研究微溶解离子在气相S(N) 2反应中的α效应
Ditte L Thomsen1, Jennifer N Reece, Charles M Nichols
1Department of Chemistry, University of Copenhagen , Universitetsparken 5, Copenhagen, DK-2100 Denmark.
Journal of the American Chemical Society
|September 20, 2013
概括
这项研究表明,α效应增强了微溶性核友的反应性,即使是单一的水分子. 这表明了α效应.
科学领域:
- 物理化学 物理化学
- 化学动力学 化学动力学
- 计算化学计算化学
背景情况:
- 阿尔法效应描述了由于相邻的单一对而增强的核反应性.
- 之前的工作证明了氧化离子 (HOO−) 在气相S(N) 2反应中的α效应.
- 溶解对α效应的影响需要进一步研究.
研究的目的:
- 为了研究微溶解对气相S(N) 2反应中的α效应的影响.
- 为了比较微溶化氧化离子 (HOO−(H2O)) 与微溶化氧化离子 (RO−(H2O)) 的反应性.
- 为了阐明差异溶解在阿尔法效应中的作用.
主要方法:
- 使用流动的光后选择离子流管仪器研究气相离子分子反应.
- 研究了微溶解核 (HOO-H2O和RO-H2O) 与甲基化物 (CH3Cl) 的S(N) 2反应.
- 进行理论计算来分析过渡状态结构和溶解相互作用.
主要成果:
- 微溶解的氧化离子 (HOO−(H2O)) 呈现出增强的反应性,证实了水分子存在时的α效应.
- 微溶性核友的Brønsted βnuc值较大,这表明与未溶性核友相比,它们的过渡状态较晚.
- 观察到过渡状态的差异解法,这表明对α效应的贡献.
- 产品分布显示,裸体离子 (Cl−) 优先形成,而不是水合集群 (Cl−(H2O)).
结论:
- 阿尔法效应甚至存在于微溶的α核爱好者中,并且具有重要意义.
- 溶解显著影响阿尔法效应,差异溶解起着关键作用.
- 氧化离子 (HOO−) 在微溶解集群反应中仍然是活跃的核.
相关概念视频
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...
02:26SN2 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...
01:17Reactions of α-Halocarbonyl Compounds: Nucleophilic Substitution
Nucleophilic substitution in α-halocarbonyl compounds can be achieved via an SN2 pathway. The reaction in α-haloketones is generally carried out with less basic nucleophiles. The use of strong basic nucleophiles leads to the generation of α-haloenolate ions, which often participate in other side reactions.
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,...
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.
01:14Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1
Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano...
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano...