Order, in water

For aqueous solutions, ascorbate can be included in the hierarchy, while a-tocopherol has to be replaced by its water-soluble analogue trolox, which is often assumed to have the same standard reduction potential. The ordering of the antioxidants based on the two different determinations of E in water is rather similar, and it should be noted that ascorbate is the antioxidant which will regenerate the other antioxidants, with the ascorbate itself ending up being oxidised. In contrast to what was observed for DMF, the ordering in water predicts that quercetin could regenerate a-tocopherol from its oxidised form. 

Both complexes (867) and (868) promote the hydrolysis of urea in a two-step process.2080 Heating of (867) or (868) in acetonitrile solution produced ammonia with kinetic first-order dependence on complex concentration and an observed rate constant of (7.7 0.5) x 10-4 h-1 to yield a cyanate complex as the reaction product. When the reaction was carried out in 50% aqueous acetonitrile solution, ammonia was produced at the same rate but without buildup of the cyanate-containing product, suggesting that the latter is hydrolyzed in the presence of water. The hydrolysis rate was also first order in water, indicating that it occurred by attack of an external water on the coordinated cyanate.2080... 

Feed concentration of diacetal is 1 kmole/m3. Note. Data are hypothetical. The reaction may be assumed to be zero-order in aldehyde and alcohol, and apparently zero-order in water. 

Experiments have shown that the reaction is first order in (CH3)3CBr, and zero order in water. In other words, the rate of the reaction does not depend, at all, on the concentration of water. The rate law equation is written as follows ... 

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates (1) with X-benzylamines (2) in acetonitrile at 55 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T, with a hydrogen-bonded four-centre type transition state (3). The results of studies of the aminolysis reactions of ethyl Z-phenyl carbonates (4) with benzylamines (2) in acetonitrile at 25 °C were consistent with a four- (5) and a six-centred transition state (6) for the uncatalysed and catalysed path, respectively. The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction was found to be third order in water. The kinetic solvent isotope effect was (A h2o/ D2o) = 2.90 0.12. Proton inventories at each molarity of water studied were consistent with an eight-membered cyclic transition state (7) model. 

Consider the cationic polymerization of isobutylene using S11CI4 as the coinitiator and water as the initiator. Under certain reaction conditions, the polymerization rate was found to be first-order in SnCLj, first-order in water, and second-order in isobutylene. 

Kinetics There have been few comprehensive studies of the kinetics of selective oxidation reactions (31,32). Kinetic expressions are usually of the power-rate law type and are applicable within limited experimental ranges. Often at high temperature the rate expression is nearly first order in the hydrocarbon reactant, close to zero order in oxygen, and of low positive order in water vapor. Many times a Mars-van Krevelen redox type of mechanism is assumed to operate. 

It has been mentioned that the rate constant for photohydration of uracil shows a sigmoid variation with increasing pH over the pH range of 1.5-8.64 The rate is independent of (UH ), is independent of neutral salt concentration (showing that one species in the reaction complex is neutral), and is, as shown, first order in water concentration. The point of inflection of the rate versus pH curve is about 4, compared... 

If the rate-determining step of the reaction involves only the protonated ester and a water molecule or molecules, the dependence of the rate of hydrolysis of the protonated ester on the activity of water may be obtained by plotting log 0hs/[BH+] versus log aH2o- Lane s plot is reproduced as Fig. 9. The slope should give the order in water, and is close to 2 for hydrolysis and for O-exchange also. 

Analogous experiments with mixtures of identical combustion temperature containing various excess amounts of carbon monoxide showed that the flame velocity is proportional to [CO ]1/2, where [CO ] is the carbon monoxide concentration in the reaction zone. From this it follows that the chemical reaction in a flame is first order in carbon monoxide. The role of water in the combustion of carbon monoxide is well known. Analysis of available data shows that the flame velocity is proportional to [H20]1//2, i.e., the reaction is first order in water vapor content. The influence on combustion of such flegmatizers as CC14 may be ascribed to the binding of hydrogen by halogen with the formation of a molecule of HC1, which is dissociable only with difficulty. However, the latest experiments by Kokochashvili in our laboratory show that the influence of the... 

The kinetic order in water for the spontaneous hydrolysis reaction, n, and the hydronium ion catalyzed reaction, m, varies depending on the structure of the silane ester and the solvent conditions [36, 40]. The difficulty in determining the kinetic order of water in aqueous-organic solvents arises from the observation that as the concentration of water is varied, the polarity of the solvent and the activity of the acid change [40], A plot of the logarithm of the rate constant vs. the logarithm of water concentration often does not yield a straight line. These... 

The solvent also affects the order of the reaction. For example, the order in water for the spontaneous hydrolysis varies from n = 0.8 in aqueous propan-2-ol to n =4.4 in 30-40% aqueous dioxane. For the hydronium ion catalyzed hydrolysis, the order of water varies from m = 0 in aqueous propan-2-ol to m = 2.6 in 30-40% aqueous dioxane [36, 40]. More water molecules are required for the spontaneous hydrolysis reaction than for the hydronium ion catalyzed reaction. The order in water for the hydroxide anion catalyzed hydrolysis term p and for the general base catalyzed hydrolysis term q has not been determined [36-39,41 -43]. 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

The fact that the hydrolysis is second order overall, first order in TMMS, and first order in water suggests that the rate-determining step of the hydrolysis reaction is the nucleophilic attack of water on the silicon atom. The data also indicate that the reaction is acid-catalyzed. One possible mechanism for add-catalyzed hydrolysis of TMMS begins with the rapid protonation of the methoxy group to form a better leaving group. The protonation is followed by the... 

The hydrolysis reaction was found to be first order in acid, zero order in water and to have a Hammett p of -1.42 when catalyzed by sulfuric acid. These results are consistent with current opinion that the reaction mechanism is SN, and involves a positive intermediate, possibly a siliconium intermediate. 

The hydrolysis reaction was first order in acid and zero order in water. This reaction was found to have a Hammett p value of -1.42. This suggests that the electron donating groups speeded up the reaction, presumably by stabilizing the developing positive charge of the siliconium intermediate. 

The results obtained with sodium and water differ somewhat from those described above. Three runs were made, and except for three traces of the last run only a single rate was observed. Figure 3 shows a plot of log k (where k is the pseudo-first-order rate constant) vs. log [water concentration]. The order in water found from this plot is 1.6 compared to a value of 1.1 for the slowest rate of cesium with water also shown in Figure 3. 

All reactions were found to be first-order in the absorbance, and all reactions except that of sodium solutions with water seem to be first-order in water. In considering the apparent 3/2 order in water for the latter, it must be recalled that sodium solutions show only a single absorption band with a maximum at 660 m/z. The nature of this reaction is... 

Dissolution of alkali metal cations such as Cs+ results in short-range liquid order in water as a primary solvation shell of about eight water molecules is established about the metal cation. Lithium, however, exerts a much greater polarising power and is capable of organising a first- and second-coordination sphere of about 12 water molecules about itself, resulting in a much larger hydrated radius for the ion and hence decreased ionic mobility. 

A recent study was designed to determine more specifically the effect of water on the electrohydrodimerization of diactivated olefins. During the reactions of DEF, the radical anion of [12] it was found that the reaction in DMF is second order in radical ion as expected from previous work and first order in water, according to rate law (116) (Parker, 1981c). A small deuterium... 

In dry MeCN, n = 0.6 was found for 51, but the value increased to 0.94 on addition of water up to 10% [13]. For 52a in very dry MeCN, the mechanism of the reductive dimerization was examined and the experimental results were interpreted as an RS mechanism under these conditions [125]. However, addition of water increased the rate of reaction considerably, and in the presence of water the kinetic measurements were in accord with the RR mechanism [126]. In DMF, addition of water also accelerates the dimerization process for 52 [7,10], similar to what is observed for many monoactivated alkenes (Sec. II.A.7). The accelerating effect of water in DMF on the dimerization of 52a has been studied in greater detail [15]. On the basis of a reaction order in water close to 1, it was suggested that the dimerization reaction takes place between a free radical anion and a hydrogen-bonded radical anion [15]. The involvement of hydrogen bonding between radical anions and water may also account for the low activation energies found for the reductive dimerization of 52a in MeCN [126] and in DMF [10] (see Table 11). 

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