Kinetics of Hydration and Dehydration Reactions

All these reactions will have a common mechanism, which for the hydration reaction can be written as follows (Bell and Higginson, 1949)  

Of these four reactions (ii) and (iv) involve simple proton transfers to and from oxygen atoms, and experience shows that such equilibria will be set up very rapidly. The rate-limiting steps then become (i) and (iii), which involve greater structural changes and are likely to be slow. They are both formally termolecular reactions, and it is of interest to enquire whether either of them can be split up into consecutive bimolecular processes, one of which is rate-limiting. The only possibilities are as follows  

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

Some variants on the above mechanism have been proposed. Gibert 

At present there appears to be no evidence which distinguishes between these two possibilities, and they become indistinguishable in terms of the cyclic transition states proposed above. 

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However, this simple picture only applies to gases that do not undergo reactions in the boundary layers. For gases that do react, for example through hydration and acid-base reactions, the net flux depends on the simultaneous movement of all the solutes involved, and the flux will not be the simple function of concentration expressed in Equation (3.25). An example is CO2, which reacts with water to form carbonic acid and carbonate species-H2C03, HCOs and COs . The situation is complicated because the exchange of H+ ions in the carbonate equilibria results in a pH gradient across the still layer, and it is therefore necessary to account for the movement of H+ ions across the still layer as well as the movement of carbonate species. The situation is further complicated in the case of CO2 by the kinetics of hydration and dehydration, which may be slow in comparison with transport. 

The kinetics of the hydration and dehydration reactions are slow in comparison with some processes in the water. The reactions are... 

For isoenzymes I and II, the CO2 hydration rates are independent of buffer at high buffer concentrations, indicating thereby that a reaction step other than the buffer-dependent step becomes rate limiting. Studies of both hydration and dehydration reactions at high concentrations of buffers in H20 and DoO indicated that the kinetic parameter, kCSLt, for isoenzyme II has large isotope effect (k jkV) 3-4) (45b). This is consistent with involvement of H+ transfer in the rate-limiting step. The H+ transfer half-reaction is composed of at least two steps,... 

Carbonic anhydrase catalyses the hydration and dehydration reactions of CO2 and accelerates this reaction rate approximately 5000-fold. The results of earlier attempts to use carbonic anhydrase to improve sensor response time were mixed, probably because the response times of previous sensors were limited by slow diffusion through the gas-permeable membrane, and not by reaction kinetics [29, 30]. 

It was then shownthat in dioxan and acetonitrile solution the kinetic orders with respect to water for the hydration and dehydration reactions are very close to 3 and 2 respectively when no acid or base catalyst is added. This accords well with the reaction scheme (99), and further confirmation is obtained from studies of activation entropiesand of hydrogen isotope effects. 

Early (1930 to 1940) kinetic studies of dehydrations contributed much to the development of the concept of the reaction interface as the important feature of nucleation and growth reactions [2]. Kinetic equations applicable to the decompositions of a vnde range of crystalline substances were developed. Large, well-formed crystals of hydrates could be prepared relatively easily and studies of these were particularly rewarding. The interpretation of kinetic data was supplemented by microscopic evidence concerning the formation and development of product nuclei. Recent work on dehydrations has included more precise determinations of the crystal structures of reactants, products and their interrelationships, including interface textures, in the attempt to resolve unanswered questions. 

Heyden et al. suggested that hydrated and dehydrated monomolecular iron sites in Fe-ZSM-5 are responsible for N2O decomposition. They proposed that Z [FeO]+ is a key intermediate. Furthermore, water strongly adsorbs to give Z Fe(OH)2 "h This deactivates the Z [FeO]+ site. The activation energy for N2O decomposition in the presence of water increases steeply compared with the anhydrous situation, because water has to desorb from Z Fe(OH)2 in order for N2O reduction to occur. Hydration and subsequent dehydration of the oxy-iron complex may provide an alternative explanation for the oscillatory reaction found by El-Malki et al. shown in Fig. 4.28. If the reaction is not isothermal, the temperature fluctuations arising from the exothermic N2O decomposition reaction may lead to fluctuation in the water adsorption. This may provide an alternative explanation of the oscillatory kinetic behavior in the Fe +-ZSM-5 system. 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Measurements of the kinetics of the individual nucleation and growth steps in the reactions of several hydrated sulphates have been referred to in Sect. 1.2 though, perhaps surprisingly, these data were not combined in a kinetic analysis for the overall reaction in studies of the alums [51,431, 586] or NiS04 7 H20 [50]. Indeed, Lyakhov and Boldyrev [81], in one of the few reviews of the field, maintain that the satisfactory topochemi-cal description of dehydrations is a problem which at present remains... 

Non-isothermal measurements of the temperatures of dehydrations and decompositions of some 25 oxalates in oxygen or in nitrogen atmospheres have been reported by Dollimore and Griffiths [39]. Shkarin et al. [606] conclude, from the similarities they found in the kinetics of dehydration of Ni, Mn, Co, Fe, Mg, Ca and Th hydrated oxalates (first-order reactions and all values of E 100 kJ mole-1), that the mechanisms of reactions of the seven salts are probably identical. We believe, however, that this conclusion is premature when considered with reference to more recent observations for NiC204 2 H20 (see below [129]) where kinetic characteristics are shown to be sensitive to prevailing conditions. The dehydration of MnC204 2 H20 [607] has been found to obey the contracting volume... 

X-ray powder diffractometry can be used to study solid state reactions, provided the powder pattern of the reactant is different from that of the reaction product. The anhydrous and hydrated states of many pharmaceutical compounds exhibit pronounced differences in their powder x-ray diffraction patterns. Such differences were demonstrated earlier in the case of fluprednisolone and carbamazepine. Based on such differences, the dehydration kinetics of theophylline monohydrate (CvHgN H20) and ampicillin trihydrate (Ci6H19N304S 3H2O) were studied [66]. On heating, theophylline monohydrate dehydrated to a crystalline anhydrous phase, while the ampicillin trihydrate formed an amorphous anhydrate. In case of theophylline, simultaneous quantification of both the monohydrate and the anhydrate was possible. It was concluded that the initial rate of this reaction was zero order. By carrying out the reaction at several... 

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 following calculations show the range of effects from infinitely slow hydration-dehydration to infinitely fast. Emerson (1975) and Kirk and Rachhpal-Singh (1992) and have made calculations allowing for the kinetics of the uncatalysed hydration-dehydration reactions, giving intermediate results. 

For example, for formaldehyde (R = H) at neutral pH, the pseudo-first-order rate constant for the hydration reaction (forward reaction), k =k [H20], is about 10 s 1 and the first-order rate constant for dehydration, k2, is about 5x 10-3 s-1. In Chapter 20 we will use this example to show that the reactivity of compounds can influence the kinetics of air/water exchange if both processes (reaction and exchange) occur on similar time scales. 

A rapid-reaction technique was used to study the pH dependence of the reversible addition of water across the 3,4-double bond of eighteen quinazolines and four triazanaphthalenes. The pH range of 0-13 was covered, at 20°. When the rate constants for hydration were plotted against pH, a paraboloid curve was obtained with the minimum rate near neutrality. It was calculated that there is a strong acceleration of hydration in acidic solution due to the successive formation of mono-and dications (the attacking species is the water molecule). The increasing rate of hydration in alkaline solution was seen as the catalytic effect of the hydroxyl ion on the neutral species.30 The kinetics of dehydration in neutral solution proved to be 105 times faster than those for hydration. For quinazoline, the two curves crossed at pH 3.5, below which hydration ran much the faster. Substituent and positional effects, particularly the slowing effect of a substituent in the 4-position, were quantified.30... 

The atmospheric chemical kinetics of linear perfluorinated aldehyde hydrates, Cx-F2x+iCH(OH)2, have been measured for x = 1,3, and 4, focusing on formation (from aldehyde, by hydration), dehydration, and chlorine atom- and hydroxyl radical-initiated oxidation.211 The latter reaction is implicated as a significant source of perfluorinated carboxylic acids in the environment. 

The quantitative high-temperature chemistry of chlorine oxysalts is rather underdeveloped. There are very few thermodynamic data for these compounds above 298 K. Even when they exist, they must be applied cautiously, since there may be kinetic rather than thermodynamic factors that determine decomposition behavior. Although the thermal decomposition of a few compounds has been studied very carefully (e.g., the KC104 literature extends back for more than a hundred years because of the compound s use in explosives), the bulk of the available information is qualitative or semiquantitative. In recent years this has changed somewhat with increasing use of automated techniques such as DTA and TGA. Many of the reactions are complex, with mechanisms frequently controversial and not completely worked out. Decomposition products may depend on experimental conditions e.g., salts are frequently prepared by dehydration of their hydrates, and residual water may affect the course of the decomposition. 

Equilibrium isotopic fractions rebect the combined, unidirectional kinetic isotopic fractionations. In considering the one-way buxes as in Equation (12), estimates of the one-way kinetic fracbonations are needed. Knowledge of kinetic effects is obtained from controlled experiment with pure CO2 and water or with salt solutions, but for the chemically complex system that is ocean-water, empirical values are adopted. Eor example, laboratory experiments show that the hydration of aqueous CO2 to bicarbonate involves fractionation of 13%c and the dehydration reaction fractionate by 22%c (O Leary et al., 1992). The difference between these two kinetic fractionations, 9%c, corresponds to the equilibrium fractionation depicted above (Marlier and O Leary, 1984 O Leary et al., 1992). In practice, it was estimated... 

Reversible reactions. Many solid-gas reactions are reversible, e.g., dehydration of crystal hydrates, so that rate equations for such processes should include terms for the rate of the reverse reaction. If the rates of contributing forward and reverse reactions are comparable, the general set of kinetic models (Table 3.3.) will not be applicable. The decomposition step in a reversible reaction thus needs to be studied [94] under conditions as far removed from equilibrium as possible (e.g. low pressures or high flow rates of carrier gas) and sensitive tests are required for determining whether the kinetics vary with the prevailing conditions. Sinev [95] has calculated that, for the decomposition of calcium carbonate, the rate of the reverse reaction is comparable with that of the forward reaction even when small sample masses (10 mg) and high flow rates (200 cm s ) of inert gas are used. Interpretation of observations becomes more difficult and the reliabihty of conclusions decreases if local inhomogeneities of kinetic behaviour develop within the reactant mass. 

Few studies have been specifically directed towards identifying the factors which control nucleation in dehydration reactions [7]. The possibility that (i) elimination of water from the reactant structure and (ii) recrystallization of the product to form the crystalline lower hydrate or anhydrous phase may be separable kinetic processes has been discussed [34]. 

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