和溶剂对反应动态的影响对于预测化学反应性至关重要.
- 之前对化物基S(N) 2反应的研究得出了可能受实验条件影响的结论.
研究的目的:
- 通过计算来研究化物与乙烯和新烯化物及其α-衍生物的气相S(N) 2反应.
- 为了量化三甲基基对激活能量的固态影响.
- 用不同的计算模型评估各种溶剂 (DMSO,甲醇,水) 对反应能量的影响.
主要方法:
- 使用B3LYP和PDDG/PM3方法进行密度函数理论 (DFT) 计算.
- 完整的基础集 (CBS-QB3) 计算高精度的能量学.
- 量子力学/分子力学 (QM/MM) 蒙特卡洛模拟用于溶剂效应.
- 可极化连续模型 (CPCM) 用于溶剂效应计算.
主要成果:
- 计算预测,由于硬质障碍,与甲基组相比,三甲基基组增加了约6kcal/mol的激活能量.
- 溶剂效应,包括DMSO,甲醇和水,显著增加了这些S(N) 2反应的激活能量.
- 溶剂对乙烯和新烯基基底和它们的蓝衍生物的激活能量的差异效应被发现是最小的.
结论:
- 固体效应在调节S(N) 2反应的激活能量方面发挥着重要作用,这可以通过三丁基的影响来证明.
- 溶剂的极性在很大程度上影响了反应能量,增加了激活障碍.
- 理论发现挑战了以前仅基于气相速度测量的解释,强调了在S(N) 2反应研究中考虑溶剂贡献的重要性.
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相关概念视频
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,...