Hydrolysis kinetics half lives

Discrimination between exposed and unexposed areas in this process requires the selection of thia zolidine compounds that do not readily undergo alkaline hydrolysis in the absence of silver ions. In a study of model compounds, the rates of hydrolysis of model /V-methyl thia zolidine and A/-octadecyl thiazolidine compounds were compared (47). An alkaline hydrolysis half-life of 33 min was reported for the /V-methyl compound, a half-life of 5525 min (3.8 days) was reported for the corresponding V/-octadecyl compound. Other factors affecting the kinetics include the particular silver ligand chosen and its concentration (48). Polaroid Spectra film introduced silver-assisted thiazolidine cleavage to produce the yellow dye image (49), a system subsequentiy used in 600 Plus and Polacolor Pro 100 films. 

Ta 1.5 X 10 2, K3 2.1 X 10 and 2.4 x and the corresponding negative logarithms are pA" 1.0, pA"2 1.8, pA"3 6.57 and pA"4 9.62. The P—O—P linkage is kinetically stable towards hydrolysis in dilute neutral solutions at room temperature and the reaction half-life can be of the order of years. Such hydrolytic breakdown of polyphosphate is of considerable importance in certain biological systems and has been much studied. Some factors which affect the rate of degradation of polyphosphates are shown in Table 12.10. 

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,... 

Further investigations revealed additional advantages conferred by the association of the partners in the T. thermophilus transamidosome. The kinetics of the hydrolysis of the aa-tRNAs in free form or bound in the complex show that the transamidosome stabilizes the ester bond of the Asp-tRNA intermediate by increasing its half-life about twofold (half-lives 315 and 204 min) and that of the end product Asn-tRNA fourfold (half-lives 18 and 66 min). Finally, measurements of the thermostability of the protein partners reveal a significant increase in the stability of GatCAB and the ND-AspRS at 85 °C, the optimal growth temperature of T. thermophilus, when associated in the transamidosome. 

Chemical/Physical. Under laboratory conditions, carbon tetrachloride partially hydrolyzed in aqueous solutions forming chloroform and carbon dioxide (Smith and Dragun, 1984). Complete hydrolysis yields carbon dioxide and HCl (Ellington et al., 1993 Kollig, 1993). The estimated hydrolysis half-life in water at 25 °C and pH 7 is 7,000 yr (Mabey and Mill, 1978) and 40.5 yr (Jeffers et al., 1989 Ellington et al, 1993). The estimated hydrolysis half-life reported by Mabey and Mill (1978) was based on second-order neutral kinetics. Jeffers et al. (1996) reported that hydrolysis of carbon tetrachloride is first-order, contrary to findings of Mabey and Mill (1978). Jeffers et al. (1996) report that the extrapolated environmental half-life at 25 °C is 40 years. 

First, the computer calculated uncertainties shown for the calculated values of kj, k and kg are an indication that the model has considerable validity for describing the kinetics of the system, at least over one half-life in the disappearance of chlorpyrifos. Second, the values of k and kj are all similar and their magnitude indicates that in this case the assumption of rapid sorption/desorption kinetics compared to hydrolysis is valid. 

A study Is In progress which Is Investigating the kinetics and mechanism of hydrolysis of EDB at pH 5, 7, and 9, and three elevated temperatures (97). Preliminary Indications are that EDB s half-life at ambient ground water temperatures and pH Is >6 years, and the most probable hydrolysis product Is vinyl bromide. 

The half-life of Fe2(OH)2" at room temperature is a few seconds. An improved model for the kinetics of dissociation of this dinuclear cation recognizes significan articipation by Fe2(OH)3 + at higher pHs, thus clearing up earlier slight anomalies in this area. Phosphate ester hydrolysis at the di-iron center of uteroferrin has now been shown to involve nucleophilic attack by bridging hydroxide (as proposed but not conclusively demonstrated for several M—OH—M-containing catalytic species) rather than by hydroxide bonded to just one Fe. ... 

In fact, the very recent 195Pt NMR results of Bancroft et al. (41) indicate that, in agreement with Miller and House (36c), most likely [cis-Pt(NH3)2Cl(H20)]+ is the predominant species that reacts with biomolecules (at least with DNA). Other Pt amine compounds that are antitumor active have different kinetics of the hydrolysis reactions, and usually react much slower. The second-generation drug CBDCA (Fig. 2) is known to hydrolyze (in a 1 mAf solution) with a half-life at 37°C of a few days (41a) (compared to only 1 hour for cis-Pt). 

Recently, Pankov and Morgan (1981a,b) emphasized the importance of various mechanisms for regulating kinetics in the aquatic environment. Examples showed the wide range of first- and second order rate constants (kf) and half lifes (ti) for different reactions that might take place in natural waters. The rate constants for several first order trace metal hydrolysis reactions, second order redox- and complexation reactions of interest for aquatic studies are summarized by Hoffmann (1981). His comparison of kinetic data on the oxidation of HS- under only slightly different conditions shows considerable variations e.g., t ranges from 7 -600 min for seawater media. 

Chlorohydrins are compounds characterized by alpha halo-alpha alkoxy groups bound to a common carbon atom. These compounds undergo rapid hydrolysis at this shared carbon atom. Bis(2-chloroisopropyl)ether, a chlorohydrin, has two such carbon atoms, and both react very rapidly with water. In fact, the reactions are so fast that acid and alkaline contributions have not been determined. It is likely, however, that base accelerates the reaction kinetics. The proposed reaction pathway for this compound is based on the reported pathway for bis(chloroethyl)ether (Figure 13.4). The reported rate constant for bis(chlorome-thyl)ether, of 0.23 sec-1 was based on an observed half-life of a few minutes. Similarly, for bis(2-chloroisopropyl)ether, both of the chloro substituents are reactive, and a half-life of a few minutes can be assigned to this compound, as well. 

The formation of a number of lanthanide texaphyrin complexes has been reported [95]. In all cases, metal insertion and oxidation proceeds smoothly (Scheme 16) [95]. The complexes demonstrate fair water solubility and good stability towards hydrolysis. Detailed kinetic studies of complex 147, for instance indicated that the half-life for decomplexation and/or decomposition of this complex is 37 days in a 1 1 mixture of MeOH H20 (pH7). Thus, it appears that gadolinium (III) complexes of texaphyrins could provide the basis for a new approach to paramagnetic MRI contrast reagent development [95]. 

Kinetic analysis of hydrolysis revealed a half-life of 2 min for MIC in aqueous solution, which is much slower than that for aryl isocyanates (Brown et al, 1987). Interaction of isocyanates with cholinesterases is reversible and MIC is far less potent than aryl isocyanates in this respect (Brown et al, 1987). At the same time MIC can act as a hapten leading to generation of antibodies in both animals and humans, although it results in low titers (Karol and Kamat, 1988 Karol et al, 1987). 

The reactivity of 2-fluoropyrazine with aqueous sodium hydroxide to give 2-hydroxypyrazine has been investigated (882, 884). In 1.07N sodium hydroxide at 26° the reaction followed pseudo-first-order kinetics with a half-life of 43 minutes, whereas under the same conditions 2-chloropyrazine had a half-life of 18 days, and 2-iodopyrazine and 2-fluoropyridine remained unchanged (882, 884). Thus, under the above conditions, 2-fluoropyrazine was 640 times more reactive than 2-chloropyrazine (882). Hydrolysis of 2-fluoropyrazine in 61V hydrochloric acid proceeded at a much slower rate with a half-life of 4 days at room temperature (884). Some literature preparations of hydroxypyrazines by hydrolysis of halogenopyrazines (chloropyrazines with aqueous sodium or potassium hydroxide unless otherwise specified) are as follows 2-hydroxy (150°) (818) 2-hydroxy-3-methyl (reflux) (680) 2-hydroxy-3,5-dimethyl (reflux) (978) 3-hydroxy-2,5-dimethyl (reflux) (98, 312, 680, 740) [at 120° (978)] 3-hydroxy-2,5-di- -butyl (powdered potassium... 

Show that the hydrolysis follows first-order kinetics and calculate (a) the rate constant, and (b) the half-life. 

Functional groups in both alcohols and ethers are generally resistant to hydrolysis (Harris 1990). Therefore, hydrolysis of 2-butoxyethanol, which contains both alcohol and ether functional groups, is not expected. The estimated hydrolysis half-life (first order kinetics) for 2-butoxyethanol acetate of >1,000 days (ASTER 1995a) indicates that the hydrolysis of 2-butoxyethanol acetate in water is also not expected. 2-Butoxyethanol does not absorb light of wavelength >290 nm (Silverstein and Bassler 1963). Therefore, photolysis of this compound by absorption of sunlight is not important. Aerobic... 

The reaction of sarin with hydrogen chloride has been reported and kinetics determined by NMR imaging (Bard et al., 1970). With rate constants determined at 25°C, 81.5°C, and 100°C, Arrhenius analysis led to a calculated activation energy of 17.8 kcal/mole. The base-induced hydrolysis of sarin analogs and tabun was studied by Larsson (1958b) and the half-life of GA has been estimated to be 1.5 min at pH = 11 at 25°C. Ultimately, and depending on conditions (pH, reaction times, and so forth), hydrolysis products may include fluoride ion (or hydrogen fluoride), the 1-methylethyl ester of methylphosphonic acid, methylphosphonic acid, and 2-propanol. 

Attack as a Nucleophile. Hydrolysis of esters or amides can occur through the nucleophilic attack of metal-bound hydroxide ions, as exemplified by N (49, 50). In most cases, however, this mechanism is not easily differentiated from the kinetically equivalent attack by hydroxide ion at the metal-bound carbonyl carbon (A) (38). In the case of substitutionaUy inert complexes of Co(lll), 0-tracer experiments revealed that both of the two mechanisms occur in the hydrolysis of the bound amino acid esters and amides (36, 49, 51, 52). At a pH 7-8, ionization of Co(III)-bound amide I (R = H) produced N in >90% of the total concentration, and N was hydrolyzed with a half-life of 100 min at 20°C (36). 

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