Thermodynamics kinetics compared

Such a system could proceed to chiral takeover, for example, if the active alkylating agent is the tetrahedral zinc complex (Fig. 11.4), if the initial total concentration of the chiral zinc complexes is small compared to that of the aldehyde reactant, if the tetrahedral D,i-zinc complex is more stable thermodynamically than are the tetrahedral d,d- and L,r-zinc complexes, and if the reaction of the d,l- complex with the pyrimidyl aldehyde is kinetically sluggish compared to the reaction rate of the... 

The Hammond Postulate applies only if both forward reactions are fast. Obtain energies for the transition states leading to 1 -propyl and 2-propyl radicals (propane+F end andpropane+F center). Draw an energy diagram for each hydrogen abstraction reaction (place the diagrams on the same axes). Do these diagrams indicate that use of the Hammond Postulate is justified Calculate the barrier for each reaction, and calculate the relative concentrations of 1-propyl and 2-propyl radicals that would form at 298 K if each reaction were irreversible. Use equation (2). How does this (kinetic) ratio compare to the equilibrium (thermodynamic) ratio of these radicals ... 

Figure 1.2 gives the comparative graphical interpretations of an elemen tary chemical reaction in commonly accepted energetic coordinates and in the thermodynamic coordinates under the discussion. Note that the traditional energetic coordinates are always related to the fixed (typically, unit) reactant concentrations and, therefore, identify the behavior of standard values of the plotted parameters. As for the thermodynamic coordinates, they illustrate the process that proceeds under real conditions and are not restricted by the standard values of chemical potentials or thermodynamic rushes of the reac tants. The thermodynamic (canonical) form of kinetic equations is conve nient for a combined kinetic thermodynamic analysis of reversible chemical processes, especially for those that proceed in the stationary mode. 

In contrast, thioethers of the type R2S are generally poor donors to metals centers, the stability of complexes decreasing in the order > RS > R2S (Scheme 1). Macrocychc ligands tend to form complexes of increased thermodynamic stability and kinetic inertness compared to their open-chain analogs see Macrocyclic Ligands) ... 

Since larger free radicals are more stable than those with small molecules, the fragmentation in the middle of the polymeric chain is favored thermodynamically compared to the formation of small molecules. However, kinetic factors also may play a role in determining the abundance of a specific compound. The formation of small radicals from the end of a polymeric chain can be kinetically favored, and, as a result, formation of small radicals in the initiation step is more common than expected based on the thermodynamic criteria. Taking as an example polystyrene, an end chain p-scissions to the aromatic ring can be written as follows ... 

Finally, Lin (1988) advocates a more detailed elucidation of the differences between the kinetic, as compared with the thermodynamic, control of reactions. This, he suggests, is a way to avoid poor understanding about the nature of the products of a reaction in given conditions. 

Drug stability is an important consideration in pharmacenti-cal unit operations and prolonged storage. It is possible that one polymorph is unstable/metastable (a kinetic polymorph) compared to another form of the same dmg (the thermodynamic polymorph). For example, differences in the chemical and thermal/photochenfical stability of polymorphs are observed for drugs such as carbamazepine, furosemide, ... 

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

Provided that thermodynamic properties such as internal energy and Gibbs free energy of materials with permanent dipole moments undergo significant changes under the influence of microwave fields, shifts in the reaction equilibrium and in kinetics, as compared to thermal fields at the same temperature, can be foreseen [11]. At present, it seems that final conclusions about non-thermal effects can be arrived at only after much more carefully performed experiments have been carried out. However, it is clear that sub-THz radiations - and especially microwaves - can serve as a tool for rapid and efficient heating, with no further influence on most reactions [10]. 

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. 

The idea of kinetic versus thermodynamic control can be illustrated by discussing briefly the case of formation of enolate anions from unsymmetrical ketones. This is a very important matter for synthesis and will be discussed more fully in Chapter 1 of Part B. Most ketones, highly symmetric ones being the exception, can give rise to more than one enolate. Many studies have shown tiiat the ratio among the possible enolates that are formed depends on the reaction conditions. This can be illustrated for the case of 3-methyl-2-butanone. If the base chosen is a strong, sterically hindered one and the solvent is aptotic, the major enolate formed is 3. If a protic solvent is used or if a weaker base (one comparable in basicity to the ketone enolate) is used, the dominant enolate is 2. Enolate 3 is the kinetic enolate whereas 2 is the thermodynamically favored enolate. 

The product of nucleophilic attack can be anticipated by examining the lowest-unoccupied molecular orbital (LUMO) on protonated cyclopentene oxide. From which direction (top or bottom) would a nucleophile be more likely to approach each epoxide carbon in order to transfer electrons into this orbital Explain. Does one carbon contribute more to the LUMO, or is the orbital evenly spread out over both epoxide carbons Assuming that LUMO shape dictates product stereochemistry, predict which stereoisomers will be obtained, and their approximate relative amounts. Is the anticipated kinetic product also the thermodynamic product (Compare energies of 1,2-cyclopentanediol stereoisomers to tell.)... 

Examine atomic charges and display the electrostatic potential map for 2,7-octadione. Are you able to say which hydrogens (at Ci or at C3) are more likely to be abstracted by base, and conclude which is the kinetically-favored enolate Which enolate (2,7-octadione, Cl enolate or C3 enolate) is the lower in energy What do you conclude is the thermodynamically-favored enolate Is this also the enolate in which the negative charge is better delocalized Compare electrostatic potential maps to tell. 

Kinetic investigation of the reaction of cotarnine and a few aromatic aldehydes (iV-methylcotarnine, m-nitrobenzaldehyde) with hydrogen eyanide in anhydrous tetrahydrofuran showed such differences in the kinetic and thermodynamic parameters for cotarnine compared to those for the aldehydes, and also in the effect of catalysts, so that the possibility that cotarnine was reacting in the hypothetical amino-aldehyde form could be completely eliminated. Even if the amino-aldehyde form is present in concentrations under the limit of spectroscopic detection, then it still certainly plays no pfi,rt in the chemical reactions. This is also expected by Kabachnik s conclusions for the reactions of tautomeric systems where the equilibrium is very predominantly on one side. 

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). 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

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