Dehydration rate

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

Fig. 12. Schematic representation of variations in dehydration rates (ft) with prevailing water vapour pressure (Ph2o) These examples include Smith—Topley behaviour and indicate correlations with phase stability diagrams. (After Bertrand et al. [596], reproduced with permission, from Journal of Inorganic and Nuclear Clemistry.)...

More detailed consideration of the sensitivities of dehydration rates to reaction conditions (PH2o> temperature) are given in the articles cited reported values of E, at various PH2Q, are summarized in Fig. 13. From kinetic observations, it was concluded that between 353 and 383 K, the dehydration of CaS04 2 H20 [590] involved nucleation ( in Fig. 13) and boundary control (o) but for 383—425 K, a diffusion mechanism ( ) operated. The kinetics of dehydration of a-CaS04 ] H20 [590] (X,+)... 

Pritchard PH, Gripe CR, Walker WW, et al. 1987. Biotic and abiotic dehydration rates of methyl parathion in freshwater and estuarine water and sediment samples. Chemosphere 16 1509-1520. 

The mass loss rates for the boric acid samples were comparable to the untreated samples, despite a higher char yield (Table IX) for the treated samples. This was unexpected since the role of a wood fire retardant is to increase the char by increasing the dehydration reaction (1,3,7). Thus, a fire retardant treated sample will actually pyrolyze at a lower temperature. Data from Table III suggests that boric acid may form more char by suppressing formation of flammable volatiles instead of by increasing the dehydration rate. 

Table 2. Representative values of the dehydration rate constant and the complex formation rate constant for a singly and a doubly charged anionic ligand for several metals in aqueous solutions. An ionic strength of 0.01 mol dm 3 was assumed. T] is the half dissociation time (equivalent to O J/ka) for a singly charged anionic ligand with a stability constant of 106 dm3 mol. Based on [5,164,172]...

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

The sulfate anion-radical is not a very strong hydrogen acceptor. It acquires the atomic hydrogen from organic substrates at significantly smaller rates as compared with the rates of one-electron oxidations. For instance, dehydration rate constants are 10, 10 , and 10 L mol s for methanol, tert-bntanol, and acetic acid, respectively (Goldstein and McNelis 1984, Zapol skikh et al. 2001). Snch a peculiarity is very important for the selectivity of ion-radical syntheses with the participation of S04. ... 

The rate of dehydration of uridine hydrate (and the hydrates of various uridine phosphates) has been reported by Logan and Whitmore,52 at 86°C and a pH of 8.4. The concentration of hydrate was unspecified (it was prepared by photolysis of uridine solution and not isolated), and the fraction of complete recovery was not specified (about 97.5% recovery is shown). The main purpose was to compare the dehydration rates of uridine, various uridine phosphates and polyuri-dylic acid. The uncertainty about the nature of the products formed in the dehydration of irradiated uridylic acid (see below), however, makes interpretation of the observed rates quite difficult. It would be better to measure the rates of formation of a particular product than to rely too heavily on measurements of absorbance change. 

Logan and Whitmore report that the dehydration rates for irradiated oligo U and poly U are functions of the chain length (Fig. 30) and pH. Furthermore, the dehydration rate for poly U which had been irradiated and dehydrated and then irradiated again was faster than that of the first dehydration. It was observed that the elution pattern on Sephadex of the recycled poly U had changed. 

Fig. 30. Dehydration rates of irradiated oligo U and poly U as functions of the chain length (Logan and Whitmore52).

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

The dehydration rate depends very strongly on substitution on Ca. Large differences in reactivity of primary, secondary and tertiary alcohols over solid catalysts were reported as early as in 1931 by Dohse [90]. Also, substituents on Cp affect the rate. Both influences can be quantitatively described by the Hammett and Taft relationships the published correlations are summarised in Table 4. Of special interest is the extensive set of alcohols of the type R R2R3COH [56], which includes primary, secondary and tertiary alcohols and gives a single Taft correlation with an excellent fit. The values of p and p which can give information about the mechanism and catalyst nature will be discussed in the following sections. 

An interesting example of kinks in kinetic data is obtained from Scheurer and co-workers (133), who studied the dehydration of carbonic acid. Figure 6 is a plot of the log (dehydration rate) vs. reciprocal of absolute temperature. Note the relatively abrupt change near 31 °C. 

Write the steps in the mechanism for the dehydration of a given alcohol. Given alcohols of different classes, tell which dehydration mechanism is most likely, and what the relative dehydration rates will be. 

Bellincontro, A., De Santis, D., Botondi, R., Villa, I., and Mencarelli, F. (2004). Different postharvest dehydration rate affects quality characteristics and volatile compounds of Malvasia, Trebbiano, and Sangiovese grapes for wine production. ]. Sci. Food Agric. 84, 1791-1800. 

Spectra of ZrC>2-supported WOx species were recorded by Baertsch et al. (2002) after 1 h under 2-butanol dehydration conditions (0.5 kPa reactant, 323 K). The relative abundance of reduced centers was estimated from the Kubelka-Munk function in the range 1.5-3.2 eV (824—388 nm). Dehydration rates were obtained in a separate quartz reactor at 373 K. UV-vis band area and rate increased with the tungsten density up to a particular loading. Equivalent experiments with WOx/ A1203 were performed by Macht et al. (2004). A parallel increase of the initial dehydration rate at 373 K and the relative abundance of reduced centers at 423 K were pointed out. 

Dehydration of alcohols proceeds by heterolytic cleavage of the C-O bond, yielding carbocation and hydroxide anion, and the dehydration rate is determined by the stability of the thus-formed carbocation. Therefore tertiary alcohols such as tcrt-butyl alcohol (2-methyl-2-propanol) are more easily dehydrated. When these solvents are used for the solvothermal reaction, the essential nature of the reaction may be identical to that of the hydrothermal reaction. 

Examples of reactions in which acid-base catalysed dehydration was combined with acid-base equilibria either preceding or subsequent to the dehydration process are quoted for the waves of pyridine aldehydes (Tirouflet and Laviron, 1959 Volke, 1958 ManouSek and Zuman, 1964) and of glyoxalic acid (Kuta, 1959). Different dehydration rates were found for pyridinium ions and free pyridine derivatives, as well as for the free glyoxalic acid and its anion. For numerous aldehydic substances the effect of hydration has been observed but a quantitative treatment has not yet been applied. 

Linear Free Energy Relationships. - Kibby and Hall studied the dehydration of fifteen acyclic alcohols on a stoicheiometric (HA) hydroxyapatite [Caio(P04)6(OH)2] and a non-stoicheiometric (NHA) hydroxyapatatite for which Ca/P= 1.58. The former gave both dehydrogenation and dehydration but the latter gave only dehydration. In the case of the NHA catalyst the dehydration rate constants correlated with the Taft constants for a-carbon substitution giving p = — 5 at 230 °C, a-propanol being the reference alcohol so that the Taft equation was of the form (equation 2). 

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