OXYGEN EXCHANGE AS EVIDENCE FOR THE EXISTENCE OF AN INTERMEDIATE IN ESTER HYDROLYSIS
OXYGEN EXCHANGE AS EVIDENCE FOR THE EXISTENCE OF AN INTERMEDIATE IN ESTER HYDROLYSIS
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DOI：
10.1021/ja01148a063
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发表时间：
1951-01-01
影响因子：
15
通讯作者：
BENDER, ML
中科院分区：
文献类型：
--
作者：
BENDER, ML
苯甲酸乙酯、苯甲酸丙酯和苯甲酸丁酯已用O18标记在羰基上。测定了这些酯的水解动力学和伴随着水解的氧交换动力学。在水解过程中，酯的羰基氧和溶剂之间的氧交换在每种情况下都发生了，这一事实有力地表明，在水解过程中发生了真正的中间体，在该中间体中，羰基氧以可逆的方式参与。该中间体最可能的结构必须包括酯的非电离水合物CeH6-C(OH)2(OR)的贡献，因为中间体必须具有能够交换的对称结构。在33%二氧六环中，苯甲酸乙酯的kh/k为10.6；苯甲酸异丙酯为3.7；苯甲酸异丁酯为7.6。这三种酯的kh/kt很相似，而苯甲酸乙酯的酸和碱水解的kh/ke实际上是相同的，这表明在所有情况下都有类似的中间体。从许多关于这些简单的羧酸酯的水解的提议中，有两套突出的机理，一套是Hammett3提出的，另一套是Day和InGold提出的。4这两位作者的公式符合文献中提供的动力学、旋光性和同位素交换数据。使用O18的实验表明，在碱性5和酸6的水解过程中，简单的羧酸酯的断裂都伴随着酰基离子的分裂发生。根据这一证据和其他证据，Hammett提出，碱性水解包括将羟基离子加到羰基碳上，然后或伴随着烷氧基的释放，而酸解可能涉及将水加到以原酸单酯为中间体的羰基双键上。Day和InGold说明了碱水解有两种机制，一种涉及加成中间体，另一种涉及过渡态。它们用一个涉及过渡态的机制说明了酸的水解。在讨论碱催化的机理时，戴和英戈尔德指出，I和II是不同的
Ethyl benzoate,¿-propyl benzoate and¿-butyl benzoate have been labeled in the carbonyl group with O18. The kinetics of the hydrolysis of these esters, and of the oxygen exchange accompanying hydrolysis, havebeen determined. The fact that oxygen exchange occurs in every case between the carbonyl oxygen of the ester and the solvent during the process of hydrolysis strongly suggests that a true intermediate, in which the carbonyl oxygen participates in a reversible fashion, occurs during the hydrolysis. The most probable structure ofthis intermediate must include a contribution from the un-ionized hydrate of the ester, CeH6-C (OH) 2 (OR), since theintermediate must assume a symmetrical structure capable of exchange. In 33% dioxane, kh/k, for ethyl benzoate is 10.6; for¿-propyl benzoate it is 3.7; for¿-butyl benzoate it is 7.6. Thesimi-larity of kh/kt for these three esters and thevirtual identity of kh/ke for both the acid and alkaline hydrolysis of ethyl benzo-ate sugggest a similar intermediate in all cases.From a multitude of proposals for the hydrolyses of those simple carboxylic esters which occur by means of a bimolecular reaction involving acylion fission, two outstanding sets of mechanisms have been put forth, one by Hammett3 and another by Day and Ingold. 4 The formulations of both these authors fit the kinetic, optical activity and isotope exchange data available in the literature. Experiments utilizing O18 have demonstrated that the rupture of simple carboxylic esters takes place with acyl-ion fission during both alkaline5 and acid6 hydrolysis. Hammett proposes, on the basis of this and other evidence, that alkaline hydrolysis involves the addition of hydroxyl ion to the carbonyl carbon, followed or accompanied by the release of the alkoxyl group, and that acid hydrolysis probably involves the addition of water to the carbonyl double bond with the monoester of the ortho acid as an intermediate. Day and Ingold illustrate alkaline hydrolysis with two mechanisms, one involving an addition intermediate, the other a transition state. They illustrate acid hydrolysis with a mechanism involving a transition state. In discussing the base-catalyzed mechanism, Day and Ingold state that I and II “differ