Mechanism ester hydrolysis

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

Table 4, Classification of esterification mechanisms and of ester hydrolysis mechanisms according to Ingold248)... 

Taft, following Ingold," assumed that for the hydrolysis of carboxylic esters, steric, and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base (see the discussion of ester-hydrolysis mechanisms. Reaction 10-10). Rate differences would therefore be caused only by the field effects of R and R in RCOOR. This is presumably a good system to use for this purpose because the transition state for acid-catalyzed hydrolysis (7) has a greater positive charge (and is hence destabilized by —I and stabilized by +1 substituents) than the starting ester. 

CHOLESTERYL ESTER HYDROLASE ESTER HYDROLYSIS MECHANISMS ETA (7,)... 

TETRAHEDRAL INTERMEDIATE ESTER HYDROLYSIS MECHANISM CHYMOTRYPSIN SERPINS (Inhibitory Mechanism)... 

Wladkowski, B.D., L.A. Svensson, L. Sjolin, J.E. Ladner, and G.L. Gilliland. 1998. Structure (1.3 A) and charge state of a ribonuclease A-uridine vanadate complex Implications for the phosphate ester hydrolysis mechanism. J. Am. Chem. Soc. 120 5488-5498. 

We can illustrate the interpretation of pH rate profiles by considering the pH rate profile for the hydrolysis of aspirin. The pH rate profile and that of its methyl ester are shown in Figure 3.30. The methyl ester is a classic case in which there are both acid-and base-catalyzed regions and an uncatalyzed region as described by Equation (3.58). These features are characteristic of the normal ester hydrolysis mechanism outlined on pp. 325-326. Under the reaction conditions, only the more reactive O-aryl ester group is hydrolyzed. Aspirin, however, shows a region between pH 2 and pH 9 that reveals one downward and two upward bends. Thus there must be a fourth variant of the mechanism (since there are a total of three upward bends). 

TABLE 7.9 Ester Hydrolysis Mechanisms in Concentrated Sulfuric Acid Solutions ... 

Acid-catalysed ester hydrolysis mechanism (based on Sykes, 1986 ). 

With Fischer esterification and ester hydrolysis, we can see the principle of microscopic reversibility by comparing the mechanism for Fischer esterification. (See Primer II (page 699) and the ester hydrolysis mechanism box immediately... 

The reaction of dimethylsulfoxonium methylide with enantiopure esters provided the corresponding carboxylic acids at room temperature without racemization. 0-labelled reagents showed that the reaction does not proceed through an ester hydrolysis mechanism. This methodology is general and can be considered a valid alternate route for ester cleavage when a substrate is sensitive to hydrolysis conditions. 

These copolymers, by exhibiting differences in their ester hydrolysis mechanisms, permit an increase in the apolar nature of the catalyst by incorporation of the poly[4(5)-vinylimidazole] groups while simultaneously incorporating the 1-alkly substituted imidazole capable of rapid substrate turnover. 

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