Disappearance rate constants for

In anoxic hypolimnion samples collected from Lower Mystic Lake, MA, hexachloroethane was abiotically transformed into tetrachloroethylene via reductive elimination and to pentachloro-ethane via hydrogenolysis. Tetrachloroethylene accounted for 70% of hexachloroethane in unaltered lake water and 62% in filter-sterilized water after 10 d. Trichloroethylene and pent-achloroethane accounted for <1 and 2% in unaltered lake water and filter-sterilized water, respectively. Disappearance rate constants for hexachloroethane were 0.33/d for unaltered water and 0.26/d for filter-sterilized water. At least 80% of the hexachloroethane disappearance in unaltered water was abiotic in origin due to the reactions with naturally occurring aqueous polysulfides, H2S and (Miller et al, 1998a). 

In contrast, 1-phenylethylidene (74), which can undergo 1,2-hydrogen migration to form styrene (75), was found to be thermally stable in argon or xenon matrix at 10 K. The carbene decay to give styrene only when warming to 65 K in xenon matrix (Scheme 9.18). From the disappearance rate constant for 74, a energy barrier... 

No direct determination of A oh for molybdenum was made. Smith18 determined the rate constant of hydroxyl disappearance on Pyrex at various temperatures. Avramenko19 determined the oxygen atom disappearance rate constant for pure molybdenum. Assuming that it is equal to that of hydroxyl, an equation may be written... 

Baughman (1992) measured the disappearance rate constants for a number of solvent and disperse azo, anthraquinone, and quinoline dyes in anaerobic sediments. The half-lives ranged from 0.1 to 140 days. Product studies of the azo dyes showed that reduction of the azo linkages and nitro groups resulted in the formation of substituted anilines. The 1,4-diaminoanthraquinone dyes underwent complex reactions thought to involve reduction and replacement of amino with hydroxy groups. Demethylation of methoxyanthraquinone dyes and reduction of anthraquinone dyes to anthrones also was observed. 

Figure 5. The pseudo-first-order disappearance rate constants for ground-state CtO are plotted as a function of 02 pressure for two pressures of C3Ot. The results fall on a straight line and the slope gives a second-order disappearance rate constant for C20 reacting with 02 of 3.30 0.12 X 10 13 cm3 moleculesec 1 (9), 10 mtorr Cs02 (+), 5 mtorr C302. K" — 3.30 0.12 X 10 13 cm3 sec 1.

The disappearance rate constants for a large number of halogenated hydrocarbons, which include halogenated alkanes and alkenes, in anoxic sediment-water systems were correlated with several molecular descriptors (Peijnenburg et al.,... 

Figure Log plot of the measured disappearance rate constants for C (a 7T V = 0, 1, and 2) with 0 vs. l/T.

Figure Log plot of the disappearance rate constant for C (X 2 5 V = O) with OpVs. l/T. The straight l ne is the result...

In order to deduce a disappearance rate constant for the reactant ions from the experimental data, a hydrodynamic analysis must be made of the flow down the afterglow tube. Fite has outlined the physics of this problem, for which no exact solution may be obtained. In the absence of such analysis, the crudest model, assuming slug flow and uniform addition of the neutral reactant, gives... 

More recently, the reaction advancement of resole syntheses (pH = 8 and 60°C) was monitored using high-performance liquid chromatography (HPLC), 13C NMR, and chemical assays.55,56 The disappearance of phenol and the appearances of various hydroxymethyl-substituted phenolic monomers and dimers have been measured. By assessing the residual monomer as a function of reaction time, this work also demonstrated the unusually high reactivity of 2,6-dihydroxymethyl-phenol. The rate constants for phenolic monomers toward formaldehyde substitution have been measured (Table 7.6). 

Thus, we may write the pseudo first-order rate constant for disappearance of CD4 as n(CH5 iD -+) = 4.40 X 10 4 sec.-1 Appropriate rate equations are... 

Because VD and Vfi were taken to be unequal for the general case, the disappearance rate constant (a(3) is not equal to the appearance rate constant (a P )- However, the rate constants become identical when the volumes of the compartments are also identical. 

If one monitors the rate of disappearance of the original reactant species the general differential and integral approaches outlined in Section 3.3 may be used to determine the rate expression for the initial reaction in the sequence. Once this expression is known one of several other methods for determining either absolute or relative values of the rate constants for subsequent reactions may be used. 

Values of /c2, the maximal rate constant for disappearance of penicillin at pH 10.24 and 31.5°, and Ka, the cycloheptaamylose-penicillin dissociation constant are presented in Table VII. Two features of these data are noteworthy. In the first place, there is no correlation between the magnitude of the cycloheptaamylose induced rate accelerations and the strength of binding specificity is again manifested in a rate process rather than in the stability of the inclusion complex. Second, the selectivity of cycloheptaamylose toward the various penicillins is somewhat less than the selectivity of the cycloamyloses toward phenyl esters—rate accelerations differ by no more than fivefold throughout the series. As noted by Tutt and Schwartz (1971), selectivity can be correlated with the distance of the reactive center from the nonpolar side chain. Whereas the carbonyl carbon of phenyl acetates is only two atoms removed from the phenyl ring, the reactive center... 

The reciprocal of the FRET-unperturbed donor lifetime, td, is given by the sum of all rate constants for deactivation. These parameters have been extensively discussed in earlier chapters. We note in passing that the constants with extreme values in Eq. (12.1) disappear if one expresses the absorption (excitation) spectrum of the acceptor in terms of the molecular absorption cross-section, o (2) = 1017ln[10] Njy x e (2)(nnr/moleculc). 

The rate of SECOND-ORDER REACTIONS depends on the concentration of both substrates. If the concentration of B is constant. A disappears in a first-order fashion, but the rate constant for A disappearance depends on the concentration of B. 

Since glutathione is synthesized in cells in relatively huge amounts, it is seldom applied as pharmacological antioxidant. Furthermore, the mechanism of its antioxidant activity is not so simple as that of vitamins E and C. The major reason is that the GS radical formed during scavenging of free radicals by GSH does not disappear by dimerization but participates in the chain reaction, producing superoxide (Reactions (20)-(23)). Furthermore, it has recently been shown that contrary to previous findings the rate constant for the reaction of GSH with superoxide is relatively small (200-1000 lmol-1 s-1) [211,223],... 

In the same vein is the observation that the lifetime of dipropylcarbene (59) in CH2C12 or cyclohexane is 0.3 ns,84 which, after statistical correction is 48 times less than the lifetime ( 21 ns) of Me2C in pentane.22 This reflects promotion by the propyl bystander groups of 59 of the 1,2-H shift to Z- and E-3-heptene.84 (Dipropylcarbene can be photolytically generated from either an oxadiazoline (diazoalkane)84 or diazirine85 precursor, but RIES lowers the efficiency of carbene production in either case.) Recently reported LFP lifetimes for Et2C and MeCEt in cyclohexane or benzene are 0.6-3 ns (cyclohexane) or 1-5 ns (benzene),14 in accord with the lifetimes of S822 and S9.84 The rate constants for carbene disappearance in cyclohexane ( 3 x 108 to 2 x 109 s 1) are presumably limited by 1,2-H shifts.14... 

Figure 26.7. Only after the mineral has almost disappeared does the silica concentration begin to decrease. Since the surface area and rate constant for cristobalite are considerably greater than those of quartz, the fluid remains near equilibrium with cristobalite until it in turn nearly disappears. Finally, after several hundred thousand years of reaction, the fluid approaches saturation with quartz and hence thermodynamic equilibrium.

The rate constants for metabolism and excretion were further combined to a rate constant for the disappearance of the drug, k. Four dosage forms were studied and it was found that was constant for all forms except for a coarse, slowly dissolving powder. 

Portmann and co-workers then studied the kinetic pathways in man for hydroxynalidixic acid, the active primary metabolite.(26) The rate constants for glucuronide formation, oxidation to the dicarboxylic acid and excretion of hydroxynalidixic acid were calculated. Essentially total absorption of hydroxynalidixic acid was found in every case. Good agreement between experimental and theoretical plasma levels, based on the first order rate approximations used for the model, was found. Again, the disappearance rate constant, kdoi was found to be very similar for each subject, although the individual excretion and metabolic rate constants varied widely. The disappearance rate constant, k was defined as the sum of the excretion rate constant, kg j and the metabolic rate constants to the glucuronide and dicarboxylic acid, kM-j and kgj, respectively. 

Samples 1-4 correspond to VPO treated in steam for 92, 312h, in N2 and activated base catalysts, respectively, k, are pseudo-first-order rate constants for the disappearance of butane. The constants are measured in a microreactor on a larger amount ( 1 g) of catalyst at 633 K. k (intrinsic) are based on the BET surface area. 

Various rate constants which enter into the expression for k i, Equation 14.14, have now been discussed. kunj as defined in Equation 14.13 has the appearance of a first order rate constant for the disappearance of A molecules but it is actually only a pseudo first order rate constant since it explicitly depends on the concentration of M, the species involved in the activation and deactivation of A molecules. In the limit of high concentration, [M] oo, kuni reduces to an apparent first order process, lim (kuni j oo) = ka(E)(8ki(E)/k2)[A] = kt(Apparent)[A], while at low concentration the reduction is to an apparent second order process, lim(kunij[M]->.o) = 8k (E)[A][M] = k2 (Apparent) [A] [M],... 

Owing to this large concentration of OH relative to O and H in the early part of the reaction zone, OH attack on the fuel is the primary reason for the fuel decay. Since the OH rate constant for abstraction from the fuel is of the same order as those for H and O, its abstraction reaction must dominate. The latter part of the reaction zone forms the region where the intermediate fuel molecules are consumed and where the CO is converted to C02. As discussed in Chapter 3, the CO conversion results in the major heat release in the system and is the reason the rate of heat release curve peaks near the maximum temperature. This curve falls off quickly because of the rapid disappearance of CO and the remaining fuel intermediates. The temperature follows a smoother, exponential-like rise because of the diffusion of heat back to the cooler gases. 

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