Kinetic and Theoretical Studies on Pyridinolysis of 2,4-Dinitrophenyl X-Substituted Benzoates: Effect of Substituent X on Reactivity and Mechanism
作者:Ik-Hwan Um、Eun-Hee Kim、Li-Ra Im、Masaaki Mishima
DOI:10.5012/bkcs.2010.31.9.2593
日期:2010.9.20
Second-order rate constants ($k_N$) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-$NO_2$) with a series of Z-substituted pyridines in 80 mol % $H_2O$/20 mol % DMSO at $25.0\pm}0.1^\circ}C$. The Br$\o}$nsted-type plots exhibit downward curvature (e.g., $\beta_2$ = 0.89 ~ 0.96 when $pK_a$ < 9.5 while $\beta_1$ = 0.38 ~ 0.46 when $pK_a$ > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The $pK_a}^o$, defined as the $pK_a$ at the center of Br$\o}$nsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of $k_N$ into the microscopic rate constants $k_1$ and $k_2/k_-1}$ ratio has revealed that $k_1$ is governed by the electronic nature of the substituent X but the $k_2/k_-1}$ ratio is not. Theoretical calculations also support the argument that the electronic nature of the substituent X in the benzoyl moiety does not influence the $k_2/k_-1}$ ratio.
已通过分光光度法测量 2,4-二硝基苯基 X 取代的苯甲酸酯(X = 4-MeO、H 和 4-$NO_2$ 的反应)的二阶速率常数 ($k_N$) ) 与一系列 Z-取代吡啶在 80 mol % $H_2O$/20 mol % DMSO 中在 $25.0\pm}0.1^\circ}C$< /TEX>。 Br$\o}$nsted 型图呈现向下曲率(例如,当 $pK_a$ 时,$\beta_2$ = 0.89 ~ 0.96 > < 9.5 while $\beta_1$ = 0.38 ~ 0.46 when $pK_a$ > 9.5),表明反应通过逐步机制进行,速率决定发生变化步骤(RDS)。 $pK_a}^o$,定义为 Br$\o}$nsted 曲率中心的 $pK_a$,无论苯甲酰基部分中取代基 X 的电子性质如何,分析结果为 9.5。将 $k_N$ 分解为微观速率常数 $k_1$ 和 $k_2/k_-1}$ 比率表明 $k_1$ 受取代基 X 的电子性质控制,但 $k_2/k_-1}$ 比率则不然。理论计算也支持这样的论点:苯甲酰基部分中取代基 X 的电子性质不会影响 $k_2/k_-1}$ 比率。