STERIC EFFECTS .3. BIMOLECULAR NUCLEOPHILIC-SUBSTITUTION
STERIC EFFECTS .3. BIMOLECULAR NUCLEOPHILIC-SUBSTITUTION
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DOI：
10.1021/ja00846a023
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发表时间：
1975-01-01
影响因子：
15
通讯作者：
CHARTON, M
中科院分区：
文献类型：
--
作者：
CHARTON, M
涉及 XCHjY 类型化合物（其中 X= H 或烷基，Y 为离去基团）的 18 组双分子亲核取代的速率数据与方程 log kx=\+ 0ctrx++ h 和 log kx=+ h 相关。相关性结果表明，X 基团的影响即使不是完全是空间位阻的，也是占主导地位的。由于 u 常数是根据羧酸的酯化定义的，并且代表对近似 sp3 杂化过渡态形成的空间效应，因此它们可能不能提供双分子亲核取代中空间效应的最佳表示，其中 X 基团对近似 sp2 杂化过渡态产生空间效应。因此，新的空间效应取代基常数 u' 是根据 XCHjBr 与 Br~ 在 MeAc 中于 25 下反应的速率常数定义的。给出了八个烷基的 u' 值。双分子亲核取代中的空间效应的大小大于酯的酯化和酸水解或酯的碱水解中的空间效应的大小。然而，对于大多数基团，i/的值小于u的值。考虑化合物XCHjY的集合，其中X是烷基或氢原子，并且Y是离去基团。已经指出，X基团对双分子亲核取代速率的影响包括空间增量和极性增量。 1 冈本、新田、井本和新-
Rate data for 18 sets of bimolecular nucleophilic substitution involving compounds of the type XCHjY, where X= H or alkyl, and Y is a leaving group, were correlated with the equation, log kx=\+ 0ctrx++ h, and log kx=+ h. The results of the correlations show that the effect of the X group is predominantly if not entirely steric. As the u con-stants were defined from the esterification of carboxylic acids and represent a steric effect on the formationof an approximately sp3 hybridized transition state, they probably do not provide the best representation of the steric effect in bimolecular nucleophilic substitution where the X group exerts a steric effect upon an approximately sp2 hybridized transition state. New steric effect substituent constants, u', were therefore defined from the rate constants for the reaction of XCHjBr with Br~ in MeAc at 25. Values of u'for eight alkyl groups are given. The magnitudeof the steric effect in bimolecular nucleophilic substitution is greater than the magnitude of the steric effect in esterification and acid hydrolysis of esters, or in basic hydrol-ysis of esters. Values of i/are smaller than values of u for most groups, however.Consider the set of compounds XCHjY in which X is an alkyl group or hydrogen atom, and Y is a leaving group. It has been stated that the effect of the X group on the rate of bimolecular nucleophilic substitution includes both steric and polar increments. 1 Okamoto, Nitta, Imoto, and Shin-