= p = 7.3. The latter Brønsted plot is consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T±) on the reaction path, and a change of the rate-determining step, from breakdown to formation of T±, as pyridine basicity increases. For the reactions of 1 the β value indicates that the mechanism is also stepwise with expulsion of the nucleofuge from T± as the rate-determining
4-
硝基苯基和2,4-
二硝基苯基S-甲基
硫代
碳酸酯(分别为1和2)与一系列3-和/或4-取代的
吡啶在
水溶液中,在25.0°C和离子强度为0.2的反应对M(KCl)进行动力学研究。通过分光光度法研究4-
硝基苯氧根(400 nm)或2,4-
二硝基苯氧根(360 nm)阴离子的释放来研究反应。在胺过量的情况下,发现伪一阶速率系数(k obsd)。的曲线图ķ实测值VS [
吡啶]是线性的并且不依赖pH的,用斜率ķ Ñ。布朗斯台德图(log k N vs p K一个
吡啶鎓离子),用于的反应中1是线性的,斜率β= 1.1,与此相反的情节用于反应2,这是双相的,斜率β 1 = 0.25(高
磷ķ一个)和β 2 = 0.90(低p K a),曲率中心在p K a = p = 7.3。后一个布朗斯台德图与逐步机理一致,反应机理是通过两性离子四面体中间体(T ±)在反应路径上进行的,并且随着
吡啶碱度的提高,速率确定步骤从分解到形成T