Hydrolysis rate constant, pseudo-first-order

Basis for the kinetic model is a standard batch hydrolysis experiment ( ). Fig. lA shows the standard hydrolysis curve for soy protein isolate - Alcalase. The reaction constant (pseudo first order rate constant) is calculated from the standard curve by fitting the inverse curve in a small DH-range 1 3 ) to a se-... 

These CD-based polymers were engaged in hydrolysis reaction of various p-nitrophenol esters derivatives. The reaction was monitored by UV at 400 mn. Among the CD-based polymers, only poly- 3-CD-A showed a high hydrolysis activity. The pseudo first-order rate constant k measured for the ester hydrolysis catalyzed by poly- 3-CD-A is compared to the rate constant measured using the native 3-CD. The rate enhancements resulting from the use of poly- 3-CD-A are presented in Table 2.1. 

Fig. 8.P3I. Plot of the pseudo-first-order rate constants for hydrolysis of thioesters A (O), B ( ), C (A), D (A) as a fiinction of pH at 50°C and ionic strength 0.1 (KCI). Lines are from fits of the data to = kon(K /H+)) + (k KJK + [//+])) where koH is the hydroxide term and is the intramolecular assistance term for B and C and from linear regression for A and D. Reproduced from problem reference 31 by permission of the American Chemical Society.

Table 2-4 gives data for the alkaline hydrolysis of phenyl cinnamate under pseudo-first-order conditions, with calculations made in order to apply the Guggenheim method. The plot according to Eq. (2-55) is shown in Fig. 2-9. From the slope the pseudo-first-order rate constant is 3.37 x 10 s . ... 

These are pseudo-first-order rate constants for the alkaline hydrolysis of ethyl / -nitrobenzoate at 25°C. 

Quantitative structure-reactivity analysis is one of the most powerful tools for elucidating the mechanisms of organic reactions. In the earliest study, Van Etten et al. 71) analyzed the pseudo-first-order rate constants for the alkaline hydrolysis of a variety of substituted phenyl acetates in the absence and in the presence of cyclodextrin. The... 

The overall catalytic rate constant of SNase is (see, for example, Ref. 3) kcat — 95s 1 at T = 297K, corresponding to a total free energy barrier of Ag at = 14.9 kcal/mol. This should be compared to the pseudo-first-order rate constant for nonenzymatic hydrolysis of a phosphodiester bond (with a water molecule as the attacking nucleophile) which is 2 x 10 14 s corresponding to Ag = 36 kcal/mol. The rate increase accomplished by the enzyme is thus 101S-1016, which is quite impressive. 

Pseudo-first-order rate constants (k0t,s) determined from the linear relationship of In (Am-At) with time, were calculated for different concentrations of catalyst. A pseudo-first-order rate constant was also calculated for the uncatalyzed hydrolysis at pH 8.5, and was substracted from the values found for the catalyzed hydrolysis. 

The identification of the structures responsible for D1 and D2 and their stabilities toward hydrolysis are further supported by investigations of the hydrolysis behavior of the corresponding isolated ring molecules, octamethylcyclotetrasiloxane (OMCT) and hexamethylcyclotrisiloxane (HMCT) (51.). OMCT is stable in aqueous environments, whereas HMCT hydrolyzes with a pseudo-first order rate constant 3.8 ( 0.4) x 10 3 min 1 (tt, 2 — 3.0 hours). This latter value is comparable to the rate constant for D2 hydrolysis, 5.2 ( 0.5) x 10 3 min"1 (t1/2 - 2.2 hours) and is 75x greater than the rate constant describing hydrolysis of unstrained, conventional a-Si02 (as estimated by extrapolation of the data in reference (52.). 

Figure 1. Hydrolysis pH-rate profiles of phenyl acetate (lower) and a substituted 2-phenyl-l,3-dioxane (HND). Phenyl acetate profile constructed from data of Mabey and Mill (32), HND profile from data of Bender and Silver (33). Phenyl acetate reacts via specific-acid catalyzed, neutral, and base-catalyzed transformation pathways. The pseudo-first-order rate constant is given by Kobs = K(h+) [H+] + Kn + K(qh-) [0H—]. HND hydrolyzes only via an acid-catalyzed pathway the phenolate anion is some 867 times more reactive than its conjugate acid.

Thereafter, the reaction between the coordinated dienoate ligand in 106 and acrylonitrile was examined (Scheme 12). Surprisingly, this reaction is complete within 56 h (pseudo-first-order rate constant k — 1.4 x 10-5 s-1, ti/2 ca. 0.5 day) and affords only two products 107a and 107b in a ratio of 57 43 (or the correepsonding acids 103a,b via acid hydrolysis). Thus, in... 

The most outstanding example illustrating this strategy came from the team of Alain Friboulet and Daniel Thomas, who produced anti-idiotype antibodies against a monoclonal antibody AE2 that was a competitive inhibitor of acetylcholine esterase. One of the selected antibodies, 9A8, catalyzes the hydrolysis of acetylthio-choline with a pseudo first-order rate constant /feat = 81 and a factor of acceleration of 4.2 x 10 . These remarkable parameters, which are only two orders of magnitude lower when compared to those of the enzyme, make abzyme 9A8 the most powerful abzyme known until now. 

The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyl-trimethylammonium bromide (TTABr) and sodium dodecyl sulfate (SDS) on the rates of alkaline hydrolysis of securinine (223) were studied at a constant [HO ] (0.05 m). An increase in the total concentrations of CTABr, TTABr and SDS from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, and 7, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelles. Cationic micelles of CTABr speed attack of hydroxide ion upon coumarin (224) twofold owing to a concentration effect. ... 

CASRN 101200-48-0 molecular formula C15H17N5O6S FW 365.39 ChemicaPPhysical. The pseudo-first-order hydrolysis rate constants of tribenuron-methyl in aqueous buffers solutions at 40 °C were 1.1.38/d at pH 7 and 3.66 x lO /d at pH 10. At pH 4 and 22 and 40 °C, the reaction was too fast to determine the hydrolysis rate constant. (Berger and Wolfe, 1996). 

In which kg Is the pseudo-first-order rate constant for OP hydrolysis In the absence of PB. With PB In great excess. In buffered solution, kg and [HO2 ] are constant. Then pseudo-first-order kinetics result and we get equation (8),... 

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. 

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