Base catalyzed hydrolysis rate constant

Figure 4. Bronsied plot of the logarithm of the general base catalyzed hydrolysis rate constants vs. the p/C of the conjugate acids of the oxygen and nitrogen bases (buffers) for phenyl tris-(2-methoxyethoxy)silane ( ) and phenyl bis-(2-methoxyethoxy)silanol ( ) in aqueous solution at 30 C. and for tert-butyldimethyl-3-nitrophenoxysilane ( ) in 70% water/dioxane at 37°C.

The acid-catalyzed hydrolysis of diethyl succinate conforms to the above generalizations, as shown in the last row of Table VI, but the base-catalyzed hydrolysis presents a quite different situation. The rate constant for the base hydrolysis of diethyl succinate approxi-... 

This property of organophosphate esters may be of environmental importance since phosphoric acid diesters are much more soluble and very little is known concerning the environmental toxicity of these compounds. The available data do not provide sufficient descriptions of the experimental methods to determine if the rates are reliable (Barnard et al. 1961 Ciba-Geigy 1984e, 1986 Howard and Deo 1979 Mayer et al. 1981 Wolfe 1980). The majority of reports provide only a minimum of information and exclude important facts such as the duration of the experiments and the concentration of buffers. Despite the lack of experimental detail, published rate constants for base-catalyzed hydrolysis appear to be reasonably consistent and suggest that the hydrolytic half-life of triphenyl phosphate will vary from... 

Most pyrethroids undergo acid- and base-catalyzed hydrolysis to form the corresponding acid and alcohol (Fig. la), typically with U-shaped pH-rate profiles [8, 40]. The hydrolysis of pyrethroids in water basically obeys first-order kinetics with a half-life simply calculated from hydrolysis rate constant (A obs) as 0.693/kobs. Pyrethroids are generally stable under the acidic and neutral conditions at pH 4—7,... 

Lactams undergo both acid- and base-catalyzed hydrolysis [79]. The pH vs. log(rate constant) profiles of penicillins are V-shaped with rate constants at a minimum of ca. pH 7, whereas, for cephalosporins, these plots are U-shaped with minimum rate constants in the pH range of 3-7. The basis for this difference is that penicillins undergo no significant uncatalyzed (pH-independent) reaction, while, in contrast, for cephalosporins, a significant degree of spontaneous, pH-independent reaction between pH 3 and 7 is often observed [80],... 

The Hammett substituent constant o is a measure of the electron-with-drawing capacity of substituents directly conjugated to the reaction center [84], Since the values of this parameter were obtained from the rate constants of base-catalyzed hydrolysis of other aryl esters, it can be concluded from Eqn. 8.5 that the same electronic factors are involved in both cases. 

Figure C. Rate constants for base-catalyzed hydrolysis of Si(acac)z+...

The kinetic effects of C02 in the base catalyzed hydrolysis of some carboxylato amine cobalt(III) complexes have been reported (80-82). In the base catalyzed hydrolysis of oxalatopentaammine-cobalt(III) (80), C02 retarded the reaction due to the formation of a virtually unreactive ion-pair, f (N H .) r, 2 2 COi ]. The equilibrium constant for formation of carbonate ion-pairs with (glycinato-O) (tetraethylene-pentamine)cobalt(III), (81) and (o-methoxybenzoato) (tetraethylenepentamine)cobalt(III) (82) were, however, much smaller than for the oxalatopentamminecobat(III) and a very weak rate retardation and virtually no effect was observed in the base catalyzed hydrolysis of the latter two complexes. 

Table 12.1 Reaction Rate Constants for the Base-Catalyzed Hydrolysis of Benzoic Acid Ethyl Ester (Ethyl Benzoate) in Various Organic Solvent-Water Mixturesa...

Figure 13.15 Effects of substituents on the base-catalyzed hydrolysis of benzoic acid ethyl esters in ethanoliwater (85 15) at 25°C. Relative reaction rates are correlated with Hammet Oj constants (data from Tinsley, 1979).

An example in which rate constants are related to equilibrium constants involves the base-catalyzed hydrolysis ofN-phenyl carbamates (Fig. 13.16). As discussed above, these compounds hydrolyze with the dissociation of the alcohol moiety being the rate-determining step. Hence, by using the pA a s of the leaving groups (phenols and aliphatic alcohols), we find a nice correlation to the rates of these reactions. 

When comparing the hydrolysis rate constants of a series of carboxylic acid esters (Table 13.8), it can be seen that the values for the acid-catalyzed reactions are all of the same magnitude, whereas the rate constants for the base-catalyzed reactions vary by several orders of magnitude. Explain these findings. 

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. 

Because the electronic nature of substituents has little effect on the rate of acid-catalyzed hydrolysis of meta- or para-substituted benzoates (e.g., p for the acid hydrolysis of XC6H4COOR esters is close to zero), Taft suggested that the electronic nature of substituents will also have little effect on acid-catalyzed hydrolysis of aliphatic esters (Lowry and Richardson, 1987). Nevertheless, a strong electronic effect occurs in basic hydrolysis, as can be examined from the large p values for base-catalyzed hydrolysis of meta- or para-substituented benzoates. Hence, the effect of X on acid hydrolysis is purely steric but is a combination of steric and electronic effects in basic hydrolysis. Taft defined Es, a steric substituent constant, by Equation (5.11) ... 

Aqueous hydrolysis rate constants for esters, carbamates, epoxides, halomethanes, and selected alkyl halides. Acid- and base-catalyzed rate constants are estimated, but not neutral hydrolysis rate constants. 

FIGURE 5.26 pH-rate constant profile for the specific acid-base catalyzed hydrolysis of a water-soluble diaziridinyl benzoquinone derivative. [Graph reconstructed from data by Jain et al., AAPS National Meeting, Paper No. 2268, Indianapolis, IN, Oct. 29, 2000.]... 

Acetylsalicylic acid (aspirin) (pKa= 3.62) undergoes acid-base catalyzed hydrolysis. The pseudo-first-order rate constants were determined at four different temperatures (25, 35, 50.3, and 60.3°C) as shown in Figure 5.41. The hydrolysis of aspirin follows Equation (5.167) with k0o = 0 and k0 = 0. Each rate constant is given by ... 

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... 

The effect of increasing the hydrocarbon chain length from methyl to octadecyl on the acid catalyzed, neutral, and base catalyzed hydrolysis of n-alkyl sulfate esters has been examined (Kurz, 1962). The rate constants for the neutral hydrolysis decrease smoothly from methyl to dodecyl sulfate and hence are unaffected by micellization of the longer chain esters. The rate constants for the acid-catalyzed hydrolysis, however, are relatively constant for the non-micellar ester but increase dramatically with micelle formation (Table 7). Conversely, the hydroxide... 

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