Thermodynamic and kinetic factors

Several kinetic and thermodynamic factors, as well as processing parameters, determine the microstmctures of the ceramics by using liquid-phase sintering. 

The capability of the liquid to wet and spread over the surface of the solid particle determines the effectiveness of liquid-phase sintering. The wetting capability of a liquid is dependent on its surface tension, i.e., lower the surface tension, the high the wetting capability the liquid has, which is usually characterized by a parameter, known as contact angle. 

The contact angle is determined by the various interfacial energies of the soUd-liquid-vapor systems, which is demonstrated using a droplet of liquid on the surface of a flat solid, as shown schematically in Fig. 5.29. When the specific energies 

The solid-solid interfacial tension yss is the same as the interfacial tension in the grain boundary ygb in the solid-state sintering. 

The penetration of the liquid into the grain boundary is described by the dihedral angle which is determined by the ratio yss/ysL- With yss/ysL 2, the dihedral angle has values of 0-180° and the liquid cannot penetrate the grain boundary, as shown in Fig. 5.31 [74]. In this case, solid-state processes are actually dominant. 

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The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

Both thermodynamic and kinetic factors are of importance for antioxidant capacity. The antioxidant has to be located in the right position at the right time in order to prevent oxidative damage to vital cell components and will need to be regenerated from the one-electron oxidised form in a recycling process ... 

It is important to have an understanding of the competing thermodynamic and kinetic factors that govern crystallization. Situations exist where one polymorph formation is kinetically controlled, while another is thermodynamically controlled. 

Despite the adjustments made to the CHETAH method, there is still the disadvantage of only taking into account themiodynamic Actors of instability. Stuih suggested a method, which attempts to combine the thermodynamic and kinetic factors. 

Both thermodynamic and kinetic factors affect the inhibition of hydrate deposits. 

All these studies demonstrated that water stability of MOFs can be improved by incorporating specific factors (e.g., metal-ligand strength, thermodynamic and kinetic factors, etc.) which govern the structural stability of the framework. 

Chemistry can be viewed as a balance between thermodynamic and kinetic factors which dictate the course of chemical reactions and the stability of compounds. Chemists seeking to achieve particular goals, manipulate these factors using chemical or physical means. The papers in this symposium on "High Energy Processes in Organometallic Chemistry" describe recent attempts to apply mainly physical means to get around the thermodynamic and kinetic constraints of conventional organometallic chemistry. 

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

Solvent molecules interact with reactants, reactive intermediates, and products as well as transition structures. The efficiency of intermolecular reactions is controlled by various thermodynamic and kinetic factors, particularly the concentration of a product-determining molecular complex and its reactivity. However, the requisite molecular association is often prevented by the strong association... 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

In general, polymorphism can be enthalpically and entropically driven and, in general, is susceptible to both thermodynamic and kinetic factors. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors. As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. 

A wide variety of cyclic monomers have been successfully polymerized by the ring-opening process [Frisch and Reegan, 1969 Ivin and Saegusa, 1984 Saegusa and Goethals, 1977]. This includes cyclic amines, sulfides, olefins, cyclotriphosphazenes, and IV-carboxy-oc-amino acid anhydrides, in addition to those classes of monomers mentioned above. The ease of polymerization of a cyclic monomer depends on both thermodynamic and kinetic factors as previously discussed in Sec. 2-5. 

Polymerization reactions can proceed by various mechanisms, as mentioned earlier, and can be catalyzed by initiators of different kinds. For chain growth (addition) polymerization of single compounds, initiation of chains may occur via radical, cationic, anionic, or so-called coordinative-acting initiators, but some monomers will not polymerize by more than one mechanism. Both thermodynamic and kinetic factors can be important, depending on the structure of the monomer and its electronic and steric situation. The initial step generates... 

Put another way, epimerization is the mechanistic event, racemization is the observation. True racemization, the actual production of a racemic mixture, is rarely seen in peptide synthesis. Instead, it is the extent of epimerization that defines the stereochemical outcome of a peptide-bond-forming reaction. In order to assess the probability of epimerization under a given set of conditions, one must be aware of the mechanisms of epimerization, as well as the thermodynamic and kinetic factors that affect this process. 

In the case of vinyl monomers (styrene, acrylonitrile, acrlylamide, isobutylene, etc.) copolymerization is generally spontaneous however, the reaction products are determined by the kinetic constants - a case of interplay between thermodynamic and kinetic factors. 

Discuss the differences between the thermodynamic and kinetic factors that determine the structure and stability of dispersions. Give examples of dispersions with stability that are... 

If both thermodynamic and kinetic factors operate simultaneously in favor of open-chain forms like 460 and 463, then 2//-pyran 459 may not be identifiable. This may be the reason why attempts to prepare unsubstituted 2//-pyran (4) from cis-2,4-pentadienal (466) have failed (for other attempts to prepare 4, see reference 386). Thus even if 466 had been generated by thermolysis of epoxycyclopentene 465, no traces of 4 were detected.28 Moreover, 466 seems to be thermodynamically more stable than cyclic form 4 according to ab initio MO calculations.56... 

Considerable insight into the future of selective reactions of elemental fluorine may be gleaned from consideration of the thermodynamics and kinetic factors of the low-temperature reactions of elemental fluorine. As is stated earlier, there is, under normal circumstances, no selectivity of the reactions of fluorine with carbon-hydrogen bonds and most other bonds at room tempera-... 

The experimental results do not permit us to establish the relative importance of the above thermodynamic and kinetic factors. Two arguments favor the hypothesis that thermodynamic factors AG° and AG°C play a more important role than kinetic factors AAG or AAG C. 

IfK2, kl9 k u and k2 had the same values as K 2, k u k l9 and k 2, then the optical purity of the product, RH, would be determined solely by the value of the diastereomeric equilibrium constant Kv If, however, the primed and unprimed constants were different, the final optical yield could be determined by both thermodynamic and kinetic factors, and in one extreme could result in the observation that the preferred enantiomer of the product originated in the minor dia-stereomer. Clearly, kinetic factors can be important since the steric interactions of the initial two diastereomers are different and these could affect the rate constants of the reaction. Moreover, the o--alkyl intermediate is chiral, as shown for one of the initial olefin diastereomers in Figure 4, and the rate of hydrogen addition and insertion... 

Preparative routes to homoleptic alkyls/aryls must take account of the sensitivities of the products, as well as the usual thermodynamic and kinetic factors. The following will serve as examples. 

In order to optimize the antitumoral properties of radicicol, particularly in vivo, the same group synthesized the analogous cycloproparadicicol, where the epoxide function is replaced by a cyclopropane [64]. Submitted to the conditions of the previous RCM reaction (CH2C12, 42°C, 19h), cydopropyl triene 94 leads to the expected macrolide 95 in only 16% yield, along with 30% of the corresponding 28-membered dimeric macrocycle (Scheme 2.37). After numerous assays, the best conditions tested (toluene, 110 °C, 10 min) brought the yield up to 55%. In this case, the balance between thermodynamic and kinetic factors seems decisive for the course of the reaction. The fact that the monomeric product is predominant at elevated temperature indicates that this form is entropically favored. 

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