DIRECTIVE EFFECTS IN AROMATIC SUBSTITUTIONS .14. SUBSTITUENT CONSTANTS FOR AROMATIC SUBSTITUTION
DIRECTIVE EFFECTS IN AROMATIC SUBSTITUTIONS .14. SUBSTITUENT CONSTANTS FOR AROMATIC SUBSTITUTION
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DOI：
10.1021/ja01565a039
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发表时间：
1957-01-01
影响因子：
15
通讯作者：
OKAMOTO, Y
中科院分区：
文献类型：
--
作者：
BROWN, HC;OKAMOTO, Y
Meta和帕拉取代的苯基二甲基甲酰氯的溶剂分解速率常数对Hammett常数的曲线图显示，在Meta衍生物的情况下，但帕拉取代基的严重偏差的良好的相关性。在t< 3的方向上的偏离是由于帕拉取代基与过渡态初始碳正离子的缺电子中心之间的重要共振相互作用。Meta衍生物用于确定反应的p（-4.620）。一组新的帕拉取代基的r-常数，然后计算从这个p值和所观察到的溶剂分解速率。用Hammett r-yalue方法计算了单取代芳烃的硝化速率、芳烃的卤化速率、芳基硼酸的溴解速率和芳基三甲基硅烷的质子解速率，结果有较大偏差，但与y值吻合较好。本文提出，α值可为处理芳香取代反应中的方向效应提供定量依据，Hammett方程虽然仅限于芳香体系的Meta和帕拉衍生物，但已为定量处理结构对反应速率的影响提供了最成功的途径。[4· 6]它在处理芳香体系的侧链反应方面的成功导致了许多将这种处理方法应用于核取代反应的尝试。哈米特本人在用他的值处理可用的硝化数据方面取得了一定程度的成功。4随着更多的替代数据的出现，这种简单的处理显然是不够的。因此，目前可用的芳香族硝化数据显示哈米特处理存在严重偏差。7有人指出，邻位对位导向基团通过与缺电子过渡态的共振相互作用，可能能够稳定帕拉硝化的过渡态，其稳定程度远大于根据取代基的-常数值所预期的程度。7
A plot of the logarithms of the rate constants for the solvolysis of the meta and para substitutedphenyldimethylcarbinyl chlorides against the Hammett-constants reveals an excellent correlation in the case of the meta derivatives but serious deviations for the para substituents. The deviations are in the direction t< 3 be anticipated for important resonance interactions between the para substituent and the electron deficient center of theincipient carbonium ion in the transition state. The meta derivatives are used to establish p for the reaction (—4.620). A new set of r-constants for the para substituents is then calculated from this p-value and the observed solvolysis rates. The available data, on the rates of the nitration of monosubstituted aromatics, the halogenation of aromatics, the brominolysis of arylboronic acids and the protonolysis of aryltrimethylsilanes exhibit important deviations using the Hammett¡ r-yalues, but they givegood agreement with the ay-values. It is proposed that the+-values may provide a quantitative basis for the treatment of directive effects in aromatic substitution.The Hammett equation, limited though it is to meta and para derivatives of aromatic systems, has provided the most successful approach to a quanti-tative treatment of the effects of structure upon re-action rates. 4· 6 Its success in treating the side-chain reactions of aromatic systems has led to a number of attempts to apply the treatment to nuclear substitution reactions. Hammett himself realized a measure of success in treating the avail-able nitration data6 in terms of his-values. 4 As additional substitution data have become available, it has become apparent that this simple treatment cannot suffice. Thus the presently avail-able data on aromatic nitration show serious deviations in the Hammett treatment. 7 It was pointed out that ortho-para directing groups, through resonance interaction with theelectron deficient transi-tion state, might be able to stabilize the transition state for para nitration to a far greater extent than would be expected from the value of the-constant for thesubstituents. 7