在溶解反应中直接观察电离化阶段:碳酸的电友性与电流性对比
Heike F Schaller1, Alexander A Tishkov, Xinliang Feng
1Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München, Germany.
Journal of the American Chemical Society
|February 20, 2008
概括
高度稳定的基离子与碳酸盐离子反应. 电离率和反应性没有相关性,这表明在S(N) 1反应中存在可变的内在障碍和非碳酸类过渡状态.
科学领域:
- 有机化学 有机化学
- 反应动力学反应动力学
背景情况:
- 氨基基替代的基离子 (Ar2CH+) 是高度稳定的碳酸.
- 了解它们反应的动力学和热力学对于反应机制研究至关重要.
研究的目的:
- 为了确定基离子和碳酸盐离子之间的反应的速度和平衡.
- 研究电离率 (电流性) 和反应性 (电友性) 之间的关系.
- 分析S(N) 1反应中的过渡状态结构和溶剂效应.
主要方法:
- 在乙和乙中对反应速率和平衡的光度测定.
- 直接测量S(N) 1反应的第一步,通过从共价中再生基离子.
- 构建用于电离的自由能量配置文件.
主要成果:
- 基碳酸盐再生基离子,允许直接测量S(N) 1的第一步.
- 电流性顺序与基离子的电友性顺序没有相关性.
- 对于以相似速度生成的离子,观察到超过两个数量级的活性差异.
- 可变的内在障碍解释了利率平衡关系的分解.
- 过渡状态不是类似于碳酸的,在温斯坦-格鲁诺瓦尔德方程中受到小型溶剂效应 (m = 0.35-0.55) 的支持.
结论:
- 二碳酸盐的电离涉及到过渡状态,这些过渡状态并不完全类似于碳酸盐.
- 小溶剂效应并不一定表明有S(N) 2的电离机制.
- 内在障碍在基离子的反应性中起着重要作用.
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