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在有机反应机理的教学中,涉及到次级同位素效应时,由于在有关的教学参考书中叙述得较为简单,分散,难以给学生一个明确消晰的概念。本文拟就有关问题作一简要的陈述。一、概述动力学同位素效应作为研究反应机理,推测过渡状态结构的重要方法之一,已经广泛地应用于有机反应机理的研究之中。在动力学同位素效应的研究中,最常采用的是用氘(D)取代化合物中的氢,测定其所引起的反应速度的改变。由于氘的质量比氢大,对C-H(D)键来说,氘取代氢将导致键的零点振动能的降低,因而使C-D键断裂所需要的活化能比相应的C-H键更高。如果在决定反应速度的步骤中
In the teaching of organic reaction mechanism, when it comes to the secondary isotope effect, it is difficult to give the students a clear and unambiguous concept because they are relatively simple and scattered in the relevant teaching reference book. This article intends to make a brief statement on the issue. I. Overview Kinetic isotope effect as one of the important methods to study the reaction mechanism and infer the transition state structure has been widely used in the study of organic reaction mechanism. In the study of the kinetic isotope effect, the most commonly used method is to replace the hydrogen in the compound with deuterium (D) to determine the change in the reaction rate it causes. Since deuterium is of a mass greater than that of hydrogen, deuterium-substituted hydrogen will result in a decrease of the zero point vibrational energy of the bond to the C-H (D) bond, thus requiring more activation energy to cleave the C-D bond than the corresponding C-H bond. If in the decision of the reaction speed of the step