Alkaline hydrolysis rate prediction

The disappearance of a plasticizer from water can be the result of a number of abiotic and biotic processes that can transform or degrade the compound into daughter compounds that have different physicochemical properties from the parent compound. Hydrolysis is a family of chemical reactions where a plasticizer reacts with water. Phthalate esters may hydrolyze to form monoesters and then dicarboxylic acid. It has been predicted that di-(2-ethylhexyl) sebacate will form 2-ethylhexanol and decanedioic acid. Wolfe et al experimentally measured second-order alkaline hydrolysis rate constants for dimethyl, diethyl, di-n-butyl, and di-(2-ethylhexyl) phthalates, and it appears that hydrolysis may be too slow to have a major impact on the fate of most dissolved plasticizers. The estimated hydrolysis half-lives at pH 7 for 20 plasticizers were longer than 100 days. No information was located for diallyl, ditridecyl and diundecyl phthalates. Under alkaline conditions, hydrolysis may be important for tricresyl phosphate and tri-(2-ethylhexyl) trimellitate at pH 8 their predicted half-lives are 3.2 and 12 days respectively. 

Kindler [Twi., 450( 1), 1926] has studied the alkaline hydrolysis of the ethyl esters of a number of substituted benzoic acids. The m-nitro compound was found to have a rate constant 63.5 times as fast as the unsubstituted compound. What relative rate constant is predicted for the reaction of p-methoxybenzoate by the Hammett equation The value based on experimental results is 0.214. 

These conclusions have several implications for pesticide waste disposal considerations. For incidental or accidental disposal of pesticides in natural aquatic systems, the results suggest that model calculations using aqueous solution values for abiotic neutral hydrolysis rate constants can be used without regard to sorption to sediments. For alkaline hydrolysis, on the other hand, models must explicitly include sorption phenomena and the correspond ng rate reductions in order to accurately predict hydrolytic degadation. 

Finally, as an example of reaction type (f) in Table 5-4, the alkaline hydrolysis of the trimethylsulfonium ion demonstrates the predicted large rate decrease by increasing the proportion of water in an aqueous ethanolic reaction medium [70],... 

Alkaline hydrolysis of hexafluorophosphate (PFg ) occurs without accumulation of lower fluorophosphates. The rates have been examined between 160 and 190 °C, with lithium, sodium and potassium hydroxides. At unit ionic strength (nitrates) the rates were dependent on the nature of the cation (K" < Na" " ionic strength, behaviour is more predictable and the rate expression is... 

The kinetics of the alkaline hydrolysis of a series of (heteroarylmethyl)-triphenylphosphonium salts have been investigated. The rates of reaction decrease in the predicted order. The phosphorus-fnran bond is broken as expected when the heteroarylphosphonium salts (79) are treated with aqueous alkali. ... 

Using the enzyme inhibition kinetics and product identification and model studies of alkaline hydrolysis of the compounds, stmcture-activity relationships of the enzyme inhibitor interactions could be understood and predicted. With this knowledge the authors were able to design alternate substrate inhibitors with reasonable chemical stability, inhibition constants in the nanomolar range, and very slow deacylation rates (fcoff), resulting in virtually irreversible inhibition. 

FIG. 26 Scheme showing relative rates of aeid and alkaline hydrolysis of different classes of cleavable surfaetants and the effect of micellization of hydrolysis rate. Full lines represent experimentally verified results and dashed lines indicate predicted behavior. 

Relation to Alkalinity. Some explanations for this behavior can be offered. Persistent levels appear to be associated with low pH (4.5-6.5) and alkalinity (<10 mg/L) in the groundwater. Residues disappeared faster from Fields 4 and 5, where pH and alkalinities were high, especially at deeper levels (Figure 5) than in Fields 1 and 2, where pH and alkalinity were low (Figure 6). Rates of chemical hydrolysis of aldicarb and its oxides have been extensively studied (26-29). Alkalinity and pH tend to increase with depth in the aquifer under all fields. However, prediction of rates of chemical hydrolysis based on known rate constants are complicated by two phenomena 1. fluctuations in groundwater... 

The relative rates of the various steps are a function of the pH of the solution and the basicity of the imine. In the alkaline range, the rate-determining step is usually nucleophilic attack of hydroxide ion on the protonated C=N bond. At intermediate pH values, water replaces hydroxide ion as the dominant nucleophile. In acidic solution, the rate-determining step becomes the breakdown of the tetrahedral intermediate. A mechanism of this sort, in which the overall rate is sensitive to pH, can be usefully studied by constructing a pH-rate profile, which is a plot of the observed rate constants versus pH. Figure 8.4 is an example of the pH-rate profiles for hydrolysis of a series of imines derived from substituted aromatic aldehydes and t-butylamine. The form of pH-rate profiles can be predicted on the basis of the detailed mechanism. The value of the observed rate can be calculated quantitatively as a function of pH, if a sufficient number of the individual rate constants and of the acid dissociation constants of the species involved are known or can be estimated reliably. Agreement between the calculated and observed pH-rate profile can then serve as a sensitive test of the adequacy of the postulated mechanism. Alternatively, one may begin with the experimental pH-rate profile and deduce details of the mechanism from it. 

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