Thermodynamics compared with kinetics

Thermodynamic Compared with Kinetic Control in S l Reactions of AUylic Derivatives (Section 14-6)... 

The film compositional signature in E, q, and A-space allows visual diagnosis of thermodynamic compared with kinetic control and the identification of various possible phenomena these include film reconfiguration, ion and solvent trapping, relative rates of ion and solvent transfer, and relative rates of solvent entry and exit. 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

When a 1 1 mixture of NO and NO2 (i.e., NO2/NOx=0,5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6] Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction... 

Computer codes usually calculate only the thermodynamically most stable configuration of a system. Modifications can simulate nonequilibrium, but there are limitations on the extent to which codes can be manipulated to simulate processes that are kinetically (rate) controlled the slow reaction rates in the deep-well environment compared with groundwater movement (i.e., failure to attain local homogeneous or heterogeneous reversibility within a meter or so of the injection site) create particular problems. 

Figure 8. Kinetic and thermodynamic advantages of PECVD compared with c...

As the data in Fig. 5.12 show, and as was pointed out by Bruno et at. (1991), the half time for the dissolution reaction of U02 in the pH-range of most natural waters and under reducing conditions is in the order of days. If we compare this with typical residence times of undisturbed ground waters (years) we can conclude that the dissolution of U02(s) and the mobility of uranium under these conditions is thermodynamically and not kinetically controlled. 

So far only a few quantitative data on the thermodynamic stability of arenediazonium salts and crown ethers have been reported. Bartsch et al. (1976) calculated the value of the association constant of the complex of 18-crown-6 and 4-t-butylbenzenediazonium tetrafluoroborate from kinetic data on the thermal decomposition of the complex, Kt = 1.56 x 105 1 mol-1 in 1,2-dichloroethane at 50°C. Compared with the corresponding linear polyether this is at least a factor of 30 higher (Bartsch and Juri, 1979). 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

The thermodynamics and shock-tube kinetics of pyrolysis of azetidine, in argon at high dilution, have been compared with those for trimethylene oxide, sulfide and imine. ... 

Is the stability of 8Ad due to unfavorable kinetics, i.e., the bulky adamantyl groups blocking reaction, or to unfavorable thermochemistry, i.e., loss of aromaticity of the imidazole ring as a result of reaction, or both The distinction is potentially important as understanding could assist in designing stable carbenes. To decide, compare the kinetics and thermodynamics of the insertion of 8Ad into the central CH bond in propane with reactions of 8Me, which should also be aromatic but lacks shielding groups , and 9, which is neither aromatic nor crowded. 

In Figure 10.1 the time course of thermodynamically and kinetically controlled processes catalysed by biocatalysts are compared. The product yield at the maximum or end point is influenced by pH, temperature, ionic strength, and the solubility of the product. In the kinetically controlled process (but not in the thermodynamically controlled process) the maximum yield also depends on the properties of the enzyme (see next sections). In both processes the enzyme properties determine the time required to reach the desired end point. The conditions under which maximum product yields are obtained do not generally coincide with the conditions where the enzyme has its optimal kinetic properties or stability. The primary objective is to obtain maximum yields. For this aim it is not sufficient to know the kinetic properties of the enzyme as functions of various parameters. It is also necessary to know how the thermodynamically or the kinetically controlled maximum is influenced by pH, temperature and ionic strength, and how this may be influenced by the immobilization of the biocatalysts on different supports. 

In the following sections of this article we first define the terms necessary to identify a chemical system. After this, the use of an algebraic technique is developed for the expression of general reaction mechanisms and is compared with the previous treatments just mentioned. Next, a combinatorial method is used to determine all physically acceptable reaction mechanisms. This theoretical treatment is followed by a series of examples of increasing complexity. These examples have been chosen to illustrate the technique and for comparison with previous studies. They do not constitute a survey of all the most significant studies concerned with the mechanisms illustrated. Finally, a discussion is presented of the relationship of the present treatment to studies concerned with thermodynamics, and of the relationship between kinetics and mechanisms. 

A well understood case is that of quinoline reaction at position 2 is kinetically favored as compared with reaction at position 4, but the adduct from the latter is thermodynamically more stable. This situation, where the site of attack leading to the more stable adduct is the y position, is analogous with those regarding the formation of Meisenheimer adducts from benzene and pyridine derivatives and RCT nucleophiles. Presumably, with quinoline kinetic control favors the position that is more strongly influenced by the inductive effect of the heteroatom. The fact that position 2 of quinoline is the most reactive toward nucleophilic reagents is probably related to the lower 71-electron density at that position.123 However, the predominance of the C-4 adduct at equilibrium can be better justified by the atom localization energies for nucleophilic attachment at the different positions of quinoline. Moreover, both 7t-electron densities and atom localization energies indicate position 1 of isoquinoline to be the most favored one for nucleophilic addition. 

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