Solid reactants

Mixtures of triglycerides, triglycerides plus free fatty adds or triglycerides plus fatty acid alkyl esters are used as reactants in fat modification processes. These mixtures are exposed to lipases supported on macroporous particles in the presence of a small amount of water. Liquid substrates (oils) can be reacted without use of a solvent, but with solid reactants (fats) it is necessary to add a solvent to ensure that the reactants and products are completely dissolved in the organic phase. Various water immisdble solvents can be used, but hexane is preferred for commercial operation because this solvent is already used industrially for the processing of oils and fats. 

An apparatus with four necks makes possible the introduction of solid reactants without removal of the condenser or dropping funnel. 

Melting. Loss of crystallinity of a solid reactant through melting, or eutectic formation, or dissolution in a product often results in an enhanced rate of decomposition as a consequence of the relaxation of the bonding forces responsible for lattice stabilization. The appearance of a... 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

X-Ray and electron diffraction measurements have been most usually used to characterize the phases present in any reactant mixture, and provide a means of identification of solid reactants, intermediates and products. In addition to such qualitative analyses, the method can also be used quantitatively, with suitable systems, to determine the amounts of particular solids present [111], changes in lattice parameters during reaction, topotactical relationships between reactants and products, the presence of finely divided or strained material, crystallographic transformations, etc. 

Doping of solid reactant involves the introduction of a controlled amount of an impurity into solid solution in the host lattice. Such impurities can be selected to cause the generation or destruction of those electronic or structural defects which participate in the rate process of interest. Thus, the influence of the additive on kinetic behaviour can provide evidence concerning the mechanism of reaction [46,47]. Even if the... 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

The predominant gaseous products of the decomposition [1108] of copper maleate at 443—613 K and copper fumarate at 443—653 K were C02 and ethylene. The very rapid temperature rise resulting from laser heating [1108] is thought to result in simultaneous decarboxylation to form acetylene via the intermediate —CH=CH—. Preliminary isothermal measurements [487] for both these solid reactants (and including also copper malonate) found the occurrence of an initial acceleratory process, ascribed to a nucleation and growth reaction. Thereafter, there was a discontinuous diminution in rate (a 0.4), ascribed to the deposition of carbon at the active surfaces of growing copper nuclei. Bassi and Kalsi [1282] report that the isothermal decomposition of copper(II) adipate at 483—503 K obeyed the Prout—Tompkins equation [eqn. (9)] with E = 191 kJ mole-1. Studies of the isothermal decompositions of the copper(II) salts of benzoic, salicylic and malonic acids are also cited in this article. 

In none of these examples is there any evidence that the catalytic behaviour results from direct chemical interaction between the solid reactant and the solid additive. 

The simplest solid—solid reactions are those involving two solid reactants and a single barrier product phase. The principles used in interpreting the results of kinetic studies on such systems, and which have been described above, can be modified for application to more complex systems. Many of these complex systems have been resolved into a series of interconnected binary reactions and some of the more fully characterized examples have already been mentioned. While certain of these rate processes are of considerable technological importance, e.g. to the cement industry [1], the difficulties of investigation are such that few quantitative kinetic studies have been attempted. Attention has more frequently been restricted to the qualitative identifications of intermediate and product phases, or, at best, empirical rate measurements for technological purposes. 

STRATEGY he n. ictinti is endothermic and, because a gas is produced from solid reactants, cxctirs with an increase in entropy, because A//° > 0 and A 5° 0, the formation... 

STRATEGY The reaction is endothermic and, because a gas is produced from solid reactants, occurs with an increase in entropy. Because AHc > 0 and AS° > 0, the formation of products from pure reactants becomes spontaneous at temperatures for which AH° TAS°. The temperature at which the tendency for the reaction to occur begins solves to T = AH7AS°. We use data from Appendix 2A (and remember that the standard enthalpies of formation of elements are zero). 

The description of the association of heterocychc chemistry and microwave irradiation has also shown that performing microwave-assisted reactions should be considered with special attention. A few of these considerations can be applied generally for conducting microwave-assisted reactions and include the following (a) the ratio between the quantity of the material and the support (e.g., graphite) or the solvent is very important (b) for solid starting materials, the use of solid supports can offer operational, economical and environmental benefits over conventional methods. However, association of liquid/solid reactants on solid supports may lead to uncontrolled reactions which may result in worse results than the comparative conventional thermal reactions. In these cases, simple fusion of the products or addition of an appropriate solvent may lead to more convenient mixtures or solutions for microwave-assisted reactions. 

Solids. Reactants which are solid at room temperature present more of a problem since they must be heated to their vaporization temperature, which in some cases may be relatively high. This is particularly true with metal halides as shown in Ch. 3, Table 3.3. It is often preferable to generate the reactant in situ. An example is the deposi-... 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

In heterogeneous solid-state reactions where the composition of both solid reactants does not change, the electrode s eqnilibrinm potential depends only on the nature of the two phases, not on their relative amonnts. Hence, dnring the reaction the potential does not change. It also remains constant when the cnrrent is interrupted after partial reduction or oxidation. 

State of subdivision. The smaller the pieces of a solid reactant—the smaller the state of subdivision—the faster the reaction. Wood shavings burn faster than solid wood, for example, because they have more surface area in contact with the oxygen with which they are combining (for a given mass of wood). In a sense, this is also a corollary of factor 4. 

MW irradiation conditions [80]. More recently, however, Varma and Kocevar s group have shown that a solvent-free and catalyst-free reaction of hydrazines with carbonyl compounds is possible upon MW irradiation (Scheme 6.24) [81]. Interestingly, the general reaction proceeds smoothly even for solid reactants and is completed below the melting points of the two reactants possibly via the formation of a eutectic. The reactions have been conducted in household MW oven and the control experiments are conducted concurrently in separate open beakers the reactions can be essentially followed by visual observation when a melt is obtained [82],... 

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