Rate constant, base hydrolysis systems

Hay and Nolan407 have carried out a detailed kinetic study on the hydrolysis of N-2-pyridyl-methyleneaniline (117 = L) and its copper(II) complex (118). Very substantial rate accelerations were observed in this system. Base hydrolysis of [CuL(OH2)2]2+ (118) is some 105 times faster than base hydrolysis of L at 25 °C. The rate constants for this system are summarized in Table 26. 

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. 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Other systems are ambiguous and require a careful consideration of the magnitudes of the derived rate constants before a conclusion can be drawn. An extreme case can be found in the pH dependence of the solvolysis of m-[Co(en)2(H20)Cl]2+.330 The rate is independent of pH in the range 7—9, where the complex is almost entirely in the form of ris-[Co(en)2(OH)Cl]+ and it is usually, and probably correctly, assumed that the pH independent rate constant is that for the uncatalyzed aquation of this species.180 However, consideration ought to be given to the possibility that the observed process is the base catalyzed hydrolysis of the aquo complex in which a primary amine proton is removed. Problems of this sort are discussed in ref. 301, p.84. 

A different approach was used by Emgenbroich and Wulff [42] to develop imprinted polymers with enantioselective esterase activity. The system was based on the use of an amidinium functional monomer (33), already developed earlier by the same group, but in this case a chiral phosphonate (62) was used as imprinting TSA in order to catalyse the hydrolysis of the corresponding chiral ester (63). The polymer imprinted with the L-enantiomer was able to enhance the esterolytic activity 325-fold when compared with the background and 80 times compared to the non-imprinted polymer. The ratio between the two rates constant, Lai-i/L ii-i) = 1 -4, can be taken as a measure of the enantioselective efficiency of the reaction. 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

The MEMED technique has been used to study the hydrolysis of aliphatic acid chlorides in a water/l,2-dichloroethane (DCE) solvent system [3]. It was shown unambiguously that the reaction proceeds via an interfacial process and shows saturation kinetics as the concentration of acid chloride in the DCE increases the data were well fitted to a model based on a pre-equilibrium involving Langmuir adsorption at the interface. First-order rate constants for interfacial solvolysis of CH3(CH2) COCl were 300 150(n = 2), 200 100(n = 3) and 120 60 s-1( = 8). 

Both the automatic coulometric titration of petroleum streams and the continuous monitoring of pesticides and sulfur-halogen compounds indicate that the coulometric titrator method is amenable to the automatic maintenance of the concentration of a component in a solution system. A manual version of this approach has been used to study the kinetics of hydrogenation of olefins as well as to determine the rate of hydrolysis of esters.12 The latter system is a pH-stat that is based on the principles of coulometric titrations. Equations (4.9)-(4.11) indicate how this approach is applied to the evaluation of the rate constants for ester hydrolysis. A similar approach could be used to develop procedures for kinetic studies that involve most of the electrochemical intermediates summarized in Table 4.1. The coulometric titration method provides a convenient means to extend the range of systems that can be subjected to kinetic study in solution. 

Relatively small retardations of the rate of the base catalyzed hydrolysis of methyl-1-naphthoate by sodium dodecylsulfate (NaLS) and hexadecyltrimethylammonium bromide (CTAB) in 50 wt percent dioxane-water have been observed. These effects were attributed to micelle formation in this solvent system since plots of In where 2 and are the second order rate constants in the presence and absence of the organic salts, vs. the concentration of the salts were non-linear... 

Most of the Hammett-type constants pertain to aromatic systems. In evaluating an electronic parameter for use in aliphatic systems, Taft used the relative acid and base hydrolysis rates for esters. He developed equation 1.38 as a measure of the inductive effect... 

In principle, extrathermodynamic relationships that deviate from the simple Hammett equation (equation 8) can be treated by equation 14. The major problem is the determination of the different sets of o s, (e.g., set and 0 set) in a way that will indeed reflect their relation to independent properties. An example of such a procedure is the separation of polar and steric effects (10). The need for such a separation arose when a nearly complete lack of correlation was observed between substituent effects represented by the Hammet a constants and the rates for alkaline hydrolysis of aliphatic systems (12). Inspection of the structures indicated that the proximity of the substituents to the reaction site was a common feature. The steric interaction between R and X had to be considered separately from the electronic effects. Polar substituent constants were thus defined as the difference between the rate constants of base and acid catalyzed hydrolysis of esters. From the structural similarity of the transition states for these reactions (equation 15) it was assumed that the difference in their charge reflects only the polar effect of the substituent... 

The dramatic increases in reaction rates that occur in enzyme-catalyzed reactions can be seen for representative systems in the data given in Table 2.2.4 The hydrolysis of the representative amide benzamide by acid or base yields second-order rate constants that are over six orders of magnitude lower than that measured for ben-zoyl-L-tyrosinamide in the presence of the enzyme a-chymotrypsin. An even more dramatic rate enhancement is observed for the hydrolysis of urea The acid-catalyzed hydrolysis is nearly 13 orders of magnitude slower than hydrolysis with the enzyme urease. The disprotionation of hydrogen peroxide into water and molecular oxygen is enhanced by a factor of 1 million in the presence of catalase. 

Enzymatic action can be defined on three levels operational kinetics, molecular architecture, and chemical mechanism. Operational kinetic data have given indirect information about cellulolytic enzyme mode of action along with important information useful for modeling cellulose hydrolysis by specific cellulolytic enzyme systems. These data are based on measurement of initial rates of enzyme hydrolysis with respect to purified celluloses and their water soluble derivatives over a range of concentrations of both substrate and products. The resulting kinetic patterns facilitate definition of the enzyme s mode of action, kinetic equations, and concentration based binding constants. Since these enable the enzymes action to be defined with little direct knowledge of its mechanistic basis, the rate equations obtained are referred to as operational kinetics. The rate patterns have enabled mechanisms to be inferred, and these have often coincided with more direct observations of the enzyme s action on a molecular level [2-4]. 

This means that a complete analysis of this system involves determination of ten rate constants two for direct proton transfer, two for the protolysis reaction, two for the hydrolysis reaction, and four for proton transfer reactions such as (7.3.14) and (7.3.15). The relationship among these reactions is shown in fig. 7.4 for a general reaction involving the acid HA and the base B in water. 

Me2pyo[14]trieneN4 (CR) ligand (Fig. 6a) catalyzes the hydrolysis of the triester diphenyl 4-nitrophenyl phosphate in aqueous acetonitrile solution.221 This reaction is first-order in zinc complex and phosphate ester. On the basis of pH-rate studies, which revealed a kinetic pifa value of 8.7,40 the active zinc complex is proposed to be [(CR)Zn-OH]+. A hybrid mechanism in which the zinc center of [(CR)Zn-OH]+ serves to provide the hydroxide nucleophile, and also electrophilically activates the phosphoryl P O bond, is favored for this system. This type of bifunctional mechanism was proposed based on the fact that the second-order rate constant for the [(CR)Zn-OH]+-catalyzed reaction (2.8 x 10 1 M-1 s 1) is an order of magnitude larger than that of free hydroxide ion-catalyzed hydrolysis (2.8 x 10 2M 1 s 1). As OH- is a better nucleophile than the zinc-coordinated hydroxide, Lewis acid activation of the substrate is also operative in this system. 

Chemical Reactivity. Among other things, reactivity plays a vital role in determining fate of pollutants in the environment. A prototype computer system to estimate reactivity of systems based on stmcture, SPARC (SPARC Performs Automated Reasoning in Chemistry), is described (242). As of this writing, procedures for hydrolysis rate constants, ionization uv—vis light absorption spectra, and several physical properties have been developed for SPARC. 

Taft equation. Taft (1956) has extended the Hammett-type correlation to aliphatic systems. Because steric effects of substituents in aliphatic systems cannot be ignored as they were for m- and / -substituted benzene compounds, Taft recognized the need to develop separate terms for the polar and steric effects for substituent constants. Based on the observation that the acid-catalyzed hydrolysis of meta- and para-substituted benzoic acid esters are only slightly affected by the electronic nature of the substituent group (p values are near 0), Taft concluded that the acid-catalyzed hydrolysis of aliphatic esters would also be insensitive to polar effects of substituent groups. Any effect on rate due to substituent groups could therefore be attributed to steric effects. Taft defined a steric substituent constant, E, by ... 

Taft proposed a substituent constant, polar effect of alkyl substituents in aliphatic systems. This method is based on the assumption that resonance is unimportant in aliphatic systems and that steric effects are the same for ester hydrolysis whether in acid or base, so only the polar effect of the substituent is different under the two reaction conditions. The value of a for a substituent, R, was based on the rate constants for acid-catalyzed and base-promoted hydrolysis of the ester RCO2R relative to those for CH3CO2R. A factor of 2.48 was used to relate cr values to the Hammett cr values. Thus,... 

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