Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates: change in RDS versus resonance contribution.
Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates: change in RDS versus resonance contribution.
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2,4-二硝基苯基 X-取代苯甲酸酯吡啶分解的非线性哈米特图：RDS 变化与共振贡献。
DOI：
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发表时间：
2010
影响因子：
3.2
通讯作者：
J. Shin
中科院分区：
文献类型：
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作者：
I. Um;Li;Eun;J. Shin
在25.0 ± 0.1 ℃，80mol%H（2）O/20mol%DMSO中，测定了2，4-二硝基苯X-取代苯甲酸酯（1a-j）与Z-取代吡啶的亲核取代反应的二级速率常数（k（OH）~-）。1a-j与吡啶反应的Hammett图由两条相交的直线组成，即，对于在苯甲酰基部分中具有给电子基团（EDG）的底物（1a-c）的反应，ρ值大，而对于具有吸电子基团（EWG）的底物（1 e-j），ρ值小。非线性Hammett图归因于通过给电子取代基和羰基官能团之间的共振相互作用稳定底物1a-c的基态，因为相应的Yukawa-Tsuno图显示出与大r值的良好线性相关性。已经表明，底物的反应性并不比从Hammett取代基常数所预期的更高，而是底物1a-c表现出比所预期的更低的反应性。1a-j的吡啶解的Brønsted型图与β（nuc）= 0.74-0.98呈线性，表明反应通过分步机制进行，其中第二步是RDS。它已得出结论，取代基X的苯甲酰基部分的电子性质不影响RDS，但在过渡态的键形成的程度（或亲核位点上的有效电荷）变得更加显着的取代基X从一个强EDG到一个强EWG的变化。
Second-order rate constants (k(OH)-) have been measured for nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-j) with Z-substituted pyridines in 80 mol% H(2)O/20 mol% DMSO at 25.0 +/- 0.1 degrees C. The Hammett plots for the reactions of 1a-j with pyridines consist of two intersecting straight lines, i.e., a large rho value for the reactions of substrates (1a-c) possessing an electron-donating group (EDG) in the benzoyl moiety and a small one for substrates (1e-j) bearing an electron-withdrawing group (EWG). The nonlinear Hammett plots have been attributed to stabilization of the ground state of substrates 1a-c through resonance interactions between the electron-donating substituent and the carbonyl functionality, since the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with large r values. It has been shown that substrates are not unusually more reactive than would be expected from the Hammett substituent constants, but rather, substrates 1a-c exhibit lower reactivity than would be predicted. The Brønsted-type plots for pyridinolysis of 1a-j are linear with beta(nuc) = 0.74-0.98, indicating that the reaction proceeds through a stepwise mechanism in which the second step is the RDS. It has been concluded that the electronic nature of the substituent X in the benzoyl moiety does not influence the RDS, but the degree of bond formation (or the effective charge on the nucleophilic site) in the transition state becomes more significant as the substituent X changes from a strong EDG to a strong EWG.