Rate of hydration

Equilibrium Constants (AChydr) and Relative Rates of Hydration of Some Aldehydes and Ketones... 

Table 6.1. Rates of Hydration of Some Alkenes in Aqueous Sulfuric Acid ...

Subsequent investigations proved that identical hydration reactions occur on bare aluminum surfaces and bonded surfaces, but at very different rates of hydration [49]. An Arrhenius plot of incubation times prior to hydration of bare and buried FPL surfaces clearly showed that the hydration process exhibits the same energy of activation ( 82 kJ/mole) regardless of the bare or covered nature of the surface (Fig. 11). On the other hand, the rate of hydration varies dramatically, de-... 

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

Effects of Structure on Rate Electronic and steric effects influence the rate of hydration in the sfflne way that they affect equilibrium. Indeed, the rate and equilibrium data of Table 17.3 parallel each other almost exactly. 

M s , in agreement with independent measurements of the rate of hydration. Thus, it appears that the oxygen exchange rate can be completely accounted for by the hydration reaction. 

The rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

When two sites of hydration are favored in a molecule (see Section IV for the influences involved), occasionally the relative rates of hydration are such that the flrst hydrate formed is not the more stable one. Thus 2,6-dihydroxypteridine (7) adds water flrst across the 3,4-position to give substance 8, but hydration across the 7,8-position takes place so effectively that after 2 hr most of the material is present as the isomer (9). The latter substance is the more stable but its formation involves overcoming a higher energy barrier. ... 

The effects of alkyl substituents in positions 6 and 7 on the rates of hydration and dehydration of 2-hydroxypteridine are not very great. The observed decrease in the equilibrium ratio of [HY]/[HX] has been... 

When the rates of hydration of pteridine and its methyl derivatives are plotted against pH, a similar V-shaped curve (Fig. 5) is obtained. Results for a limited series of rate measurements on the hydration of pteridine, measured polarographically, accord with these values. 

It is the rapid increase in rates of hydration with increasing hydrogen ion concentration that prevents measurement with existing apparatus of the -pKa values of anhydrous bases such as pteridine. For example, at pH 1, hydration of the anhydrous cation is half-complete in 0.01 sec at 20°. Conversely, it is the comparative slowness of the reactions in near-neutral solutions that makes it possible, by adding acid solutions to near-neutral buffers, using the stopped-flow technique, to determine the p STa values of the hydrated species. 

The hydration of 5-amino-3-cyano-l-(2,6-dichloro-4-trifluoromethylphenyl)-4-ethynylpyrazole was performed with p-toluenesulfonic acid monohydrate in acetonitrile (2 h, room temperature) to give the corresponding 4-acetyl derivative. An alkyl substituent at the triple bond decreases the rate of hydration the conversion of 5-amino-3-cyano-l-(2,6-dichloro-4-trifiuoromethylphenyl)-4-(prop-l-yn-l-yl) pyrazole to the 4-propanoylpyrazole was completed after 18 h (98INP9804530 99EUP933363). 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

As the alkyl groups R are varied on compounds 2-5, the rates of hydration... 

However, the rates of hydration of these compounds are about a factor of ten to one hundred faster than predicted by means of the Taft relation. Anchimeric assistance by the hydroxyl group in the transition state for protonation, 9, has... 

The relative rates of hydration of arylacetylenes in acetic acid water sulfuric acid to the corresponding acetophenones were investigated by Bott et al. (17), and they are listed in Table I. 

Noyce and Schiavelli (21) also measured the rate of hydration of the above phenylacetylenes in deuteriosulfuric acid solution. Values of the observed solvent isotope effects, kn jo/l D,o. ar summarized in Table III. 

The rates of hydration of substituted phenylpropiolic acids give a rho of —4.77 when plotted against a, comparable to Ihe acid-catalyzed isomerization of czs-cinnamic acid, with a rho value of —4.3. The solvent deuterium isotope effects are 3.7-S.2 for the isomerization of cinnamic acids at... 

Rate of hydration of the polymeric materials has been shown to be an important consideration in regard to drug release. Gilding and Reed (24) demonstrated that water uptake increases as the glycolide ratio in the copolymer increases. The extent of block or random structure in the copolymer can also affect the rate of hydration and the rate of degradation (25). Careful control of the polymerization conditions is required in order to afford reproducible drug release behavior in a finished product. Kissel (26) showed drastic differences in water uptake between various homopolymers and copolymers of caprolactone, lactide, and glycolide. 

Under hydrothermal conditions, alumina suffers from increased rates of hydration and hydrolysis of the oxides, resulting in the weakening of conventional supports according to Equation 1. Silica can suffer a similar hydrothermal fate. Process pH can exacerbate the problem (Fig. 34.5). Hence hydrothermal stability is a paramount criterion. 

Similar studies utilizing 180 exchange have been used for measuring the rate of hydration of C02 (and dehydration of HC03-) in aqueous solution. See ... 

Substituents on the 2- and 6-positions of phenol rings greatly influence QM reactivity. Reaction rates for QMs derived from several of the phenols, shown in Fig. 10.1, were determined in methanolic or aqueous solutions and are listed in Table 10.1. Replacing a tert-butyl substituent of BHT by a methyl group (i.e., BDMP-QM) increased the rate of hydration by 60-70-fold at pH 7.4 and this... 

Thus the rate of hydration of an alkene, directly with water,... 

The rate of hydration of obsidian, which is diffusion limited, forms the basis for Obsidian Hydration Dating [f]. A date refers to "the time a fresh surface of obsidian was created, either naturally or by man.. ..Laboratory and field studies have confirmed that the time indicated by a hydrated layer is proportional to the thickness squared of the layer. The hydration rate is independent of the relative humidity of the environment, but the chemical composition of the obsidians affect the rate by orders of magnitude. Si02 increases the rate, whereas CaO, MgO, and H20 decrease it. A 6 - 8 °C temperature increase causes doubling of the rate." The method is quite inexpensive, and it is applicable to ages between a few hundred and several million years. 

The half-life (t 1/2) of a reactant is the time required for its concentration to decrease to one-half its initial value. The rate of hydration of ethylene oxide (A) to ethylene glycol (C2H4O + H2O - C2H6O2) in dilute aqueous solution is proportional to the concentration of A with a proportionality constant kA = 4.11 X 10-5 s-1 at 20°C for a certain catalyst (HCIO4) concentration (constant). Determine the half-life (ti/2), or equivalent space-time (T1/2), in s, of the oxide (A) at 20°C, if the reaction is carried out... 

The rate of hydration of ethylene oxide (A) to ethylene glycol (C2H40 4- H20 C2H602)... 

ESR studies of other paramagnetic metal ions (Mn " ", VC)2+) as surface probes have confirmed that cation rotational mobility on fully wetted smectites with interlayer spacings of 1.0 nm or more is not more than 50% reduced relative to aqueous solution (12, 14). Apparently, about four molecular layers of water in the interlayer provide sufficient space for solution-like rotation rates of hydrated metal ions. 

The only kinetic isotope effects so far reported for these reactions are those given by Pocker (1960), without experimental detail. He reports closely similar values for the rates of solvent-catalysed hydration of the species CHg. CHO, CD3. CHO, CH3. CDO and CD3. CDO in water at 0° C the replacement of CH3 by OD3 increases the velocity by about 7%. The same effect is reported for solutions in deuterium oxide at 0° C, presumably super-cooled. A comparison was also made of rates of hydration in HjO and DgO at 0°C, giving the following values for k(H.z0)lk(T>20) in presence of different catalysts H+/D+, 1 -3 AcOH/AcOD, 2 5 AcO , 2-3 H2O/D2O, 3-6. Almost exactly the same ratios were obtained by measuring rates of dehydration at 25° C in dioxan containing 10% of H2O or D2O and various catalysts. The presence of a considerable solvent isotope effect is consistent with the mechanism given in Section IV,B, and it would not be expected that substitution of deuterium on carbon would have an appreciable effect on the rate. 

The rate of hydration of an olefin increases with acidity. It has recently been demonstrated that the hydration of isobutylene follows the Ho acidity function, i.e. dlogkj — dHo) = 1, where Hq is the acidity function defined by equation (10). The behaviour of secondary and tertiary alcohols in 0-55 % H2SO4 is very similar (Beishlin, 1963). 

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