Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany.
Chemistry. 2010 Jul 5;16(25):7469-77. doi: 10.1002/chem.200902669.
Ionization rates of para-substituted triphenylmethyl (trityl) acetates, benzoates, and para-nitrobenzoates have been determined in aqueous acetonitrile and aqueous acetone at 25 degrees C. Conventional and stopped-flow techniques have been used to evaluate rate constants ranging from 1.38 x 10(-5) to 2.15 x 10(2) s(-1) by conductimetry and photospectrometry methods. The varying stabilities of the differently substituted tritylium ions account for a gradual change of reaction mechanism. Poorly stabilized carbocations are generated slowly by the ionization of their covalent precursors and trapped fast by water. Better stabilized carbocations are generated more rapidly and accumulate, so that ionization and trapping by water can be observed as separate steps in a single experiment. Finally, highly stabilized tritylium ions do not react with water, and only the rates of their formation could be measured. The ionization rate constants correlate linearly with Winstein's ionizing powers Y; the low slopes (0.17<m<0.58) indicate non-carbocation-like transition states. While the correlation between the ionization rates and Hammett-Brown's sigma(+) parameters is excellent for symmetrically substituted tritylium derivatives, deviations for unsymmetrically substituted systems are observed. The failing rate-equilibrium relationship between the rates of ionizations (log k(ion)) and the stabilities of the carbocations in aqueous solution (pK(R(+))) may be explained by the late development of resonance between a p-amino group and the carbocationic center of the tritylium ion during the ionization process.
对位取代的三苯甲基（三苯甲）乙酸酯、苯甲酸酯和对硝基苯甲酸酯在 25°C 的水-乙腈和水-丙酮中的离解速率已被确定。采用传统和停流技术，通过电导率法和分光光度法评估了从 1.38×10(-5)到 2.15×10(2) s(-1)的速率常数。不同取代的三苯甲基离子的稳定性变化导致反应机制逐渐变化。它们的离子化的慢生成的不稳定碳正离子通过其共价前体缓慢生成，并被水快速捕获。更好稳定的碳正离子更快地生成并积累，因此在单次实验中可以观察到水的离子化和捕获是两个独立的步骤。最后，高度稳定的三苯甲基离子不与水反应，只能测量其形成的速率。离解速率常数与 Winstein 的离解力 Y 呈线性相关；斜率较低（0.17<m<0.58）表明非碳正离子过渡态。对于对称取代的三苯甲衍生物，离解速率与 Hammett-Brown 的 sigma(+)参数之间的相关性非常好，但对于不对称取代的体系则存在偏差。在水相中，离解速率（log k(ion)）与碳正离子稳定性（pK(R(+)))之间的速率平衡关系失效，可能是由于在离解过程中 p-氨基和三苯甲基离子的碳正中心之间的共振后期发展所致。
