SN2移行状態における二次運動同位体効果に対する屈曲の影響に関する計算的研究
Faraj Hasanayn1, Andrew Streitwieser, Rasha Al-Rifai
1Department of Chemistry, American University of Beirut, Beirut, Lebanon. fh19@aub.edu.lb
Journal of the American Chemical Society
|February 17, 2005
まとめ
二次性デウテリウム運動同位体効果 (KIEs) は,SN2の逆転と保持機構を区別することができる. 大きなKIEsは保持を示し,小さいKIEsは,CH3FとF-.のような反応の逆転を示唆しています.
科学分野:
- 物理化学 物理化学
- 化学動力学 化学動力学
- コンピューティング・ケミストリー
背景:
- SN2反応機構は有機化学において極めて重要であり,逆転と保持の両方の経路が可能である.
- これらのSN2メカニズムを実験的に区別することは困難である.
- キネティック・イソトープ効果 (KIE) は,反応のダイナミクスに敏感な探査機を提供します.
研究 の 目的:
- メチルフッ化物 (CH3F) を含むSN2反応における二次デウテリウム運動同位体効果 (KIE) を計算および分析する.
- KIEが,SN2の逆転と保持メカニズムを区別するための信頼できる観測値として役立つかどうかを判断する.
- イオン分子反応とイオン対反応の両方でKIEsを調査する.
主な方法:
- 計算には従来の移行状態理論が用いられました.
- 二次デウテリウムKIEは,F−を持つCH3Fの逆転と保持SN2経路のために計算された.
- 計算は,CH3FとLiFを含むイオンペア反応に拡張されました.
主要な成果:
- バランスの取れたエントロピーとエンタルピーの条件に起因する逆転SN2メカニズムのために,小さなKIE (0.98 at 298 K) が計算されました.
- 保持SN2メカニズムに対して大きなKIE (1.5) を計算した.
- また,イオンペア反応における逆転経路と保持経路の両方で大きなKIEsが観察されました.
- 大規模なKIEsに関連する移行構造は,曲げられたFCFの角度とデュテリウムに敏感な空想周波数を示した.
結論:
- 二次デウテリウムKIEは,SN2の逆転と保持メカニズムを区別するための実用的な実験観測物です.
- 計算されたKIE値は,メカニズム差別の理論的サポートを提供します.
- 大規模なKIEs,曲げられた移行構造,および敏感な想像的周波数間の観測された相関は,反応ダイナミクスの洞察を提供します.
さらに関連する動画
関連する概念動画
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.
01:08Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
01:06Nuclear Overhauser Enhancement (NOE)
Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically bonded or involved in J-coupling. This phenomenon, called the nuclear Overhauser enhancement (NOE), results from through-space interactions between the nuclear spins. The NOE effect decreases with increasing internuclear distance and is generally not observed beyond 4 angstroms. In NOE, dipole-dipole interactions between neighboring spin-active...