在气相和溶液中的反应性和机制的直接比较
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
概括
将化离子与酸的气相和溶液反应进行比较,可以发现溶剂对过渡状态的影响. 虽然溶解规则预测了溶液的行为,但由于静电力,它们无法预测气相反应.
科学领域:
- 物理有机化学 有机化学
- 反应机制 反应机制
- 溶剂效应 溶剂效应的作用
背景情况:
- 了解核替代 (SN2) 和消除 (E2) 反应机制在有机化学中至关重要.
- 溶剂特性显著影响反应速率和途径,特别是对于离子中间体和过渡状态.
- 动态同位素效应 (KIE) 作为强大的探测器,用于阐明过渡状态结构和反应机制.
研究的目的:
- 直接比较气相和溶液中的阴离子系统的反应性和机械路径.
- 调查溶剂极性和亲性对化离子和各种酸之间的SN2反应的影响.
- 通过将实验KIE与计算模型进行比较,评估"SN2反应溶解规则"的预测能力.
主要方法:
- 测量速率常数和动态同位素效应 (KIEs),包括 perdeutero,二次alpha-deuterium和β-deuterium KIEs.
- 研究的反应包括甲基,乙基,异烯和三甲基-丁化与气相中的化离子.
- 溶液研究涉及乙烯和化离子在二甲基硫氧化物/甲醇,二甲基硫氧化物和四.
- 实验数据与计算过渡状态几何,KIE和分支分数进行比较.
主要成果:
- 在溶液中,乙酸离子SN2反应在不同的溶剂中显示出一致的过渡状态结构,符合"SN2反应溶解规则".
- 气相反应表现出明显较小的 (更反向的) KIEs,表明过渡状态更紧密,而"溶解规则"无法预测.
- 更紧密的气相过渡状态归因于充电离子和基化物之间更强的静电相互作用.
- 尽管过渡状态有差异,但SN2和E2通路之间的竞争在两个阶段都是相似的,SN2在乙烯酸中占主导地位 (>99%).
结论:
- 溶剂效应在SN2反应的过渡状态结构调节中发挥着关键作用,正如KIE变异所证明的那样.
- "SN2反应的溶解规则"准确地预测了溶液的行为,但不足以描述气相机制.
- 与溶液相比,静电力在确定气相中的过渡状态密度方面更为占主导地位.
- 即使在这些反应中从溶液过渡到气相时,基本的SN2机制仍然普遍存在.
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