Base catalyzed hydrolysis reaction rate

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. 

Abiotic hydrolysis of pollutants in subsurface waters is pH dependent. The predominant pathways are acid-catalyzed, base-mediated, and neutral (pH-independent) hydrolysis. The acid-catalyzed hydrolysis reaction rate is dependent on proton concentration increases with a decrease in pH. This behavior occurs because the proton is not consumed in the reaction. 

In contrast, Perdue and Wolfe (1982) observed that DOM retarded the basic hydrolysis of the octyl ester of 2,4-D. They found that the base-catalyzed rate constant was reduced in proportion to the fraction of the hydrophobic ester that was associated with the DOM. Perdue (1983) has proposed a micelle-type model to rationalize these results and those presented for atrazine. Perdue compares the physical characteristics of DOM, which is negatively charged at environmental pHs, to those of anionic surfactants. Anionic surfactants have been demonstrated to increase hydrolysis rates for acid-catalyzed processes and decrease rates for base-catalyzed processes (Fendler and Fendler, 1975). Rate enhancements for acid-catalyzed hydrolysis reactions are attributed to stabilization of the positive charge that is developed in the transition state, whereas base-catalyzed hydrolysis reactions are impeded due to destabilization of the negatively charged transition state. Although this is an attractive model, it remains largely untested. 

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. 

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. 

This review, then, reports results of experiments which provide information that can be used to test the hypothesis that hydrolysis reactions proceed at substantially reduced rates when the molecules undergoing hydrolysis are sorbed to sediments. Results are reported for a variety of pesticides and for model compounds that are similar in structural features to pesticides. Included are neutral, base-catalyzed and, to a limited extent, acid-catalyzed hydrolysis reactions. 

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)... 

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

Base-Catalyzed Hydrolysis. Let us now look at the reaction of a carboxylic ester with OH", that is, the base-catalyzed hydrolysis. The reaction scheme for the most common reaction mechanism is given in Fig. 13.11. As indicated in reaction step 2, in contrast to the acid-catalyzed reaction (Fig. 13.10), the breakdown of the tetrahedral intermediate, I, may be kinetically important. Thus we write for the overall reaction rate ... 

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. 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

The apparent rate of hydrolysis and the relative abundance of reaction products is often a function of pH because alternative reaction pathways are preferred at different pH. In the case of halogenated hydrocarbons, base-catalyzed hydrolysis will result in elimination reactions while neutral hydrolysis will take place via nucleophilic displacement reactions. An example of the pH dependence of hydrolysis is illustrated by the base-catalyzed hydrolysis of the structurally similar insecticides DDT and methoxy-chlor. Under a common range of natural pH (5 to 8) the hydrolysis rate of methoxychlor is invariant while the hydrolysis of DDT is about 15-fold faster at pH 8 compared to pH 5. Only at higher pH (>8) does the hydrolysis rate of methoxychlor increase. In addition the major product of DDT hydrolysis throughout this pH range is the same (DDE), while the methoxychlor hydrolysis product shifts from the alcohol at pH 5-8 (nucleophilic substitution) to the dehydrochlorinated DMDE at pH > 8 (elimination). This illustrates the necessity to conduct detailed mechanistic experiments as a function of pH for hydrolytic reactions. 

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