Rate constant of neutral hydrolysis

It is well-known that at-PMMA maintains a compact-coil conformation below pH 4.5 and an extended-coil conformation above pH = 5.5. The conformational transition takes place in the intermediate pH region. Thus, the kinetic effects will reflect the influence of the compact coil of PMAA. Changes in the pseudo-first-order rate constants (/Jobsd) neutral hydrolysis of p-methoxyphenyl dichloroacetate... 

Two equivalents of acid are produced that neutralize one molecule of diamine. Consequently, a factor taking the hydrolysis reaction into account has to be subtracted from the growth rate dL/dt. The hydrolysis reaction depends on the film thickness L and on a constant k which is proportional to the rate constant of the hydrolysis reaction ... 

The rate constants of the hydrolysis of P(OEt)3 were studied at different pH values (Scheme 2.109) [143]. In highly diluted aqueous solutions, rapid hydrolysis to the monoethylester was observed. The reaction followed first-order kinetics with an activation energy of 18.7 kcal mol . Hydrolysis in an aqueous KOH solution also followed first-order kinetics with an activation energy of 5.6 kcal mol . Further treatment of (Et0)2P(0)H in a neutral or acid solution resulted in the loss of the second equivalent of alcohol. It was assumed that the second step was faster than the liberation of the first alkoxy unit. The reaction is accelerated in strongly acidic solutions. 

Several 1 -phosphates of deoxyfluoro sugars were prepared, and their acid-catalyzed hydrolysis was studied. 2-Deoxy-2-fluoro- (580), 3-deoxy-3-fluoro- (582), 4-deoxy-4-fluoro- (583), and 6-deoxy-6-fluoro-a-D-gluco-pyranosyl phosphates (584) were prepared by treatment of the corresponding per-( -acetylated )9-D-glucopyranoses with phosphoric acid [the p anomer (581) of 580 was prepared by a different method]. The first and second ionization constants (pA a, and pA a2) of these compounds were determined potentiometrically, as well as by the F-n.m.r. chemical shifts at a series of pH values, and then the rate constants of hydrolysis for neutral (B) and monoanion (C) were decided. The first-order rate-constants (k) for 580-584 and a-D-glucopyranosyl phosphate (in Af HCIO4,25 °) were 0.068, 0.175, 0.480, 0.270, 1.12, and 4.10 (all as x lOVs), respectively. The rate... 

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],... 

Hydrolysis of 1,2-dichloroethane under alkaline and neutral conditions yielded vinyl chloride and ethylene glycol, respectively, with 2-chloroethanol, and ethylene oxide forming as the intermediates under neutral conditions (Ellington et al, 1988 Jeffers et al, 1989 Kollig, 1993). The reported hydrolysis half-life in distilled water at 25 °C and pH 7 is 72.0 yr (Jeffers et al, 1989), but in a 0.05 M phosphate buffer solution the hydrolysis half-life is 37 yr (Barbash and Reinhard, 1989). Based on a measured hydrolysis rate constant of 1.8 x 10 at 25 °C and pH 7, the half-life is 71.5 yr (Jeffers and Wolfe, 1996). 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

The hydrolysis of pesticides which are sorbed to sterilized natural sediments has been investigated in aqueous systems at acid, neutral and alkaline pH s. The results show that the rate constants of pH independent ("neutral") hydrolyses are the same within experimental uncertainties as the corresponding rate constants for dissolved aqueous phase pesticides. Base-catalyzed rates, on the other hand, are substantially retarded by sorption and acid-catalyzed rates are substantially enhanced. A large body of evidence will be presented which substantiates these conclusions for a variety of pesticide types sorbed to several well-characterized sediments. The significance of our results for the evaluation of the effects of sorption on the degradation of pesticides in waste treatment systems and natural water bodies will also be discussed. 

Alkali diphosphates are practically stable in alkaline and neutral solutions, but they are more and more rapidly hydrolyzed with decreasing pH. Over the whole pH range, however, they are more stable, under the same conditions, than all other condensed phosphates. Hydrolysis is accelerated by an increasing temperature or ionic strength (44, 846). The table below shows the dependence of the rate constant for the hydrolysis on pH at... 

Table 3-1. Rate constants for the hydrolysis of methyl glycinate, H2NCH2C02Me, in neutral water, in dilute acid and in the presence of copper(n) salts.

Hydrolysis k(neutral) = 1.6 x 10 4 Ir1 indicating that neutral hydrolysis is unimportant, rate constants of 7.5 x 10-3, 8.99 x 10, and 1.07 x 10 3 h 1 corresponded to half-lives of 92, 771 and 648 h in natural surface water samples from eutrophic pond, dystrophic reservoir and oligotrophic rock quarry, respectively (Saleh et al. 1982 quoted, Howard 1991)... 

Cofenlal is inert to substitution in neutral and acidic aqueous solution up to 100°C 160). Reaction with the hydroxide ion in the absence of heterogeneous catalysts occurs at reasonable rates only above 70°C. Friend and Nunn 160) studied the hydrolysis of this complex in basic conditions, at 80°C (/u, = 0.5) the reaction is first order in [OH ] and [Co(en)3 ], with a second-order rate constant of (5 1) X 10 " Assuming the activation parameters apply at... 

This method of finding the concentration of ions near the surface was applied by Davies (49,21) to the hydrolysis of ionized films of the ester monocetyl succinate. Table IX shows that the rate constant for this hydrolysis, which increased 300% if calculated using bulk concentrations of the catalytic hydroxyl ion, varied by not more than 36% when evaluated using the surface concentrations deduced from (xxv) and (xxvi). Figure 19 shows a similar effect for the addition of neutral salt, the marked catalysis by which is thus demonstrated to be due entirely to electrostatic effects. The acceleration in the rate of hydrolysis of a film cholesterol formate if the surface bears a negative charge can be predicted on the basis of the Donnan equations (xxv) and (xxvi). Values of 5 of 6 A. and 8 A. have been used, the results being compared with experiment in Fig. 21a. The calculated retardation is shown in Fig. 21b. 

It is apparent that at low moisture content (<10% for the Na-saturated clay mineral and <5% for the Ca-, or Mg-saturated clay mineral), where water is not available for hydrolysis, hydrolysis does not occur. This low moisture content corresponds with the saturation of the cation s first hydration shell. As the moisture content is increased to the upper limit of bound water (50% moisture content), a significant enhancement of the hydrolysis of the epoxide is observed. When the moisture content exceeds the upper limit of bound water (>50%), the rate constant for the hydrolysis of the epoxide was reduced by a factor of 4. It was concluded that water in excess of sorbed water diminishes the catalytic activity of clay surfaces by reducing the concentration gradient across the double layer, effectively raising the surface pH closer to that of the bulk water. In similar studies with MTC, the addition of water to oven-dried Na-montmorillonite and Na-kaolinite retarded the hydrolysis rate of the carbamate. This observation is consistent with the fact that MTC exhibits only neutral base-catalyzed hydrolysis. 

There is much evidence that the amines including polyamines accelerate hydrolyses of phosphoras acid esters via the general basic catalysis. In the unbuffered solution, a small amount of protonated amino groups are present in the PEI molecnles (<0.5-5.0% depending on the PEI molecular mass PEI with the molecular mass from 800 (PEI-800) to 60000 (PEI-60) were studied). This means that under these conditions PEIs behave as neutral polymers rather than polyelectrolytes. The pH kinetic profile danonstrates that up to pH 9.2 the observed rate constant of hydrolyses of 1 and 3 in 0.05M solution of PEI-50 does not exceed the limits of 5 x 10 s, while a further increase in the pH results in a ca. tenfold increase in the hydrolysis rate of both phosphonates. This pH dependence is rather typical and reflects the growth of the rate constant in the field of accumulation of catalytic species, namely nonprotonated amino groups, hi addition, the contribution of basic hydrolysis to kobs values at high pH should also be taken into account. 

ABSTRACT. Cyclodextrin (CD) has a hydrophobic cavity which acts like a binding site of an actual enzyme. But enzymatic turnover reaction did not occur in CD-catalyzed reactions.3-CD was modified by a histamine group to attach a reactive functional group. 3-CD-histamine accelerates the hydrolysis of p-nitrophenyl acetate. Catalytic rate constant of this reaction is close to an actual enzyme, a-chymotrypsin. Enzymatic turnover reaction is realized with this compound at around neutral pH value. 

The closeness of fit may be gauged from the experimental and theoretical rate vs. concentration curves for hydrolysis of p-nitrophenyl carboxylates catalysed by quaternary ammonium surfactant micelles (Figure 3). The shape of the curve is satisfactorily explained for unimolecular, bimolecular, and termolecular reactions. An alternative speculative model is effectively superseded by this work. Romsted s approach has been extended in a set of model calculations relating to salt and buffer effects on ion-binding, acid-dissociation equilibria, reactions of weakly basic nucleophiles, first-order reactions of ionic substrates in micelles, and second-order reactions of ionic nucleophiles with neutral substrates. In like manner the reaction between hydroxide ion and p-nitrophenyl acetate has been quantitatively analysed for unbuffered cetyltrimethylammonium bromide solutions. This permits the derivation of a mieellar rate constant km = 6-5 m s compared to the bulk rate constant of kaq =10.9m s . The equilibrium constant for ion-exchange at the surface of the micelle Xm(Br was estimated as 40 10. The... 

One of the most comprehensive studies has been carried out by Bruice et al. [19] who studied the rate of solvolysis of neutral, positively and negatively charged esters when incorporated into non-functional and functional micelles of neutral, positive and negative charges. The second-order rate constants for alkaline hydrolysis, /cqh [0H ] were found to decrease with increasing concentration of surfactant for all cases studied. The association of the esters with non-nucleophilic micelles must either decrease the availability of the esters to OH attack or provide a less favourable medium for the hydrolysis reaction to occur. This is another circumvention of the simple electrostatic rules as the kinetic effect seems to have nothing to do with the concentration or restriction of access of the hydroxyl ions in the Stern layer of the micelles. Presumably the labile ester bond is not positioned near the surface of these micelles, but the molecules are oriented as shown in Fig. 11.2. 

There are several explanations for the large concentration of monomers present under neutral and basic conditions. From Fig, 21 we see that the rate of hydrolysis of siloxane bonds increases by over three orders of magnitude between pH 4 and 7. Because hydrolysis occurs preferentially at less highly condensed Q sites [1], monomers are the primary by-product of siloxane bond hydrolysis. In a related study, Klemperer and Ramamurthi [93] have shown that siloxane bonds are broken by redistribution reactions under basic conditions (Eq. 42) that produce unhydrolyzed monomers as a by-product. In addition, from the pH-dependence of the hydrolysis reaction (Fig. 9), we see that the hydrolysis rate is minimized at neutral pH. Because the rate constant of the alcohol-producing condensation reaction is less than that of the water-producing reaction [63,95], unhydrolyzed or partially hydrolyzed monomers may persist in solution past the gel point. Presumably these combined factors contribute to the large concentrations of monomers observed under neutral and basic conditions. 

Most of the studies on the effects of metallomicelles on the rate of hydrolysis of esters involve so-called activated esters in which nucleophilic attack is the rate-determining step. The effects of copper-containing metallomicelles (formed from both copper(II)-hydrophobic ligand complex as well as from hydrophobic ligand [L = Af,Af,M-trimethyl-Ai -tetradecylethylenediamine] containing free ions) on the rate of hydrolysis of amides 19 and 20 as well as activated esters 21, 22, and 23 have been stndied at pH 7.0 and 31°C. The apparent rate enhancements (kj i) of Cn +(L) metallomicelles on the rate of hydrolysis of 19 to 23, 2-nitrophenyl acetate (2-NPA), and 4-nitrophenyl acetate (4-NPA) under various reaction conditions are summarized in Table 6.4." The actual rate enhancements due to metallomicelles, Cu +(L), under various reaction conditions are not possible to estimate because of the lack of pseudo-first-order rate constants (kobs) for hydrolysis of 19 to 23, 2-NPA, and 4-NPA in the presence of Cu -A7V,A 7V -tetramethylethylenediamine complex. However, the values of k,, for 19 and 20 are almost same at 0.002-M Cu +(L) and 0.002 M Cu +(L) + 0.001-M Triton. The presence of 0.001-Af CTABr comicelles has no effect on k, for 20 but decreases kj i from 46 to 29 for 19 (Table 6.4), which may be attributed to larger cationie mixed micellar affinity of anionic 19 than that of neutral 20. 

Sulfates having alkyl groups from methyl to pentyl have been examined. With methyl as an example, the hydrolysis rate of dimethyl sulfate iacreases with the concentration of the sulfate. Typical rates ia neutral water are first order and are 1.66 x lO " at 25°C and 6.14 x lO " at 35°C (46,47). Rates with alkaH or acid depend on conditions (42,48). Rates for the monomethyl sulfate [512-42-5] are much slower, and are nearly second order ia base. Values of the rate constant ia dilute solution are 6.5 X 10 L/(mol-s) at 100°C and 4.64 X 10 L/(mol-s) at 138°C (44). At 138°C, first-order solvolysis is ca 2% of the total. Hydrolysis of the monoester is markedly promoted by increasing acid strength and it is first order. The rate at 80°C is 3.65 x lO " ... 

Thus curvature in an Arrhenius plot is sometimes ascribed to a nonzero value of ACp, the heat capacity of activation. As can be imagined, the experimental problem is very difficult, requiring rate constant measurements of high accuracy and precision. Figure 6-2 shows a curved Arrhenius plot for the neutral hydrolysis of methyl trifluoroacetate in aqueous dimethysulfoxide. The rate constants were measured by conductometry, their relative standard deviations being 0.014 to 0.076%. The value of ACp was estimated to be about — 200 J mol K, with an uncertainty of less than 10 J moE K. ... 

Table 1—1. Rate constants k [105 sec-1] and half-life times t1/2 [min] for neutral hydrolysis of iV-acylazoles [ conductivity water, pH 7.0, 25 °C] together with IR frequencies v(C=o) in CC14 and enthalpies.

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