Hydrolysis second-order rate constants

Apply the steady-state approximation to Scheme XXII for ester hydrolysis to find how the experimental second-order rate constant qh is related to the elementary rate constants. 

Second-Order Rate Constants for the Hydrolysis of Immonium Ions, Derived from l-(2-Methylpropenyl)Pyrrolidine at 24.8°C ... 

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference k03 = 3.75 x 10 16 cm3 molecule-1 s-1 at 298 K (cw-cyclooctene, recommended, Atkinson 1997) Hydrolysis ... 

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

The individual contributions of the H20, H+, and HO- catalysts to the mechanism of the reaction were further evaluated by means of the kinetics parameters (Table 6.4). At neutral pH, Reactions a and c were both dominated by fcH2<> The second-order rate constants ku+ and kHO- were identical, indicating similar efficiencies of the H+ and HO catalysts. Interestingly, the second-order rate constants for the hydrolysis of Gly-D-Val (6.48) to yield Gly and D-Val (6.49) (Reaction b) could also be calculated (Table 6.4). The similarity to the corresponding rate constants of Reactions a and c suggests that the rate of peptide bond hydrolysis is not particularly sensitive to substitution at or protonation of the flanking amino and carboxy groups [69],... 

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... 

Second-order rate constants A for the replacement of a ring-substituted benzoate group by hydroxide ion in a number of complexes (base hydrolysis) have been carefully determined at 25°C in 40% aqueous methanol ... 

The second-order rate constants k for the base hydrolysis of a number of cobalt(lll) complexes were measured with a simple flow apparatus using conductivity as a monitoring device. Equal concentrations (Ag) of reactants were used. Show that a plot of R,/R — R, vs time is linear, having slope s, and that... 

The amide and peptide linkages are much more difficult to hydrolyze than the ester grouping. Both free and metal bound groups hydrolyze with second-order rate constants approximately 10 -10 less than for the corresponding esters. There are two potential sites for coordination in the -CONHR residue, namely at the carbonyl O in 13 and at the amide N in 14 where ionization of the amide proton is induced (Sec. 6.4.3). Cu + promotes hydrolysis of glycinamide at low pH where it is present as 13. However it inhibits hydrolysis at high pH, where it is 14, to such a degree that hydrolysis cannot be observed. ... 

Considerable attention has been paid to this transformation (which is sometimes referred to as hydration ) in the past 15 years. 2. early example of the effect was the marked acceleration of the base hydrolysis of 2-cyanophenanthroline by Ni +, Cu + and Zn " " ions. The second-order rate constant is lO -fold higher for the Ni complex than for the free ligand, residing mainly in a more positive AS An external OH attack on the chelate was favored but an internal attack by Ni(II) coordinated OH cannot be ruled out. Nickel-ion catalysis of the hydrolysis of the phenanthroline-2-amide product is much less effective, being only about 4 x 10 times the rate for spontaneous hydrolysis. ... 

The 42-residue peptide KO-42 folds in solution into a hairpin helix-loop-helix motif that dimerizes to form a four-helix bundle. On the surface of the folded motif there are six histidines with assigned piC values in the range 5.2 to 7.2 (Fig. 1) and the second-order rate constant for the hydrolysis of mono-p-nitro-phenyl fumarate is 1140 times larger than that of the 4-methylimidazole-cataly-zed reaction at pH 4.1 and 290 K [13]. The reaction mechanism was found to be pH dependent as the kinetic solvent isotope effect was 2.0 at pH 4.7 and 1.0 at pH 6.1 and the pH dependence showed that the reaction rate depended on residues in their unprotonated form with piCj, values around 5. It was thus established that there are functional cooperative reactive sites that contain protonated and unprotonated His residues. 

Chemical/Physical. Hydrolyzes in water to o-phthalic acid (via the intermediate 2-ethyl-hexyl hydrogen phthalate) and 2-ethylhexyl alcohol (Kollig, 1993 Wolfe et al., 1980). Although no pH value was given, the reported hydrolysis rate constant under alkaline conditions is 1,400/M-yr (Ellington et al., 1993 Kollig, 1993). A second-order rate constant of 1.1 x 10 /M-sec was reported for the hydrolysis bis(2-ethylhexyl) phthalate at 30 °C and pH 8 (Wolfe et al., 1980). 

The following relative second-order rate constants have been obtained for hydroxide ion-catalysed hydrolysis glycine ethyl ester, 1 protonated glycine ethyl ester, 41 and the cupric ion complex of glycine ethyl ester, F3 x 10 (Conley and Martin, 1965). The large effect of the cupric ion cannot be due entirely to electrostatic effects, but rather to catalysis by direct co-ordination with the ester function. 

Chlorpyrifos. As was the case for the neutral hydrolysis studies, the most detailed kinetic investigations of alkaline hydrolysis kinetics in sediment/water systems have been conducted using chlorpyrifos (10). As can be seen from Figure 2, alkaline hydrolysis of chlorpyrifos is not second-order, so the value selected for k cannot be calculated from the pH and a second-order rate constant. Nevertheless, since aqueous kinetics at alkaline pH s for chlorpyrifos was always pseudo-first order, careful pH measurements and Figure 2 can be used to select accurate values for k at any pH. 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

The second-order rate constant for the reaction between sarin and either 2-PAM I or II was found to be 170 L/mol per minute. If a phosphorylated or phosphonylated oxime that does not enter rapidly into the second step above is formed, that product may be an Inhibitor of cholinesterase. 7,88 Hydrolysis of sarin in the presence of 200-fold concentrations of V and II took place more rapidly in plasma from rats with the former oxime than with the... 

Comparison of Second-Order Rate Constants (M 1sec 1) in Hydrolysis (or Acetyl and Phosphoryl Transfer) of 4-Nitrophenyl Acetate, Methyl Acetate and Bis(4-nitrophenyl) Phosphate... 

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