A greatly under-appreciated fundamental principle of physical organic chemistry.
A greatly under-appreciated fundamental principle of physical organic chemistry.
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DOI：
10.3390/ijms12128316
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发表时间：
2011
影响因子：
5.6
通讯作者：
Cox RA
中科院分区：
文献类型：
--
作者：
Cox RA
如果一种物质在反应介质中没有有限的寿命，它就不可能是机械中间体。这一原则首先由詹克斯提出，作为强制机制的概念。例如，无论是伯碳阳离子还是仲碳阳离子都没有足够长的寿命存在于水性介质中，因此涉及这些底物的SN 1反应是不可能的，并且SN 2机制被强制执行。只有叔碳正离子和那些通过共振稳定的（苄基阳离子，酰基离子）是足够稳定的反应中间体。更重要的是，现在已知H3 O+和HO−在稀水溶液中都不存在。最近几次对大质子团簇的高水平计算无法定位正电荷;它被发现只是作为一个整体“在团簇上”。任何离子水的寿命都非常短，最多只有几个分子振动;最好的实验研究，使用现代红外仪器，最有可能的水合质子结构是H13 O 6+，但估计只有四分之一的质子在任何给定的时刻以这种形式存在。由于格劳图斯机制的链转移沿着氢键，在现实中，质子或氢氧根离子是简单地立即可用的任何地方需要它的反应。重要的机械后果的结果。任何带电氧物质（例如，四面体中间体）也不会存在足够长的时间而成为反应中间体，除非电荷以某种方式稳定，通常是通过共振。一般的酸催化是在浓酸水溶液中反应的规则。格罗特胡斯机制也意味着涉及中性水的反应是有利的;溶剂已经高度结构化，因此将几个溶剂分子带到反应中心的熵并不重要。并给出了实例。
If a species does not have a finite lifetime in the reaction medium, it cannot be a mechanistic intermediate. This principle was first enunciated by Jencks, as the concept of an enforced mechanism. For instance, neither primary nor secondary carbocations have long enough lifetimes to exist in an aqueous medium, so SN1 reactions involving these substrates are not possible, and an SN2 mechanism is enforced. Only tertiary carbocations and those stabilized by resonance (benzyl cations, acylium ions) are stable enough to be reaction intermediates. More importantly, it is now known that neither H3O+ nor HO− exist as such in dilute aqueous solution. Several recent high-level calculations on large proton clusters are unable to localize the positive charge; it is found to be simply “on the cluster” as a whole. The lifetime of any ionized water species is exceedingly short, a few molecular vibrations at most; the best experimental study, using modern IR instrumentation, has the most probable hydrated proton structure as H13O6+, but only an estimated quarter of the protons are present even in this form at any given instant. Thanks to the Grotthuss mechanism of chain transfer along hydrogen bonds, in reality a proton or a hydroxide ion is simply instantly available anywhere it is needed for reaction. Important mechanistic consequences result. Any charged oxygen species (e.g., a tetrahedral intermediate) is also not going to exist long enough to be a reaction intermediate, unless the charge is stabilized in some way, usually by resonance. General acid catalysis is the rule in reactions in concentrated aqueous acids. The Grotthuss mechanism also means that reactions involving neutral water are favored; the solvent is already highly structured, so the entropy involved in bringing several solvent molecules to the reaction center is unimportant. Examples are given.
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影响因子：
15
作者：
BENDER, ML
通讯作者：
BENDER, ML
DOI：
10.1039/p29860000619
发表时间：
1986-04-01
期刊：
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
影响因子：
--
作者：
BENTLEY, TW;HARRIS, HC
通讯作者：
HARRIS, HC
DOI：
10.1039/jr9570003402
发表时间：
1957-01-01
期刊：
JOURNAL OF THE CHEMICAL SOCIETY
影响因子：
--
作者：
BUNTON, CA;LLEWELLYN, DR
通讯作者：
LLEWELLYN, DR
影响因子：
15
作者：
COX, RA
通讯作者：
COX, RA
影响因子：
15
作者：
DIETZE, PE;WOJCIECHOWSKI, M
通讯作者：
WOJCIECHOWSKI, M