Dehydration, experimental

The position of the equilibrium is determined not only by ring size and polar and steric factors but also by the environment of the molecule. The experimental evidence for the existence of three tautomeric forms has been based on the study of their reactivity and, to a lesser degree, on physicochemical measurements 175-177). Often the existence of the corresponding carbinolamine or its acyclic tautomeric form in addition to the basic dehydrated form is quite important. 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

It is usually assumed in the derivation of isothermal rate equations based on geometric reaction models, that interface advance proceeds at constant rate (Chap. 3 Sects. 2 and 3). Much of the early experimental support for this important and widely accepted premise derives from measurements for dehydration reactions in which easily recognizable, large and well-defined nuclei permitted accurate measurement. This simple representation of constant rate of interface advance is, however, not universally applicable and may require modifications for use in the formulation of rate equations for quantitative kinetic analyses. Such modifications include due allowance for the following factors, (i) The rate of initial growth of small nuclei is often less than that ultimately achieved, (ii) Rates of interface advance may vary with crystallographic direction and reactant surface, (iii) The impedance to water vapour escape offered by... 

In a discussion of these results, Bertrand et al. [596,1258] point out that S—T behaviour is not a specific feature of any restricted group of hydrates and is not determined by the nature of the residual phase, since it occurs in dehydrations which yield products that are amorphous or crystalline and anhydrous or lower hydrates. Reactions may be controlled by interface or diffusion processes. The magnitudes of S—T effects observed in different systems are not markedly different, which indicates that the controlling factor is relatively insensitive to the chemical properties of the reactant. From these observations, it is concluded that S—T behaviour is determined by heat and gas diffusion at the microdomain level, the highly localized departures from equilibrium are not, however, readily investigated experimentally. 

The obtained value AHf(PUF3,c) = - 1585.7 + 2.9 kJ.mol-1 cannot be considered as entirely satisfactory as the reliability of the adopted value for the enthalpy of dehydration is not demonstrated. The only experimentally known enthalpies of formation for the actinide trifluorides are AHf(PuF3,c) and AHf(UF3,c) accuracy is therefore essential if these two data are used to estimate the enthalpies of formation of the other trifluorides. The low temperature heat capacity measurements of Osborne et al. (22) using 242PuF3(c) yield S°(PuF3,c 126.11+0.38 J.K"l.mol"i. 

F.M. Faus, B. Zhou, H. Matralis, and B. Delmon, Catalytic cooperation between M0O3 and Sb204 inN-ethyl formamide dehydration. III. Comparison of a mathematical model based on the remote control mechanism with experimental results, J. Catal. 132, 200-209 (1991). 

Zhang, J. Davies, W.J. (1987). Increased synthesis of ABA in partially dehydrated root tips and ABA transport from roots to leaves. Journal of Experimental Botany, 38, 2015-23. 

Madin, K.A.C. Crowe, J.H. (1975). Anhydrobiosis in nematodes carbohydrate and lipid metabolism during dehydration. Journal of Experimental Zoology, 193, 335-41. 

Kaiser, W.M., Schroppel-Meier, G. Wirth, E. (1986). Enzyme activities in an artificial stroma medium An experimental model for studying effects of dehydration on photosynthesis. Planta, 167, 292-9. 

FeCl3 6H20 turned out to be the catalyst of choice for this reaction, since the presence of water improved the yield. However, high yields of the desired ketones were obtained for electron-rich alkynes with anhydrous FeCla at room temperature. Alcohols that are sensitive to acid-catalyzed dehydration were also tolerated under the present conditions (R = Me or Et). Based upon experimental observations a mechanism for this reaction was proposed (Scheme 14). 

Figure 2. Diffuse reflectance UV spectra of dehydrated [1.5]TS-1 experimental data and fit residue (dotted lines), gaussian fit components (solid lines) insert details for two different TS-1.

The most informative characterization techniques used to determine Ti species inside dehydrated TS-1 catalysts are described in Sects. 3.1-3.7. The discussion is supported by the experimental data reported in Fig. 2. 

According to these workers the formation of COO-Me causes a small degree of dehydration, while that of COO-Me-OOC is accompanied by considerable dehydration. The experimental results showed that, when a = 10, divalent ions are bound as COO-Me-OOC, a form which favours precipitation. However, as a decreases, the COO-Me form becomes more apparent. 

We make no attempt to discuss the partitioning behavior of U-series elements between aqueous fluids and minerals at ambient conditions. Examples where this behavior is important include uptake of U-series elements by cal cite in speleothems or by bone apatite. Also we do not consider U-series behavior in hydrothermal solutions at high temperatures, such as during dehydration of subducted crust. In both cases complexation behavior in the fluid may play an important role, and at low temperatures kinetic controls may dominate. These are fruitful areas for future experimental study. 

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