Kinetic studies ester hydrolysis

For the analysis of esters by RP-HPLC, it is very hard to avoid water because it will be present in the sample extraction solvent and even in the mobile phase. However, the kinetics of ester hydrolysis in solution can be studied by stopped flow injection experiments to determine compatibility with mobile phase. The same experiment must be repeated with different pH (from pH 1.2 to pH 6.8), since it is known that ester hydrolysis is pH-dependent. An example of pH-dependent ester hydrolysis is methylphenidate HCl. Methylphenidate HCl has an ester functional group, and its hydrolysis is pH-dependent. Refer to Table A15-1 for stability of methylphenidate HCl at different pHs [5]. The hydrolysis product of methylphenidate HCl is the free acid (refer to Figure A15-3) [5]. 

Akbulatov S, Xian Y, Kapustin E, Boulatov R (2013) Model studies of the kinetics of ester hydrolysis under stretching force. Angew Chem Int Ed 52 6992... 

Solvolysis of Organic Ligands.— The catalysis of hydrolysis of organic esters by metal ions or complexes has been much studied for many years. Recent examples of kinetic studies include hydrolysis of oxalate esters catalysed by a variety of ions, and of the bis[-L-(-J-)-histidine methyl ester] complexes of copper(n) and of nickel(n). The relative catalytic effects of several copper(n) complexes, including Cu(imda), Cu(nta), and Cu(dien) +, on the hydrolysis of methyl glycinate have been determined. Rate constants for base hydrolysis of this ester correlate with stability constants for mixed complexes of this ester with the above-named copper(n) complexes. ... 

In one of the earliest kinetic studies of an organic reaction earned out m the nine teenth century the rate of hydrolysis of ethyl acetate m aqueous sodium hydroxide was found to be first order m ester and first order m base... 

Carboxyhc acid ester, carbamate, organophosphate, and urea hydrolysis are important acid/base-catalyzed reactions. Typically, pesticides that are susceptible to chemical hydrolysis are also susceptible to biological hydrolysis the products of chemical vs biological hydrolysis are generally identical (see eqs. 8, 11, 13, and 14). Consequentiy, the two types of reactions can only be distinguished based on sterile controls or kinetic studies. As a general rule, carboxyhc acid esters, carbamates, and organophosphates are more susceptible to alkaline hydrolysis (24), whereas sulfonylureas are more susceptible to acid hydrolysis (25). 

Kinetic Considerations. Extensive kinetic and mechanistic studies have been made on the esterification of carboxyHc acids since Berthelot and Saint-GiHes first studied the esterification of acetic acid (18). Although ester hydrolysis is catalyzed by both hydrogen and hydroxide ions (19,20), a base-catalyzed esterification is not known. A number of mechanisms for acid- and base-catalyzed esterification have been proposed (4). One possible mechanism for the bimolecular acid-catalyzed ester hydrolysis and esterification is shown in equation 2 (6). 

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. 

The case of intramolecular participation in ester hydrolysis has been extensively studied using acetylsalicylic acid (aspirin) and its derivatives. The kinetic data show that the anion is hydrolyzed more rapidly than the neutral species, indicating that the carboxylate group becomes involved in the reaction in some way. Three mechanisms can be considered ... 

The kinetics of alkaline hydrolysis of phenyl cinnamate were studied at 25°C, in solutions containing 0.8% acetonitrile ionic strength, 0.3 M initial ester, 8.19 X 10- M reaction followed spectrophotometrically in 5-cm cells at 295 nm. For studies at three pH values, these absorbance data were obtained. The pH was established with sodium hydroxide of the normality specified in the heading of the table (as titrimetrically determined). 

Substrate and product inhibitions analyses involved considerations of competitive, uncompetitive, non-competitive and mixed inhibition models. The kinetic studies of the enantiomeric hydrolysis reaction in the membrane reactor included inhibition effects by substrate (ibuprofen ester) and product (2-ethoxyethanol) while varying substrate concentration (5-50 mmol-I ). The initial reaction rate obtained from experimental data was used in the primary (Hanes-Woolf plot) and secondary plots (1/Vmax versus inhibitor concentration), which gave estimates of substrate inhibition (K[s) and product inhibition constants (A jp). The inhibitor constant (K[s or K[v) is a measure of enzyme-inhibitor affinity. It is the dissociation constant of the enzyme-inhibitor complex. 

There is much evidence for this mechanism, similar to that discussed for ester hydrolysis. A MO study on the mechanism of amide hydrolysis is available. In certain cases, kinetic studies have shown that the reaction is second order in OH , indicating that 107 can lose a proton to give 108. Depending on the nature... 

Recent kinetic studies on thiophosphoric aryl ester dianilides suggest analogous decomposition. The rate law observed is in agreement with a hydrolysis scheme in which both the monoanion and the dianion decompose to metaphosphorothioimi-date and its anion, respectively, which then react fast with water133). 

The behavior of metal ions in reversed micelles may be more interesting, since the reversed micelle provides less solvated metal ions in its core (Sunamoto and Hamada, 1978). Through kinetic studies on the hydrolysis of the p-nitrophenyl ester of norleucine in reversed micelles of Aerosol OT and CC14 which solubilize aqueous cupric nitrate, Sunamoto et al. (1978) observed the formation of naked copper(II) ion this easily formed a complex with the substrate ester (formation constant kc = 108—109). The complexed substrate was rapidly hydrolyzed by free water molecules acting as effective nucleophiles. 

In chymotrypsin and other serine proteases the imidazole moiety of histidine acts as a general base not as a nucleophile as is probably the case in the catalysis of activated phenyl ester hydrolysis by (26). With this idea in mind, Kiefer et al. 40) studied the hydrolysis of 4-nitrocatechol sulfate in the presence of (26) since aryl sulfatase, the corresponding enzyme, has imidazole at the active center. Dramatic results were obtained. The substrate, nitrocatechol sulfate, is very stable in water at room temperature. Even the presence of 2M imidazole does not produce detectable hydrolysis. In contrast (26) cleaves the substrate at 20°C. Michaelis-Menten kinetics were obtained the second-order rate constant for catalysis by (26) is 10 times... 

Detailed kinetic studies revealed that glycine methyl ester and phenylalanine methyl ester in glycine buffer at pH 7.3 undergo a facile hydrolysis catalyzed by cupric ion (11). Under these conditions the reactions closely follow a first-order rate law in the substrate. Using these kinetic data it is possible to compare the rates of hydrolysis of DL-phenylalanine ethyl ester as catalyzed by hydronium, hydroxide, and cupric ion (see Table III). 

The initial evidence for the formation of an acyl-enzyme ester intermediate came from studies of the kinetics with which chymotrypsin hydrolyzed analogs of its normal polypeptide substrates. The enzyme turned out to hydrolyze esters as well as peptides and simpler amides. Of particular interest was the reaction with the ester p-nitrophenyl acetate. This substrate is well suited for kinetic studies because one of the products of its hydrolysis, p-nitrophenol, has a yellow color in aqueous solution, whereas p-nitrophenyl acetate itself is colorless. The change in the absorption spectrum makes it easy to follow the progress of the reaction. When rapid-mixing techniques are used to add the substrate to the enzyme, an initial burst of p-nitrophenol is detected within the first few seconds, before the reaction settles down to a constant rate (fig. 8.8). The amount of p-nitrophe-... 

Three new macrocyclic ligands (187) when complexed with zinc(II) could promote ester hydrolysis and a kinetic study of the hydrolysis of 4-nitrophenyl acetate in Tris buffer at pH 8.63 in 10% (v/v) MeCN was earned out with these.153 The hydrolysis of lipophilic esters is also catalysed by zinc(H) in a complex of a long alkyl-pendant macrocyclic tetraamine (188) in micellar solution.154 A study with a copper chloride-containing micelle has compared its effectiveness in the hydrolysis of esters and amides.155... 

A few papers on phosphonate chemistry have appeared.235-237 The synthesis of the novel nucleoside bicyclic trisanhydrides (256 A = adenosin-f-yl) has been reported.235 A kinetic study of the alkaline hydrolysis of 4-substituted phenyl ethyl benzyl phosphonates (257) supports an associative A-E mechanism for the hydrolysis.236 The direct preparation of the esters of -nitrobenzyIpliosplionic acid from -nitrobenzyl halides has been reported.237... 

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