Base-catalyzed hydrolysis, rate

In natural waters, the base-catalyzed hydrolysis rate of a weakly HS-as-sociated pollutant (e.g., Parathion) was not significantly affected by HS, while for more strongly associated pollutants (e.g., DDT) the effect of HS was clearly potentially significant in this reaction. 

Figure 13.16 Correlation (Bronst-ed plot) of base-catalyzed hydrolysis rates (log kB) of carbamates as a function of the pKa of the alcohol moiety for a series of N-phenyl carbamates. Data from references given in Table 13.11.

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.

Sediment measured t,/2 = 450 d at 25°C, based on neutral and base-catalyzed hydrolysis rates studied in pure water and in barely saturated subsurface sediment at 25-60°C (Haag Mill 1988). 

From the data on acid- and base-catalyzed hydrolysis rates, a values were derived for the 5-substituted 2-thienyl groups (65RTC1169). 

Menger and Donohue made use of commercially available 2,5-dimethoxytetrahydrofliran (1) and benzamide to generate Cbz-protected pyrrole in their studies on the base-catalyzed hydrolysis rate of N-acylpyrroles. A slight excess of 1 and long reaction times at reflux were needed to generated the product in modest (47%) yield. 

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,... 

Aquatic HS inhibited the base-catalyzed hydrolysis of the n-octyl ester of 2,4-D (i.e., 2,4-DOE). The hydrolysis rate of 2,4-DOE at pH 9-10 decreased by a factor equal to the fraction of the ester associated with the DHS. These observations are consistent with an unreactive humic-bound 2,4-DOE in equilibrium with reactive aqueous-phase 2,4-DOE. Thus, association between Dha and 2,4-DOE inhibited the base-catalyzed hydrolysis reaction. 

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],... 

We begin the discussion with one of the major sites of structural variation, namely at C(6) in penicillins and at C(7) in cephalosporins (Table 5.4,A). Electron-withdrawing substituents at C(6) in penicillins and at C(7) in cephalosporins increase the rate of base-catalyzed hydrolysis [76]. The same substituent effect has been observed for monobactams [93] [94],... 

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. 

Variations in pH induce acid-base mediated hydrolysis. For base-catalyzed hydrolysis, for example, the rate equation for an organic contaminant is... 

Acyl-phosphates also display an unusual U -shaped pH profile for the rate of hydrolysis (see Fig. 1). This reflects the fact that acyl-phosphate compounds are susceptible to both acid- and base-catalyzed hydrolysis, and the shoulder at less acidic pH values reflects the slightly faster hydrolysis of the monoanion, compared to the dianion. Observation of a U -shape pH-rate profile for a previously uncharacterized phosphoryl-enzyme intermediate supports the inference that an acyl-phosphate has formed. 

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. 

Metal ion catalysis of salicyl phosphate hydrolysis is much more complicated than that of Sarin, since the former substrate can combine with metal ions to give stable complexes, and some of the complexes formed do not constitute pathways for the reaction. In addition the substrate undergoes intramolecular acid-base-catalyzed hydrolysis which is dependent on pH because of its conversion to a succession of ionic species having different reaction rates. Therefore a careful and detailed equilibrium study of proton and metal ion interactions of salicyl phosphate would be required before any mechanistic considerations of the kinetic behavior in the absence and presence of metal ions can be undertaken. 

Taft, following Ingold,39 assumed that for the hydrolysis of carboxylic esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base (see the discussion of ester-hydrolysis mechanisms, reaction 0-10). Rate differences would therefore be caused only by the field effects of R and R in RCOOR. This is presumably a good system to use for this purpose because the transition state for acid-catalyzed hydrolysis (7) has a greater positive charge (and is hence destabilized by - / and stabilized by + / substituents) than the starting ester, while the transition state for base-catalyzed hydrolysis (8)... 

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