Neutral hydrolysis reaction rate

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 Older to investigate the effect that unneutralized atactic poly(methacrylic acid) (at-PMAA) has on the rate of a neutral hydrolysis reaction, several acyl-activated esters (1) and a series of l-acyl-l,2,4-triazols (2) were chosen as subtrates [47] ... 

In an alkaline medium the hydrolysis reaction rate is about 12 times greater than in an acidic one so it is important not to over-neutralize the feed. Clearly it may be desirable to operate under vacuum and with a short residence time. Continuous distillation would be preferred to batchwise. 

A few studies on solvolyses by alcohols and by water are available. The hydrolyses studied include displacement of alkylamino groups from acridine antimalarials and of halogen from other systems. In all cases, these reactions appeared to be first-order in the heterocyclic substrate. By a detailed examination of the acid hydrolysis of 2-halogeno-5-nitropyridine, Reinheimer et al. have shown that the reaction rate varies as the fourth power of the activity of water, providing direct evidence that the only reactive nucleophile is neutral water, as expected. 

The mechanisms by which Pu(IV) is oxidized in aquatic environments is not entirely clear. At Oak Ridge, laboratory experiments have shown that oxidation occurs when small volumes of unhydrolyzed Pu(IV) species (i.e., Pu(IV) in strong acid solution as a citric acid complex or in 45 percent Na2Coj) are added to large volumes of neutral-to-alkaline solutions(23). In repeated experiments, the ratios of oxidized to reduced species were not reproducible after dilution/hydrolysis, nor did the ratios of the oxidation states come to any equilibrium concentrations after two months of observation. These results indicate that rapid oxidation probably occurs at some step in the hydrolysis of reduced plutonium, but that this oxidation was not experimentally controllable. The subsequent failure of the various experimental solutions to converge to similar high ratios of Pu(V+VI)/Pu(III+IV) demonstrated that the rate of oxidation is extremely slow after Pu(IV) hydrolysis reactions are complete. 

The hydrolytic depolymerisation of PETP in stirred potassium hydroxide solution was investigated. It was found that the depolymerisation reaction rate in a KOH solution was much more rapid than that in a neutral water solution. The correlation between the yield of product and the conversion of PETP showed that the main alkaline hydrolysis of PETP linkages was through a mechanism of chain-end scission. The result of kinetic analysis showed that the reaction rate was first order with respect to the concentration of KOH and to the concentration of PETP solids, respectively. This indicated that the ester linkages in PETP were hydrolysed sequentially. The activation energy for the depolymerisation of solid PETP in a KOH solution was 69 kJ/mol and the Arrhenius constant was 419 L/min/sq cm. 21 refs. 

Strongly influences rates of hydrolysis. Hydrolysis of aliphatic and alkylic halides optimum at neutral to basic conditions.43 Other hydrolysis reactions tend to be faster at either high or low pH.186... 

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],... 

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates (1) with X-benzylamines (2) in acetonitrile at 55 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T, with a hydrogen-bonded four-centre type transition state (3). The results of studies of the aminolysis reactions of ethyl Z-phenyl carbonates (4) with benzylamines (2) in acetonitrile at 25 °C were consistent with a four- (5) and a six-centred transition state (6) for the uncatalysed and catalysed path, respectively. The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction was found to be third order in water. The kinetic solvent isotope effect was (A h2o/ D2o) = 2.90 0.12. Proton inventories at each molarity of water studied were consistent with an eight-membered cyclic transition state (7) model. 

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. 

Three general classes of hydrolytic reactions in aqueous solutions have been characterized. In neutral, or pH independent hydrolysis, the rate of disappearance of a pesticide, P, is given by... 

For a given pesticide which undergoes hydrolysis, any or all of these hydrolytic pathways may be relevant at various pH s. Organophosphorothioates, for example, have measurable neutral and alkaline hydrolysis rate constants (7). Esters of 2,4-dichlorophenoxyacetic acid (2,4-D), on the other hand, hydrolyze by acid and alkaline catalyzed reactions, but have extremely small neutral hydrolysis rate constants ( ). Thus, any study of the hydrolysis of sorbed pesticides must be prefaced by an understanding of the hydrolytic behavior of individual pesticides in aqueous solution. 

The hypothesis that hydrolysis of sorbed molecules occurs at rates insignificant with respect to aqueous phase hydrolysis has been demonstrated to be incorrect for neutral (pH-independent) hydrolysis reactions. The rate-constants for sorbed state neutral hydrolysis are, on the contrary, similar in magnitude to those for hydrolysis in the aqueous phase. 

The effect of the local non-neutral environment (4) should be considered together with the detailed reaction mechanism of the hydrolysis reaction and together with the charge development in the activation process in particular. The electrostatically non-neutral environment offered by ionic micelles is generally thought to be the reason for the observation that rate-retarding effects exerted by anionic surfactants on this type of hydrolysis reaction are typically stronger than those by other surfactants. 

Proteolysis. Proteolysis is the cleavage of amide bonds that comprise the backbone of proteins and peptides. The reaction can occur spontaneously in aqueous medium under acidic, neutral, or basic conditions. This process is accelerated by proteases, ubiquitous enzymes that catalyze peptide-bond hydrolysis at rates much higher than occur spontaneously. In humans, these enzymes only recognize sequences of L-amino acids but not d-amino acids. They are found in barrier tissues (nasal membranes, stomach and intestinal linings, vaginal and respiratory mucosa, ocular epithelium), blood, all internal solid organs, connective tissue, and fat. The same protease may be present in multiple sites in the body. 

A further conclusion that we may draw from Table 13.5 is that the SN2 reactions of aliphatic halides with OH" should be unimportant at pH values below about 10. Since the hydrolysis of a carbon-halogen bond is commonly not catalyzed by acids, one can assume that in most cases, the hydrolysis rate of aliphatic halides will be independent of pH at typical ambient conditions. Hence, regardless of whether hydrolysis occurs by an SN1 or SN2 mechanism (or a mixture of both, see below), the reaction may be described by a first-order rate law. The first-order rate constant is then commonly denoted as N (= kmo. [HzO]) to express neutral hydrolysis. Note that if the... 

This calculation shows that the reaction with chloride is about twice as important as the neutral hydrolysis, while the reactions with the other two nucleophiles only make up about 10% of the overall transformation rate of CH3Br. Note that, in some cases, a minor reaction might still be important because a more persistent toxic product may be formed (in this case acetonitrile CH3CN). Since in pure water ... 

Let us now look at some examples to illustrate what we have discussed so far to get a feeling of how structural moieties influence the mechanisms, and to see some rates of nucleophilic substitution reactions of halogenated hydrocarbons in the environment. Table 13.6 summarizes the (neutral) hydrolysis half-lives of various mono-halogenated compounds at 25°C. We can see that, as anticipated, for a given type of compound, the carbon-bromine and carbon-iodine bonds hydrolyze fastest, about 1-2 orders of magnitude faster than the carbon-chlorine bond. Furthermore, we note that for the compounds of interest to us, SN1 or SN2 hydrolysis of carbon-fluorine bonds is likely to be too slow to be of great environmental significance. 

Figure 13.9 Schematic representation of the relative contribution of the acid-catalyzed, neutral, and base-catalyzed reactions to the overall hydrolysis rate as a function of solution pH (a) neutral reaction rate is significant over some pH range (b) the contributions of the neutral reaction can always be neglected.

For the three reactions represented in Fig. 12 the maximum rate of hydrolysis in acid represents only a mpdest acceleration, compared with the rate in initially neutral solution. Bunton and Hadwick89,90 explained the maximum for methyl and phenyl trifluoroacetate in terms of negative salt effects on both acid-catalyzed and neutral reactions. Consistent with this interpretation, it was demonstrated directly that the rate of neutral hydrolysis is decreased by added salts. The effect of added salt should be to decrease the activity of water, and perhaps also to salt in the ester. 

This sensitivity to substitution of neutral hydrolysis means that the pH-independent reaction gradually becomes more important than the hydroxide reaction at the high pH end of the region, and becomes much more rapidly more important than acid-catalyzed hydrolysis at low pH. Thus from Fig. 13, the acid-catalyzed reaction can be seen to be significant for the hydrolysis of ethyl acetate between pH 4 and 5, and for phenyl acetate about pH 2 but for 2,4-dinitrophenyl acetate the acid-catalyzed reaction is not detectable at pH 1, and is presumably important only in relatively strong acid. It seems certain that this fast neutral hydrolysis is at any rate a partial explanation for the low efficiency of acid catalysis in the hydrolysis of very weakly basic esters, such as the trifluoroacetates and oxalates, in moderately concentrated acid (see p. 145). 

Apart from ethyl acetate, the least reactive ester studied is N,0-diacetyl serinamide, which is hydrolyzed in a pH-independent reaction between pH 7 and 8 with a rate coefficient193 of 2.66 x 10-5 sec-1. Salmi and Suonpaa194 and Palomaa et al. 9S, have measured the rates of neutral hydrolysis of a number of chloroacetate esters, and this work has been extended more recently by Euranto and Cleve196-198, who have measured the activation parameters for the hydrolysis of several compounds. Motfat and Hunt199 have obtained the same data for the hydrolysis of a variety of alkyl and aryl trifluoroacetates, and the data for substituted phenyl acetates191 have been plotted in Fig. 14. Most of the available data are collected in Table 27. 

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. 

When an N(l)-H is avalable as in free-base systems, deprotonation at N( 1) speeds up the hydrolysis (Steenken and Jagannadham 1985). For example, the corresponding p-nitroacetophenone Ura adduct decays with 2.4 x 10s s-1 when deprotonated at N(l). The N(l)-alkylated pyrimidines also hydrolyze, but slower (e.g., k = 4.5 x 103 s-1 in the case of uridylic acid) when deprotonated at N(3). In neutral solution, the rate of hydrolysis must be considerably slower, possibly that slow that other reactions may compete. 

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