KINETIC ISOTOPE-EFFECTS FOR REACTIONS OF METHYL FORMATE-METHOXYL-O-18
KINETIC ISOTOPE-EFFECTS FOR REACTIONS OF METHYL FORMATE-METHOXYL-O-18
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DOI：
10.1021/ja00803a027
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发表时间：
1973-01-01
影响因子：
15
通讯作者：
KIRSCH, JF
中科院分区：
文献类型：
--
作者：
SAWYER, CB;KIRSCH, JF
测定了甲酸甲酯-甲氧基-NO几种反应的h>；0/h？o值。琥珀酸缓冲液中酸催化、碱催化和一般碱催化的比例分别为1.0009±0.0004、1.0091±0.0002和1.0115±0.0002。根据观测到的LEO和LSO酯之间伸缩频率的差异，计算出决定酰基碳-甲氧基氧键断裂速率的最大预期效应为1.052。在四面体中间体裂解速率控制的条件下，在低pH条件下，肼分解的同位素效应为1.0621±0.0008，而在高pH条件下，肼对酯或质子传输步骤的攻击是速率决定的，其同位素效应仅为1.0048±0.0006。甲酸甲酯的水解率与氧交换率之比在酸性时为11.4±0.1，在碱性溶液中为18.3±0.4。在酸催化的水解过程中观察到的极小的动力学同位素效应表明，酯的平衡质子化作用被水对氧碳正离子的攻击的正常动力学同位素效应所抵消。在碱性水解、一般碱催化的水解和高pH下肼的分解中观察到的微小但显著的同位素效应表明，在过渡状态下，酯中1.15的酰基碳-甲氧基氧键的级数仅略有降低。
Values of h> 0/h» o have been measured for several reactions of methyl formate-methoxyl-nO. The ratios for acid-catalyzed hydrolysis, alkaline hydrolysis, and general base catalyzed hydrolysisin succinate buffer are 1.0009±0.0004, 1.0091±0.0002, and 1.0115±0.0002, respectively. The maximum expected effect, 1.052, for rate-determining scission of the acyl carbon-methoxyl oxygen bond was calculated from the observed difference in the stretching frequencies between leO and lsO ester. The observed isotope effect for hydrazinolysis under conditions of rate-determiningbreakdown of the tetrahedral intermediate at low pH is 1.0621±0.0008, while at high pH, where attackof hydrazine on the ester or a proton transport step is rate determining, it is only 1.0048±0.0006. The values of the ratios of the rates of hydrolysis to carbonyl oxygen exchange for methyl formate are 11.4±0.1 in acidand 18.3±0.4 in alkaline solution. The extremely small kinetic isotope effect observed for acid-catalyzed hydrolysis suggests a mechanism in which equilibrium protonation of the ester is counterbalanced by the normal kinetic isotopeeffect for the attack of water on the oxocarbonium ion. The small but significant isotope effects observed for alkalinehydrolysis, general base catalyzed hydrolysis, and hydrazinolysis at highpH suggest a transi-tion state in which the order of the acyl carbon-methoxyl oxygen bond of 1.15 in the ester is only slightly reduced in the transition state.