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ガス相と溶液中の反応性とメカニズムを直接比較する
John M Garver1, Yao-ren Fang, Nicole Eyet
1Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA.
Journal of the American Chemical Society
|March 2, 2010
まとめ
シアン酸イオンのガス相と溶液の反応をアルキルイオダイドと比較すると,過渡状態に対する溶媒効果が明らかになる. 溶解ルールは溶液の振る舞いを予測しますが,静電力によるガス相反応では失敗します.
科学分野:
- 物理有機化学 物理有機化学
- 反応メカニズム 反応メカニズム
- 溶媒効果 溶媒効果とは
背景:
- 核性置換 (SN2) と除去 (E2) 反応のメカニズムを理解することは,有機化学において極めて重要です.
- 溶媒の性質は,反応速度と経路,特にイオン中介物質と移行状態に大きな影響を及ぼします.
- 運動同位体効果 (KIE) は,移行状態構造と反応機構の解明のための強力な探査機として機能します.
研究 の 目的:
- 溶液とガス相におけるアニオン系の反応性とメカニズム経路を直接比較する.
- シアン酸イオンと様々なアルキルイオジドの間のSN2反応に対する溶媒の極性およびプロティシティの影響を調査する.
- "SN2反応の溶解法則"の予測力を評価するために,実験的なKIEsを計算モデルと比較した.
主な方法:
- 速度定数と運動同位体効果 (KIEs) の測定,ペルデュテロ,二次アルファ-デュテリウム,ベータ-デュテリウムKIEsを含む.
- 研究された反応には,メチル,エチル,イソプロピル,ターチブチルヨウ酸化物と,ガス相におけるシアン酸イオンが含まれています.
- 溶液試験では,ジメチル硫酸化物/メタノール,ジメチル硫酸化物,およびテトラヒドロフランに含まれるエチルヨウ酸化物とシアン酸イオンが含まれていました.
- 実験データと計算上の移行状態幾何学,KIEs,分岐分数との比較.
主要な成果:
- 溶液では,エチルヨウ酸化物-シアン化物イオンSN2反応は,異なる溶媒間で一貫した移行状態構造を示し",SN2反応の溶解法"に準拠した.
- ガス相反応は,より小さい (より逆の) KIEs を示し,より緊密な移行状態を示し",溶解法"が予測できなかった.
- ガス相移行状態の緊密化は,電荷イオンとアルキルハリドの間のより強い静電相互作用に起因する.
- 移行状態の違いにもかかわらず,SN2とE2経路の競争は両段階とも類似しており,SN2がエチルヨウ酸化物 (>99%) の支配的であった.
結論:
- 溶媒効果は,KIEの変異によって証明されるように,SN2反応の移行状態構造を調節する上で重要な役割を果たします.
- "SN2反応の溶解法"は溶液の振る舞いを正確に予測しますが,ガス相メカニズムを説明するには不十分です.
- 静電力は,溶液と比較して,ガス相における移行状態の密度を決定する上でより優位である.
- 基本的なSN2メカニズムは,これらの反応において溶液からガス相への移行時でさえも,支配的であり続ける.
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