Esters hydrolysis reactions

As would be expected, the slopes are almost identical the intercept difference shows that methyl benzoate reacts about 1.5 times as fast as does ethyl benzoate in the standard state, a result easily attributable to the slight increase in steric crowding to the equation (42) hydrolysis in the latter case. The order of the A2 ester hydrolysis reaction in water is thus two, a result quite difficult to obtain in other ways, even in dilute solution, perhaps requiring a proton inventory study of a reaction that is very slow in water. 

Molecular dynamics free-energy perturbation simulations utilizing the empirical valence bond model have been used to study the catalytic action of -cyclodextrin in ester hydrolysis. Reaction routes for nucleophilic attack on m-f-butylphenyl acetate (225) by the secondary alkoxide ions 0(2) and 0(3) of cyclodextrin giving the R and S stereoisomers of ester tetrahedral intermediate were examined. Only the reaction path leading to the S isomer at 0(2) shows an activation barrier that is lower (by about 3kcal mol ) than the barrier for the corresponding reference reaction in water. The calculated rate acceleration was in excellent agreement with experimental data. ... 

There are few, if any, examples of intramolecular bifunctional catalysis in aqueous solution. Maugh and Bruice (1971) examined ester hydrolysis reactions where two functional groups are present in... 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Tlie neurotransmitter acetylcholine is both a quaternary ammonium compound (see Box 6.7) and an ester. After interaction with its receptor, acetylcholine is normally degraded by hydrolysis in a reaction catalysed by the enzyme acetylcholinesterase. This enzyme contains a serine residue that acts as the nucleophile, hydrolysing the ester linkage in acetylcholine (see Box 13.4). This effectively acetylates the serine hydroxyl, and is an example of transesterification (see Section 7.9.1). For continuation of acetylcholine degradation, the original form of the enzyme must be regenerated by a further ester hydrolysis reaction. 

Another colorimetric assay for testing the enantioselectivity of lipases or esterases in ester hydrolysis reactions is based on a different principle (75). To simulate the state of competitive conditions of an enzymatic process, the so-called Quick-ii-Test... 

Although the Zucker-Hammett hypothesis works rather well in most cases, and usually gives the correct answer in ester hydrolysis reactions, for example, it does lead on occasion to incorrect conclusions, and since it is now quite clear that one of its main underlying assumptions, that the protcnation behaviour of all neutral substrates is quantitatively similar, is incorrect, it has been superseded by more accurate treatments. Unfortunately, none of these is as simple to operate as the Zucker-Hammett hypothesis, nor has any yet attained widespread use. But these improved treatments have resulted in a distinct advance in our understanding of ester hydrolysis in strong acid, and one recent approach will be discussed here in some detail. 

Following this, a reaction of type A + C is considered. Many esterification and ester hydrolysis reaction, can be brought to this form. The analysis reveals that, for the latter reaction, total conversion is always possible provided that the feed concentration is sufficiently low. In contrast to this, limitations will arise for high feed concentrations similar to the previous case - that is, if the reactant has highest or lowest affinity to the solid phase. It is worth noting that these results are consistent with experimental observations [28]. 

Many practical applications deal with a reaction of type A B + C rather than 2A B + C. Typical examples, which can be brought to this form are esterification [25, 26] and ester hydrolysis reactions [9, 28]. In general the reaction mechanism is... 

S. Grtiner, A. Kienle, P. T. Mai, et ah, Analysis of ester hydrolysis reactions in a chromatographic reactor using equilibrium theory, 2004. International Symposium on Preparative and Industrial Chromatography and Allied Techniques, SPICA 2004, October 17-20, Aachen, Germany. 

In addition to broad-scope substrate specificity, 38C2 exhibits high enantioselectivity for the aldol reaction. Although this high degree of enantioselectivity has been observed for antibody-catalyzed ester hydrolysis reactions, it is certainly not a feature common to all such catalysts (Janda et al., 1989 Lo et al., 1997 Pollack et al., 1989 Tanaka et al., 1996 Wade and Scanlan, 1996). Furthermore, the rules for the enantioselectivity for 38C2-catalyzed aldol reactions are both simple and general (Hoffmann et al., 1998). For most ketone donors, attack occurs on the si side of the acceptor. However, when a ketone with an a-hydroxy substituent (such as hydroxyacetone) acts as donor, attack occurs on the reside (Scheme 5). 

The third step of this sequence, illustrated below, is an ester hydrolysis reaction. The mechanism for a base-mediated ester hydrolysis was highlighted in Scheme 7.19. 

Dinuclear Hydrolytic Catalysis. Recent biochemical and protein crystallographic studies have revealed that di- or trinuclear metal ion centers (i.e., with Zn, Mg, Mn, Fe, Co, or Ni) effect peptidase or phosphatase (i.e., amide or phosphate ester hydrolysis) reactions in a variety of systems, Table II. Protein systems that carry out amide hy-... 

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

Section IIB). Either 35 or 36 probably applies in general for other substrates, though cyclic transition states cannot be ruled out. The general acid transition state 37 can be considered unlikely in view of the fact that few intermolecular general acid-catalyzed ester hydrolysis reactions are known. Water undoubtedly does, however, play an important role in solvation of the carbonyl oxygen in the transition state. 

Presented in Table 1 is a summary of the second-order rate constants for the hydrolysis of 4-nitrophenyl acetate promoted by various zinc complexes in aqueous solution. A conclusion that may be drawn from this data is that change in the nature of the supporting chelate ligand dramatically influences the second-order rate constant for the carboxy ester hydrolysis reaction. Notably, the slowest rate is found for the four-coordinate zinc hydroxide species [([12]aneN3)Zn(OH)]+ which has three... 

Outside the scope of coverage of this contribution are Zn(II)-promoted carboxy-ester hydrolysis reactions involving multinuclear zinc complexes or systems wherein the first step involves transesterification.98,100-102,105,109-114... 

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