イオン性S(N) 2反応におけるステリックと溶解効果
Xin Chen1, Colleen K Regan, Stephen L Craig
1Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
Journal of the American Chemical Society
|October 22, 2009
まとめ
S(N) 2反応に対する溶解効果は,最小限のサイズ依存を示しています. アルキルクロロニトリルにおけるステリック阻害は,ガスと溶液の相では類似しており,補償された溶解効果を示唆しています.
科学分野:
- 有機化学 オーガニック・ケミストリー
- 物理化学 物理化学
- コンピューティング・ケミストリー
背景:
- ステリック効果は,有機化学における反応速度に大きな影響を与えます.
- S(N) 2反応における溶解の役割を理解することは,反応性の予測に極めて重要です.
研究 の 目的:
- S(N) 2反応におけるステリック効果に対する溶解の寄与を調査する.
- モデルシステムとしてアルキルクロロニトリルを用いて,ガスと溶液相におけるステリック効果を比較する.
主な方法:
- アルキルクロロニトリルによるクロリドイオン反応の実験的運動学的研究.
- 偏振連続体モデルとQM/MMシミュレーションを含む理論的調査.
主要な成果:
- ベータ炭素の置換の増加は一貫して反応性を低下させ,ステリック障害を示唆した.
- ステリック障害の大きさは,ガス相と溶液相の両方で類似していました.
- 移行状態の溶解エネルギーは,置換量に対する弱い依存を示した.
結論:
- 溶解は,これらのS(N) 2反応におけるステリック効果を著しく変化させない.
- 弱いサイズ依存は,直接的なシールドと間接的な溶解効果の補償に起因する.
- 発見は,溶解が大きさに強く依存する他のイオン系と対照的です.
関連する概念動画
02:12Solvating Effects
An understanding of the solvating effect helps rationalize the relation between solvation and acidity of the compound. In addition, this also explains the relative stability of conjugate bases for compounds with different pKa values. This lesson details, in-depth, the principle of solvating effects. The strength of an acid and the stability of its corresponding conjugate base are determined using pKa values. This observed relationship is a consequence of solvation, which is the interaction...
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.
01:19Ionic Strength: Effects on Chemical Equilibria
The addition of an inert ionic compound increases the solubility of a sparingly soluble salt. For example, adding potassium nitrate to a saturated solution of calcium sulfate significantly enhances the solubility of calcium sulfate. Le Châtelier's principle cannot predict this shift in the equilibrium. Instead, this could be explained in terms of changes in the effective concentration of the ions in solution in the presence of added inert salt.
In this solution, the primary cation—the calcium...
In this solution, the primary cation—the calcium...
02:15SN1 Reaction: Stereochemistry
This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
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...