Base hydrolysis rate constants

Table II. Base Hydrolysis Rate Constants of Carbofuran (30 ppb) at ...

Table 111, Base Hydrolysis Rate Constants of 3-Hydroxycarbofuran (200 ppb) at 15 C...

Table IV. Base Hydrolysis Rate Constants of Methomyl (25 ppb) at 15°C...

Hydrolysis not expected to be significant based on estimated half-lives for hydrolysis of acetamide of 261, 3950, and 46 yr at pH 5,7,9, respectively, which were calculated using experimental acid and base hydrolysis rate constants for acetamide (Mabey Mill 1978 selected, Howard et al. 1991). 

The hydrolysis kinetics of aliphatic esters (R, and R2 = alkyl groups) are also sensitive to electronic effects. The hydrolysis data for a series of aliphatic esters are summarized in Table 2.4. The addition of chloride substituents to the Rj group drastically increases the neutral and base hydrolysis rate constants. These data also demonstrate that as the steric bulk of R2 increases, there is a significant decrease in kb (compare ethyl acetate to t-butyl acetate). 

Figure 2.10. Plots of the log of second-order base hydrolysis rate constants of N-phenyl carbamates versus pKg of the resulting alcohol at 25°C in water. From Wolfe et al. (1978). (Reprinted by permission of Pergamon Press Ltd.)...

Rates and activation parameters associated with the values of ki, the independently determined aquation rate constants (fcaq), and the base-hydrolysis rate constants ( 2) are collected in Table 22. The activation parameters associated with the ki and k ci values are the same within experimental error. The hydroxide-ion-independent pathways lead to virtually complete trans- cis isomerization when X = Cl or Br, and to ca. 50% isomerization when X = I. The relatively small contribution of the k term in equation (16) (Jk lkx ca. 1 dm mol ) arises from the lower values of tsS k — A5" ( i) than are observed for cw-[Rh(en)2(OH)X]+ or [Rh(NHa)6X] +. Values of AA (k2) and lie at the upper end of an isokinetic plot that includes data for... 

Tris-diimine-iron(II) complexes of unsymmetrical ligands can exist in mer and fac isomers. Interconversion between such geometrical isomers is often slow, which complicates the establishment of rate constant and solubility data. Rate constants for hydroxide attack at the particularly stable and inert complex of the ligand (40) were found to decrease over a matter of hours or days as stock solutions aged. Indeed it proved possible to obtain fairly good estimates for isomerization rate constants for this complex from the time dependence of base hydrolysis rate constants. Iron(II) complexes of the triazine-diimine ligand (41), whose disulfonate ( ferene ) is an important analytical reagent for iron determination, have been rediscovered both mer and fac isomers have been observed. 

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,... 

Chemical/Physical. Under laboratory conditions, chloroethane hydrolyzed to ethanol (Smith and Dragun, 1984). An estimated hydrolysis half-life in water at 25 °C and pH 7 is 38 d, with ethanol and HCl being the expected end-products (Mabey and Mill, 1978). Based on a measured hydrolysis rate constant of 5.1 x 10 at 25 °C and pH 7, the half-life is 2.6 yr (Jeffers and Wolfe, 1996). 

Chemical/Physical. Begins to polymerize at 80.2 °C (Weast, 1986). Slowly hydrolyzes in water forming methyl alcohol and acrylic acid (Morrison and Boyd, 1971). Based on a hydrolysis rate constant of 0.0779/M-h at pH 9 at 25 °C, an estimated half-life of 2.8 yr at pH 7 was reported (Roy, 1972). The reported rate constant for the reaction of methacrylonitrile with ozone in the gas phase is 2.91 x lO cm moFsec (Munshi et al, 1989a). 

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). 

When comparing the hydrolysis rate constants of a series of carboxylic acid esters (Table 13.8), it can be seen that the values for the acid-catalyzed reactions are all of the same magnitude, whereas the rate constants for the base-catalyzed reactions vary by several orders of magnitude. Explain these findings. 

Fiqure 2.4 Determination of the rate constants for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The first-order rate constants for the hydrolysis are plotted against various concentrations of the base. The slope of the linear plot is the second-order rate constant (k2). The intercept at zero buffer concentration is the "spontaneous hydrolysis rate constant for the particular pH. A plot of the spontaneous rate constants against pH gives the rate constants for the H+ and OH" catalysis. It is seen that pyridine is a more effective catalyst than the weaker base acetate ion. [From W. P. Jencks and J. Carriuolo, J. Am. Chem. Soc. 83,1743 (1961).]... 

The serine group which becomes phosphorylated does not appear to possess any marked nucleophilic reactivity, nor is there any evidence that a histidine group participates as a general acid-general base catalyst. Rate constants for the nonenzymic hydrolysis of alkyl and aryl phosphate monoanions at 25° are in the range HP7 to 10-9 sec-1 (167), while the comparable alkaline phosphatase-catalyzed values (in this case they refer to dianions) are in the range 102 to 103 sec-1. Thus one has to account for a rate enhancement factor of 109 to 1012. Moreover, the... 

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.

An interesting correlation has been observed53 between the formation constant XCuL of the metal complex and its catalytic activity in a mixed ligand with an amino acid ester. Large values of XCUL (equation 13) lead to lower base hydrolysis rates in the ternary complex. The Lewis... 

Metal ions have quite marked effects on the hydrolysis of methyl 8-hydroxyquinoline-2-carboxylafe (50)218 and ethyl l,10-phenanthroline-2-carboxylate (51).219 Base hydrolysis of the 8-hydroxyquinoline derivative (50) was studied over the pH range 9.2-12.1 and values of fcoH determined for HA and the anion A-. Formation constants KMA+ were determined at 25 °C for the equilibrium M2+ + A MA+, as were the rate constants fcOH for the base hydrolysis of the MA+ complexes (Table 19). Quite large rate accelerations are observed (103-106) when comparisons are made with the base hydrolysis of A . The charge carried by the complex does not appear to be a major factor in determining base hydrolysis rates. Thus at 25 °C the complex CuA+ undergoes base hydrolysis (kOH= 6.3xl05 s 1) at a very similar rate to the corresponding... 

Estimation of Hydrolysis Rate Constants Based on Analogy... 

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. 

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