Hydrolysis rate constant and

Chemical/Physical. Endosulfan detected in Little Miami River, OH was readily hydrolyzed and tentatively identified as endosulfan diol (Eichelberger and Lichtenberg, 1971). Undergoes slow hydrolysis forming the endosulfan diol and sulfur dioxide (Worthing and Hance, 1991). The hydrolysis half-lives at pH values (temperature) of 3.32 (87.0 °C), 6.89 (68.0 °C), and 8.69 (38.0 °C) were calculated to be 2.7, 0.07, and 0.04 d, respectively (Ellington et al., 1988). Greve and Wit (1971) reported hydrolysis half-lives of P-endosulfan at 20 °C and pH values of 7 and 5.5 were 37 and 187 d, respectively. In a 1 pM sodium bicarbonate buffer solution at pH 8.15 and 28 °C, suspensions of sea sand, titanium dioxide, a-ferric oxide, a-FeOOH, laponite, and silicon dioxide catalyzed the hydrolysis of a-endosulfan to endosulfan diol. The uncatalyzed hydrolysis rate constant and half-life was 4.01 x lO Vsec and 0.20 d, respectively (Walse et al., 2002). 

Increases in pH as a direct proportional augmentation of the hydroxyl ion activity leads to a base-mediated hydrolysis process. In this case, the hydroxyl behaves as a nucleophile and is consumed in the reaction. Neutral and alkaline hydrolysis are the most frequent reactions over the common environmental pH ranges. The relation between first-order hydrolysis rate constants and the pH often is presented as a pH rate profile (Wolfe et al. 1990). 

Acid catalysis—hydrolysis. Several series of alkylsilane esters were studied to determine the effect of silane structure on the hydronium ion catalyzed hydrolysis reaction. The hydronium ion catalyzed hydrolysis rate constants for a series of alkyl tris-(2-methoxyethoxy)silanes in aqueous solution were used to define the modified Taft equation log(A /Ah ) = 0.39a + 1.06ES, where Ho is the rate of hydrolysis for methyl tris-2-(methoxyethoxy)silane [42], The hydronium ion catalyzed hydrolysis rate constants and the reaction half-lives are reported in Table 2. In a similar manner, the hydronium ion catalyzed hydrolysis rate constants for a series of trialkylalkoxysilanes in 55% aqueous acetone were used to obtain the modified Taft equation log(/cH//cHo) = -0.37 a + 2.48 E where kHo is the rate of hydrolysis for trimethylalkoxy-silane. 

Equation (30) represents a QSAR for the base-mediated hydrolysis of formates and acetates. The correlation is between the second-order alkaline hydrolysis rate constants and the linear combination of the shifts of the vC=0 and vC-O stretching peaks for 12 of the 41 compounds in Table 13.3. 

Part of the problem with this and other investigations where departures from a slope of 1.0 were noted may be due to the fact that most values of Ho have been determined near 25°, whereas the hydrolyses have been performed at 60-80°. Since there is a small but systematic shift in the relationships between R oRnd of concentration of acid with changing temperature for sulfuric, hydrochloric, and phosphoric acids, values of Ho and relationships between different acids calculated at one temperature will not hold true for another. Therefore, all relationships between hydrolysis rate constants and Ho should be determined at 25° (or the temperature at which Ho was determined). 

The process of hydrolysis, as defined here, converts dead particulate organic matter into bioavailable dissolved substrate. It is usually described by a simple first-order transformation. The process involves extracellular enzyme reactions breaking down high-molecular-weight particulate material that cannot be directly metabolized by microorganisms. Table 16.12 shows a simple representation of this process. It is quantified by an empirical specific hydrolysis rate constant and a temperature-dependence coefficient. 

In Eq. (91), and were the uncatalyzed (or hydrolysis) rate constant and catalyzed rate constant, respectively. Therefore, the reaction of PhCOCl and PNO in the organic phase to yield the ionic intermediate, l-(benzoyloxy)pyridinium chloride, [reaction (85)] was the rate-determining step in the PNO-catalyzed reaction path, which led mainly to the production of benzoic anhydride. The value of k obtained from the linear plot of A obs versus [PNOjjaq was generally consistent with that obtained in the uncatalyzed reaction. Therefore, reaction (89) was the main step in the uncatalyzed (hydrolysis) path, which led to the production of benzoic acid. In the following discussion, the main features of this IPTC system are described. 

Zang L., Tong P., Chen GN., Determination of the hydrolysis rate constants and activation energy of aesculin with capfllary electrophoresis end-column amperometric detection. J. Chwmatogr. A, 1098, 194-198 (2005). 

Matsumoto et al. [24] investigated to the stability of aspirin in mixtures of aspirin and PCC at various temperatures and humidities. They calculated the intrinsic rate constants of aspirin hydrolysis from the relationship between the apparent first-order hydrolysis rate constant and the amount of water adsorbed at each temperature. The apparent first-order rate constant of aspirin and the amount of adsorbed water in the heated mixture of PCC and aspirin at different temperatures are listed in Table 1. It was suggested that the apparent first-order decomposition rate constants of aspirin in the heated mixtures were closely related to the amount of adsorbed water. 

HYDROWINTM Aqueous hydrolysis rate constants and half-lives for certain substances... 

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