Dehydration Rate Constants Research Articles

Kinetics of the reversible hydration of 1,3-dichloroacetone catalysed by micellar aerosol-OT in carbon tetrachloride

The kinetics of the reversible hydration of 1,3-dichloroacetone (DCA) by Aerosol-OT (AOT) solubilized water or D2O, have been studied in carbon tetrachloride. In the presence of the surfactant, rate constants for hydration or dehydration are faster than those in water–dioxan and/or D2O–dioxan mixtures. Rate enhancements range from 1 808 to 111 for hydration by water–AOT, and from 1 619 to 124 for hydration by D2O–AOT. The corresponding ratios for the dehydration reaction range from 508 to 64 and 1 788 to 104 for water–AOT and D2O–AOT, respectively. Equilibrium constants for hydration are also higher in the presence of AOT than those in water– or D2O–dioxan mixtures. The results are discussed in terms of the unique properties of AOT solubilized water, as well as participation of the surfactant head-group in the slow water attack.

Kinetics of concomitant degradation of tetracycline to epitetracycline, anhydrotetracycline, and epianhydrotetracycline in acid phosphate solution.

The concentrations of tetracycline, epitetracycline, anhydrotetracycline, and epianhydrotetracycline in pH 1.5 phosphate solution were followed as a function of time at four temperatures. Separation and quantification of all four species were accomplished using high-pressure liquid chromatography. Through nonlinear regression analysis, rate constants for the reversible first-order epimerization of tetracycline and anhydrotetracycline and for the first-order dehydration of tetracycline and epitetracycline were obtained. Solutions to the differential equations obtained through Laplace transforms successfully predict concentrations found experimentally. The energy of activation for each reaction step was calculated and ranged from 15 to 27 kcal/mole. The rate constants for tetracycline and epitetracycline dehydration conform with those of earlier studies that used different experimental methods. The study shows that epimerization of tetracycline and anhydrotetracycline can take place at a low pH.

Solvent Isotope Effect of Pyruvic Acid Hydration

AbstractThe solvent isotope effect on the hydration constant, the dissociation constant and the rate constants for the uncatalysed and specific acid catalysed dehydration of pyruvic acid is investigated. The pK of the dissociation constant of the ketoacid is by 0.29 smaller in H2O than in D2O. The ratio of the hydration constant of undissociated pyruvic acid (K/K) is 0.83, while the ratio of the rate constant of dehydration is 2.65 for the uncatalysed reaction and 1.33 for lyonium ion catalysis. The results are discussed in terms of the possibility of intramolecular hydrogen bonding.

Studies of acid catalyzed reactions: XII. Alcohol decomposition over hydroxyapatite catalysts

Structure-reactivity relationships for alcohol elimination reactions were studied over two well-characterized hydroxyapatite catalysts. A “stoichiometric” preparation (HA), with the composition Ca 10(PO 4) 6(OH) 2, catalyzed both dehydration and dehydrogenation. A “non-stoichiometric” catalyst (NHA) with Ca P = 1.58 was active only for detrydration. The dehydration rate constants for fifteen acyclic alcohols over the NHA catalyst correlated with Taft σ α ∗ constants for substitution at the alpha carbon atom yielding ϱ α ∗ = − 5 at 230 °C. The dehydration rates were much slower over the less acidic HA catalyst, and a temperature of 400 °C was needed for a linear relation between σ α ∗ values and the logarithms of the rate constants. At that temperature ϱ α ∗ = − 2 for alpha substitution. The structure-reactivity relationships were remarkably similar to those for gas phase reactions such as HBr-catalyzed alcohol dehydration and the pyrolysis of esters. Like the latter, the eliminations over hydroxyapatite were mainly syn-eliminations. The necessary requirement is apparently the presence of a sufficiently acidic hydroxyl group at the surface. Beta elimination predominated in the alcohol dehydrations, but, over the NHA catalyst, both primary and secondary alcohols yielded products attributable to gamma elimination, and some of the primary alcohols gave cyclopropanes. Dehydrogenation of secondary alcohols over the HA catalyst could be correlated with σ ∗ values at 400 °C but in the opposite direction to dehydration; ϱ α ∗ = +1.5 for alpha substitution, suggesting that the transition state is negatively charged. An alkoxide ion is a likely precursor, with hydride transfer as the rate-limiting step.

The rates of hydration of carbon dioxide and dehydration of carbonic acid at 37°

The rates of hydration of CO 2 and dehydration of H 2CO 3 were determined using a stopped-flow spectrophotometric apparatus. The hydration rate constant, k 1, was found to be 0.145 sec −1 in imidazole buffer at 37° and the dehydration rate constant, k −1, was calculated to be 49.6 sec −1. These rate constants are compared with previously reported values in the literature. For comparison, the rate constants at 25° were also determined. At both temperatures, the ratio of k −1 and k 1 agrees reasonably well with the equilibrium constant, K, calculated from ionization constants.

Reversal mechanisms for the photohydrates of cytosine and its derivatives

1. Dehydration rate constants of the photohydrates of cytosine, cytidine and cytidine monophosphate are presented as a function of pH with a view to elucidating the reaction mechanisms. 2. Photohydrates were produced by ultraviolet irradiation at a high exposure rate at 0° and pH 8–9 in the absence of buffer. These conditions lead to a high degree of reversibility after nearly complete photolysis. 3. Buffers with p K's between 3 and 7 were found to accelerate dehydration of cytosine and cytidine hydrates in the pH range 2–7. 4. The ionization constants of the hydrates were determined by potentiometric titration. 5. A model involving general base catalysis is proposed to account for the observed rate-pH profiles. It agrees well with the experimental results. Rate constants for catalysis by various basic species are presented. 6. Reversal of the hydrate of cytidine monophosphate was found to be insensitive to added buffers. In this case the rate is greatly influenced by the phosphate group which acts as an effective internal catalyst. Our model accounts qualitatively for this effect.

Rate constants for the dehydration of single and double hydrates of cytidylyl (3'-5')--cytidine.

Abstract— Cytidylyl (3′‐5′) cytidine (CpC) was irradiated with ultraviolet light (u.v.) to produce the single hydrate (a mixture of C*pC and CpC*) and the double hydrate C*pC* which were separated by electrophoresis. These photoproducts rapidly dehydrate to CpC and deaminate to a mixture of U*pC and CpU*. The rate constants for dehydration and deamination of the hydrates were evaluated for a range of pH values from 3 to 8 at 0°C. It is observed that the rate constants for decay of C*pC* lie between those for C*pC and CpC* for all pH values studied. Both single and double hydrates show minimum stability around pH 4·5 and maximum stability around pH 8. The maximum rate constants for dehydration of C*pC*, C*pC and CpC* are 0·26, 0·145 and 0.35 hr‐1 respectively and the minimum values are 0.024, 0.011 and 0.091 hr‐1 respectively all at 0°C. The rate constants for deamination of C*pC to U*pC for a range of pH values at 0°C were measured. The amount of deamination product varies from about 2 to 10 per cent of the hydrate depending on pH with the maximum amount being produced around pH 8.

Polarographic and Spectrophotometric Studies on Trimethylpyruvic Acid

Abstract Polarographic and spectrophotometric behavior of trimethylpyruvic acid has been investigated. The limiting current of trimethylpyruvic acid shows a minimum value in pH about 0.5. The absorption spectra show the absorption maximum at about 310 mμ in aqueous solutions, indicating the decrease of the optical density in strongly acid pH. These facts have been accounted for by the hydration of carbonyl group of undissociated acid. pKa for apparent dissociation constant of trimethylpyruvic acid has been spectrophoto-metrically obtained to be 1.52 at 25°C. Hydration constant of the undissociated acid (Kh=[AHaq]/[AH]) has been kinetically obtained to be 0.46 at 25°C. The rate constant of dehydration of hydrated trimethylpyruvate in buffer solution of pH about 5 was 2.8×10−2 sec−1 at 25°C. From the temperature dependence of the hydration constant, the heat of hydration of unhydrated acid has been found to be −8.2kcal. A quantitative interpretation of the pH-dependence of the limiting current has been attempted by deriving the equation, involving the hydration and dissociation factors. The agreement of the theoretical curve with the experimental is good, except for the pH range below 0.5, where acid catalyzed dehydration of hydrated acid should be taken into consideration. Rate constants of the dissociation and recombination of trimethylpyruvic acid have been found to be kd=6.0×105 sec−1 and kr=1.4×107 1. mol−1 sec−1, respectively.