Thermodynamically allowed reactions

Redox reactions are particularly instructive. If all thermodynamically allowed reactions in liquid NH3 were kinetically rapid, then no oxidizing agent more powerful than N2 and no reducing agent more powerful than H2 could exist in this solvent. Using data for solutions at 25° ... 

The free electrons in the plasma are reactant or catalyst in chemical reactions. Plasmas shift equilibria or they help thermodynamically allowed reactions that have rates that are too low at the low gas temperature (Table 6.3). Table 8.7 lists selected compounds that have been made with PE-CVD at low temperatures with their precursors and the conditions for their synthesis. The amorphous products of PE-CVD are rarely stoichiometric and almost never pure. 

PROX reaction is characterized by CO oxidation and quite a number of thermodynamically allowed reactions summarized in Eq. (6.94) ... 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

ATP Allows the Coupling of Thermodynamically Unfavorable Reactions to Favorable Ones... 

Thus far we have explored the field of classical thermodynamics. As mentioned previously, this field describes large systems consisting of billions of molecules. The understanding that we gain from thermodynamics allows us to predict whether or not a reaction will occur, the amount of heat that will be generated, the equilibrium position of the reaction, and ways to drive a reaction to produce higher yields. This otherwise powerful tool does not allow us to accurately describe events at a molecular scale. It is at the molecular scale that we can explore mechanisms and reaction rates. Events at the molecular scale are defined by what occurs at the atomic and subatomic scale. What we need is a way to connect these different scales into a cohesive picture so that we can describe everything about a system. The field that connects the atomic and molecular descriptions of matter with thermodynamics is known as statistical thermodynamics. 

A listing of thermodynamic properties determined by a full range of methods enables the ArG° values to be determined and hence the allowed reactions and equilibrium constants for all reactions. A tabulation of some thermodynamic quantities is found in Appendix C. 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Biochemical transformations of organic compounds are especially important because many reactions, although thermodynamically feasible, occur extremely slowly due to kinetic limitations. For example, we might be interested in the question of whether benzene can be biodegraded under naturally occurring methanogenic conditions (see Illustrative Example 17.1). Such natural attenuation of this toxic aromatic substance may be thermodynamically allowed under the perceived conditions. But these conditions may not be accurate (e.g., the benzene and methane chemical activities in the system). Also other environmental factors may cause the rate to be unobservably slow. One possibility is that the relevant microorganisms are simply not active in the environment of interest. 

Because energy underlies all chemical change, thermodynamics—the study of the transformations of energy—is central to chemistry. Thermodynamics explains why reactions occur at all. It also lets us predict the heat released or required by chemical reactions. Heat output is an essential part of assessing the usefulness of compounds as fuels and foods, and the first law of thermodynamics allows us to discuss these topics systematically. The material in this chapter provides the foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions—why they occur and in which direction they can be expected to go. 

The synthesis of quadridentate imine chelates usually requires the combination of two equivalents of the carbonyl compound and a diamine, as in the formation of complexes of ligands (6),17 18 (7)19 and (8).20,21 In similar fashion, thermodynamic template reactions allow the very effective synthesis of quinquedentate and sexadentate metal complexes of ligands such as (9),22 (10),23 (ll),24 (12)25 and (13).26 Condensation of 1,1,1-tris(aminomethyl)ethane with pyridine-2-carbaldehyde alone yields... 

Thermodynamic template reactions allow the formation of hydrazones from hydrazines and carbonyl compounds, in the same manner as simple imines are formed. However, in many cases it is more convenient to form hydrazones in non-template reactions, as they are generally easier carbonyl derivatives to isolate than are simple imines. Hydrazone complexes often are capable of further manipulation and both aspects of synthesis and reactivity will be combined in this section. 

The C60H36 hydride contains 4.76 mass% of hydrogen. It can be used as hydrogen accumulator because in the temperature of 400-600 K the hydrogenation-dehydrogenation cycles are thermodynamically allowable. The thermodynamic parameters and equilibrium hydrogen pressures for the reaction... 

The results of nucleophilic substitutions in solvated ionic aromatic cations (RX+) are strongly dependent on the nature of the halogen and on the solvent cluster size. The energetics of these systems have been studied and it has been shown that the reactions are thermodynamically allowed for each solvent, even for the 1-1 complexes. The observed different behavior related to cluster size is probably governed by kinetic reasons (existence of barriers to the reactions). Two types of substitution reaction have been identified—leading either to X radical or to HX molecule formation. 

Note that the hydrolysis of two high-energy phosphate bonds in ATP provides the energy source for the reaction. The inorganic pyrophosphate, PPi, is subsequently broken down to two phosphate ions by inorganic pyrophosphatase. The action of this enzyme means that very little PPi remains in the cell, making the synthesis of the fatty acyl-CoA favored. This is an example of metabolic coupling, the process whereby a thermodynamically unfavored reaction is allowed because it shares an intermediate (in this case PPO with a favored one. 

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