KINETICS AND INSTABILITY

Ketone peroxides, 198 Ketoximinosilanes, 199 Kinetics and instability, 199 Kjeldahl method, 199 Kondratiev cycles, 199 Krypton compounds, 200... 

A discussion with 14 refs on expls and proplnts considering the thermodynamic characteristics of expl substances, the kinetics of combustion of powders and the effects of catalysts, corrosion, and instability on the kinetics, the occurrence of deflagration on detonation, and forms of solid mixts in view of the augmentation of their performance and the extension of conditions used in their mixts. The importance of modern methods of calcn is stressed... 

Several tubidimetric methods (92, 93) have been described specifically for the kinetic determination of lipase in serum but these methods suffer from lack of linearity with increasing enzyme concentration, and instability of emulsions. 

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

A value of kjkp = 17 000 has been determined for partitioning of the acetophenone oxocarbenium ion [12+] in water.15,16 It is not possible to estimate an equilibrium constant for the addition of water to [12+], because of the instability of the hemiketal product of this reaction. However, kinetic and thermodynamic parameters have been determined for the reaction of [12+] with methanol to form protonated acetophenone dimethyl ketal [12]-OMeH+ and for loss of a proton to form a-methoxystyrene [13] in water (Scheme 10).15,16 Substitution of these rate and equilibrium constants into equation (3) gives values of AMeoH = 6.5 kcal mol-1 and Ap = 13.8 kcal mol-1 for the intrinsic... 

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

P. Gray and S. K. Scott Chemical oscillations and instabilities non-linear chemical kinetics... 

P. Gray and S. K. Scott. Chemical Oscillations and Instabilities Nonlinear Chemical Kinetics. Oxford Clarendon Press, 1990. See also the Review Lecture by P. Gray. Instabilities and oscillations in chemical reactions in closed and open systems. Proc. Roy. Soc. Lond. A 415, 1-34 (1988). 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

Tobe and coworkers have extended their work to the pyridine-based cyclophynes 17 and 18 in efforts to detect the incorporation of heteroatoms into the fullerene structure [39]. Similar to the behaviour of the hydrocarbon 16, heterocyclic 17 and 18 show the successive loss of indane units and hydrogen under the conditions of LD mass spectrometry (negative ion mode) culminating in the observation in both cases of the formation of the anion C5gN2. The relative low intensity of the diazafullerene anion observed can be attributed to the kinetic and thermodynamic instability of the heterocage formed. 

Gray P, Scott SK (1994) Chemical oscillations and instabilities. Non-linear chemical kinetics. Clarendon, Oxford... 

Reactive extrusion has emerged from a scientific curiosity to an industrial process. Various types of extruders can be used, all with their specific advantages and disadvantages. Further development suffers from lack of kinetic and rheological data at high conversions and from uncertainties about heat transfer and reactor stability. Nonlinear effects in the process can give rise to instabilities that are of thermal, hydrodynamical or chemical origin. 

Combustion instability that leads to performance deterioration and excessive mechanical loads, which could result in reduced life and premature failure, is an important issue with modern gas turbine engines and ramjet and scramjet combustors. Various techniques of passive and active control to reduce combustion instabilities and improve performance are addressed. Since extensive, promising research is being carried out to develop sensors and actuators, these techniques can be used in practical combustors in the near future. The topics covered in Section 3 provide the required chemical, kinetic, and fluid dynamic understanding to help the designer who is involved in active feedback control for combustion systems. 

Group IVA—Ti, Zr. The kinetic and thermodynamic instability of Ti(IIA), Zr(IIA) and Hf(II) complexes hampers the study of the M(IIA)-M(IVA) couple with respect to oxidative addition. In the (i/ -C5R5)2M(IIA) systems (M = Ti, Zr R = H, CH3), e.g., inter-, intramolecular, ultimately irreversible deactivation of the low-valent state occurs by metal insertion into ring or methyl C—H bonds. 

According to kinetics, the instability or stability of colloidal systems is determined by the balance of the forces of attraction and repulsion between the individual particles. The forces of attraction, causing the particles to stick together, are of the same nature as intermolecular forces and increase very rapidly as the particles approach each other. The forces of repulsion may be electrical, arising as a result of selective adsorption by the phase interface of one of the ions of an electrolyte present in the system. One of the factors keeping colloidal particles apart may be the formation on the interface of a solvate shell of molecules of the environment. 

Golikeri, S. V., and Luss, D. Diffusional effects in reacting mixtures. Chem. Eng. Sci. 26,237 (1971). Gray, P., and Scott, S. P., Chemical Oscillations and Instabilities. Nonlinear Chemical Kinetics. Oxford Science Publications, Oxford, 1990. 

---