Dehydration reaction process

Theoretical Studies. Theoretical models for the Si(OR)4 hydrolysis, polycondensation, and dehydration reactions involved in sol—gel processes have been developed using semiempirical molecular orbital models. These have been reviewed (3,5). 

The dehydration reaction leads by an Ea process to 8 and is promoted by the tertiary, benzylic nature of the OH group at Ce and its antiperiplanar trans relationship to the H atom at Csg. Furthermore, one of the cannonical forms of the enolizable 0-dicarbonyl system present at Cn and Cia has a double bond in the C ring. Thus, dehydration leads to aromatization of the C ring, and this factor must provide some of the driving force for the reaction. 

Dehydration reactions are typically both endothermic and reversible. Reported kinetic characteristics for water release show various a—time relationships and rate control has been ascribed to either interface reactions or to diffusion processes. Where water elimination occurs at an interface, this may be characterized by (i) rapid, and perhaps complete, initial nucleation on some or all surfaces [212,213], followed by advance of the coherent interface thus generated, (ii) nucleation at specific surface sites [208], perhaps maintained during reaction [426], followed by growth or (iii) (exceptionally) water elimination at existing crystal surfaces without growth [62]. 

Dehydration reactions. In early studies of dehydration reactions (e.g. of CuS04 5 H20 [400]), the surfaces of large crystals of reactant were activated through the incorporation of product into surfaces by abrasion with dehydrated material. An advantage of this pretreatment was the elimination of the problems of kinetic analysis of the then little understood relationship between a and time during the acceleratory process. Such surface modification resulted in the effective initiation of reaction at all boundary surfaces and rate studies were exclusively directed towards measurement of the rate of interface advance into the bulk. 

A large batch exploded violently (without flame) during vacuum distillation at 90-100°C/20-25 mbar. Since the distilled product contained up to 12% butyroni-trile, it was assumed that the the oxime had undergone the Beckman rearrangement to butyramide and then dehydrated to the nitrile. The release of water into a system at 120°C would generate excessive steam pressure which the process vessel could not withstand. The rearrangement may have been catalysed by metallic impurities [1]. This hypothesis was confirmed in a detailed study, which identified lead oxide and rust as active catalysts for the rearrangement and dehydration reactions [2],... 

When prepared by thermal dehydration of 4-hydroxybenzenesulfonic acid, the reaction mixture begins to decompose exothermally around 240° C. Decomposition is delayed but still occurs at lower temperatures (160°C), and the presence of iron reduces the time to maximum rate of decomposition. Above 800 ppm of iron, the time to maximum rate is less than the dehydration reaction time, leading to severe control problems. Improved processing conditions were developed. 

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

Watari, F. Delavignette, P. Van Landuyt, J. Amelinckx, S. (1983) Flectron microscopic study of dehydration transformations. Part III. High resolution observation of the reaction process FeOOH — a-Fe203. J. Solid State Chem. 48 49-64... 

Dimethyl ether (DME) is produced as an intermediate compound in the conversion of methanol to gasoline as described in the earlier discussion of the Mobil M process. It can also be produced by various catalytic dehydration reactions. 

Initially, ethylene was obtained by the dehydration reaction of ethanol. Nowadays, ethylene is obtained by steam cracking from naphtha as a basic chemical. Steam cracking degrades longer aliphatic chains and introduces the double bond. Steam cracking is done at temperatures up to 900°C and leaves a wide variety of products behind. Ethylene is recovered by distillation processes. 

The reactions described and discussed here have been selected to extend the present analysis beyond surface processes and to include some consideration of certain chemical changes that also involve interactions within lattices of solids. The examples selected include reference both to surface catalytic properties and to dehydration reactions of clay minerals. 

Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293) so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294), kaolinite ( ) (293,295 298), montmorillonite (x) (294) and muscovite (O) (299). If these surface reactions are at least partially reversible,... 

All prebiotic polymerization reactions, which are dehydration reactions, are thermodynamically unfavorable. This free energy barrier can be overcome in two ways. The first is to drive the dehydration reaction by coupling it to the hydration of a high energy compound, and the second method is to remove the water by heating. In principle, visible or ultraviolet light could drive these reactions, but so far no one has demonstrated adequately such processes. 

Flaysom FIR, Phillips JM, Richards JT, Scholes G, Willson RL (1975) Pulse radiolysis of aqueous solutions of dihydropyrimidines the role of carbonium ions. In Adams GE, Fielden EM, Michael BD (eds) Fast processes in radiation chemistry and biology. Wiley, London, pp 241-246 Flazra DK, Steenken S (1983) Pattern of OFI radical addition to cytosine and 1-, 3-, 5- and 6-substi-tuted cytosines. Electron transfer and dehydration reactions of the OFI adducts. J Am Chem Soc 105 4380-4386... 

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