Thermodynamic and kinetic issues

Let us start with a general consideration of (6.6). It formally implies the following elementary reactions  

6 Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols.. 

All together, we can summarize the A// change as in (6.8) where the affected bonds are in bold  

As a result of the unfavorable thermodynamics, the equilibrium concentration of the formed carbonate turns out to be quite low, often around 1 % or even less, depending on the reaction temperature, which usually is in the range 400—450 K for a metal-catalyzed reaction. Keeping the temperature as low as possible is a key issue in this reaction. 

Adapted with permission from [21]. Copyright (2003) American Chemical Society 

---

Both thermodynamic and kinetic issues come into play, as does the involvement of geometrical isomerization when a dithiolate with inequivalent thiolate donors, such as mtp, participates in the reaction the ligand mtp is derived from mtpH2, 2-(mercaptomethyl)thiophenol. 

Electrostatic stability plays a dominant role in many separation processes, such as filtration of industrial wastewaters. Coagulation aids (known as coagulants) are routinely used to improve the effectiveness of separation processes in such cases. Polymer-induced stability is often the method of choice, particularly in the case of concentrated dispersions for example, many pharmaceutical preparations, paints, inks, and liquid toners depend on surfactants or polymer additives for ensuring stable preparations. We see in Section 13.2 that in the case of concentrated dispersions both thermodynamic and kinetic issues often become very important. 

The difficulties in synthesizing the mixed spinel seem related to both thermodynamic and kinetic issues. Hydroxides (and/or basic salts) of divalent elements are more stable than the ferrite at low temperatures. The large reactivity difference between iron and chromium explains the rapid crystallization of iron oxides or oxyhydroxides compared with the chromium compounds, as well as the segregation of both elements. 

The design of crystallization processes for the manufacture of Active Pharmaceutical Ingredients is a significant technical challenge to Process Research and Development groups throughout the Pharmaceutical and related industries. It requires an understanding of both the thermodynamic and kinetic aspects of crystallization, to ensure that the physical properties of the product will consistently meet specification. Failure to address these issues may lead to production problems associated with crystal size, shape and solubility, and to dissolution and bioavailability effects in the formulated product. 

Despite these important studies, most steps of the reaction mechanism are still only postulated. Therefore, we have decided to undertake a theoretical investigation of the Dotz reaction by discussing whether the reaction proceeds via a dissociative or an associative pathway in the initial steps of the process. We have also analyzed the central part of the reaction, the key issue being whether the reaction proceeds through a vinylketene intermediate (route A) or, instead, via a metallacycloheptadienone (route B). As will be seen, we came across a novel third pathway (route C) that turns out to be the best alternative from thermodynamic and kinetic points of view... 

In a series of papers, we have proposed the torsional mechanism of energy transduction and ATP synthesis, the only unified and detailed molecular mechanism of ATP synthesis to date [16-20,56] which addresses the issues of ion translocation in Fq [16, 20, 56], ionmotive torque generation in Fq [16, 20, 56], torque transmission from Fq to Fj [17,18], energy storage in the enzyme [17], conformational changes in Fj [18], and the catalytic cycle of ATP synthesis [18, 19]. We have also studied the thermodynamic and kinetic aspects of ATP synthesis [19,20,41,42,56]. A kinetic scheme has been developed and mathematically analyzed to obtain a kinetic model relating the rate of ATP synthesis to pHjn and pH m in the Fq portion and the adenine nucleotide concentrations in the Fj portion of ATP synthase. Analysis of these kinetic models reveals a wealth of mechanistic details such as the absence of cooperativity in the Fj portion of ATP synthase, order of substrate binding and product release events, and kinetic inequivalence of ApH and Aip. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

Many reviews deal with the main mechanistic aspects of the metathesis reaction [10]. There are three basic metathesis reactions (apart from polymerization reactions) the ring-closing metathesis (RCM), the cross-metathesis (CM) and the ringopening metathesis (ROM) [11]. Among the fundamental aspects that govern the reaction course, the thermodynamic versus kinetic issue is particularly important when considering the application of RCM to the construction of macrocydes. 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

A vast amount is now known, at the atomic level, about heterogeneous processes at gas-solid interfaces. Yet, three decades ago, research in solid surface science was mired in traditional thermodynamic and kinetic experiments that were not able to yield molecular level information. Fundamental issues, such as the molecular structure and chemical composition of the surface complex, could not be adequately addressed by the data generated from the classical methods. It was not until the development of highly surface-specific empirical tools that tangible gains in the study of the gas-solid interface were achieved. 

Nichols, G. (1999). Thermodynamic and kinetic control of conformational polymorphism a case study. 1st International symposium on aspects of polymorphism crystallization—chemical development issues, pp. 199-209, Scientific Update, Mayfield, East Sussex, United Kingdom. [lOOf, 144f]... 

We stress here that although DSC is in principle a relatively straightforward physical technique, its theoretical thermodynamical and kinetic basis is not trivial but should be well understood as it applies to equilibrium and nonequlibrium thermotropic lipid phase transitions of various types and to either heat conduction or power compensation instruments. Moreover, some care must be taken in sample preparation, selection of sample size, and sample equilibration before data acquisition in the choice of suitable scan rates, starting temperatures, and ending temperatures during data acquisition and in the analysis and interpretation of the DSC thermograms obtained. An adequate treatment of these issues is not possible in this brief... 

This chapter eoncentrates primarily on issues of heterogeneous catalysis. The thermodynamic and kinetic factors are outlined that are important in defining the surfaee chemistry of bimetallic surfaces. In addition, the various approaches will be introdueed that are utilised by surfaee scientists in an attempt to measure the composition of bimetallic surfaces under the influence of adsorbates. Furthermore, the ehapter will investigate the difficulties encountered when attempting to obtain accurate measurements on nanoseale bimetallie particles under environments typically encountered in a eatalytic reaction. By way of contrast, the relevance of much more aceurate measurements on well-defined surfaces under idealised ultrahigh vacuum (UHV) conditions will be questioned. 

The Bell-Evans-Polanyi relationship, Hammond s postulate, and the Marcus equation are all approaches to analyzing, understanding, and predicting relationships between the thermodynamics and kinetics of a series of closely related reactions. This is an important issue in organic chemistry, where series of reactions differing only in peripheral substituents are common. Each of these approaches provides a sound basis for the intuitive expectation that substituents that favor a reaction in a thermodynamic... 

Polymers other than canonical DNA and RNA oligonucleotides can also form G-quadruplexes. The ability to alter the polymer backbone may result in G-quadruplexes with a variety of potential applications in supramolecular chemistry, biotechnology, and nanotechnology. In addition, studies on nucleic acid analogs may lead to insights into the structural factors that control fundamental issues about the thermodynamics and kinetics of the G-quad-ruplex motif in the parent DNA and RNA nucleic acids. 

---