Thermodynamic control definition

In the following I will present a model to tackle this question. We are at the level of speculations, but we will use some known facts. For example, let s start from the condensation of NCA-anhydrides, a reaction which, as we have seen in the previous chapter, is considered prebiotic. In this way, oligopeptides up to (say) a length of ten can be built, possibly under thermodynamic control, but starting from a definite set of conditions (amino-acid composition in the starting mixture, pH, salinity, etc.). We can assume, and this is actually quite reasonable, that in this way copious libraries of different decapeptides have been formed, each in a significant concentration. 

When H2O deacetylates the acyl-enzyme, phenylacetic acid is formed. When nucleophiles other than H2O deacylate the acyl-enzyme, a new condensation product, in this case phenylacetyl-O-R or phenylacetyl-NH-R is formed. By definition the hydrolysis of these condensation products can be catalyzed by the same enzyme that catalyzes their formation in equation 10.1. Thus, when the acyl-enzyme is formed from phenylacetyl-glycine or phenylacetyl-O-Me, this gives rise to an alternative process to produce Penicillin G, in addition to the thermodynamically controlled (= equilibrium controlled) condensation of phenylacetic acid and 6-aminopenicillanic acid (6-APA). This reaction that involves an activated side chain is a kinetically controlled (= rate controlled) process where the hydrolase acts as a transferase (Kasche, 1986 1989). 

The condition 1) states that unequal amounts of stereoisomers are produced or destroyed by the reaction together conditions 2) and 3) assure that the increase in chiral genus is due to a stereoselectivity which is caused by chiral influences alone. Condition 1) is an essential part of the definition it is interrelated with condition 3), and together they restrict the type of chirality increasing stereoselective reactions that will be called an asymmetric synthesis34. In fact, a stereoselective reaction is never infinitely selective , because this would require an infinite free enthalpy difference of stereoisomers, stereoisomeric transition states respectively (with thermodynamic control, or with productive selectivity) in the case of an idealized destructive selectivity infinite selectivity would be reached at infinite reaction time, when none of the considered stereoisomers would be left over3S ... 

Here we are primarily concerned with the fact that this ortho -adduct may occur in the form of two diastereomers. The diastereomers are formed as a 57 43 cis/trans-mixtme in the absence of A1C13, but a 95 5 cis/trans-mixture is obtained in the presence of A1C13. In the latter case, thus, one is dealing with a Diels-Alder reaction that exhibits a substantial simple diastereoselectivity (see Section 11.1.3 for a definition of the term). Here, the simple diastereoselectivity is due to kinetic rather than thermodynamic control, since the preferentially formed ds-disubstituted cyclohexene is less stable than its irans-isomer. 

The different courses of the reactions of o-phenylenediamines and 1,8-diami-nonaphthalenes with quinones cannot be assigned to different terms for thermodynamic control. The spirocyclic isomers are most probably energy-preferable forms for both Type 10 and 11 compounds. The cyclization step is definitely kinetically... 

The high-temperature addition of hydrogen bromide to 2-bromo-2-butene has been classified as being nonstereospecific, (ref. 11, pp. 436-437) because both cis and trans reactants give identical product mixtures - 75%-dl and 25%-meso. This is not necessarily so. By our new definition for stereospecificity, given above, this reaction is not nonstereospecific - a nonstereospecific reaction would have given a 50 50 dhmeso mixture. The above kinetically-controlled HBr addition involves latent thermodynamic equilibration - hence, the non-50 50 75%-dl and 25%-meso) mixture. Ideally, the stereospecificity should be determined for transformations under kinetic control, and not, under thermodynamic control. 

The membrane process can be kinetically or thermodynamically controlled. Thermodynamic control exists if there is a limit set by the distribution coefficient or a reaction equilibrium constant which is approached asymptotically at a slow rate. The process will be kinetically controlled if it is far away from the thermodynamic limit or if this limit is removed. The removal of amaonia from an aqueous effluent by the liquid membrane process will be used as an example to illustrate the difference between the two definitions. On account of solubility of ammonia in the membrane phase the same distribution coefficient will apply at the two surfaces of the membrane phase. The internal phase thus cannot take up more ammonia than given by the distribution coefficient at the internal surface. In turn, the maximum amount of ammonia... 

A definition that may be proposed (not as broad, nor as restrictive as the two discussed above) is based on the distinction between self-assembling and engineered SPs. The former type of assemblies is the main focus of the present chapter and is characterized by thermodynamically controlled structures. By contrast engineered SPs, important for a variety of applications, are materials assembled by controlled deposition or synthesis. A more detailed definition of aU systems that have been described as SPs (including self-assembling and engineered ones) is presented in Section I.B [3]. 

Protonation of the a-carbanion (50), which is formed both in the reduction of enones and ketol acetates, probably first affords the neutral enol and is followed by its ketonization. Zimmerman has discussed the stereochemistry of the ketonization of enols and has shown that in eertain cases steric factors may lead to kinetically controlled formation of the thermodynamically less stable ketone isomer. Steroidal unsaturated ketones and ketol acetates that could form epimeric products at the a-carbon atom appear to yield the thermodynamically stable isomers. In most of the cases reported, however, equilibration might have occurred during isolation of the products so that definitive conclusions are not possible. 

This last inflammability parameter presents problems. After stating its definition it will be seen that measuring autoignition temperature proves to be a difficult exercise because its measurement is sensitive to the experimental conditions, even more sensitive than for flashpoints. Worse, this parameter seems to be controlled by kinetic factors far more complex to master than the thermodynamic factors that probably control flashpoints (in fact it is a liquid/vapour equilibrium). So whilst the influence of the nature of the cup metal on a flashpoint has never been demonstrated, this demonstration was easily made with autoignition temperatures. 

The overall effect of the preceding chemical reaction on the voltammetric response of a reversible electrode reaction is determined by the thermodynamic parameter K and the dimensionless kinetic parameter . The equilibrium constant K controls mainly the amonnt of the electroactive reactant R produced prior to the voltammetric experiment. K also controls the prodnction of R during the experiment when the preceding chemical reaction is sufficiently fast to permit the chemical equilibrium to be achieved on a time scale of the potential pulses. The dimensionless kinetic parameter is a measure for the production of R in the course of the voltammetric experiment. The dimensionless chemical kinetic parameter can be also understood as a quantitative measure for the rate of reestablishing the chemical equilibrium (2.29) that is misbalanced by proceeding of the electrode reaction. From the definition of follows that the kinetic affect of the preceding chemical reaction depends on the rate of the chemical reaction and duration of the potential pulses. 

POMs can be viewed as assemblies of discrete fragments of oxides with definite sizes and shapes. These assemblies represent usually thermodynamically stable arrangements, even though their formation, especially in the case of polytungstates, is rather under kinetic control [3]. Most of them keep their identity in aqueous and nonaqueous solutions. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

To be able to control the thermodynamic activity of each compound, the inlet gas must be characterized by its molar composition, working temperature, and the total pressure of the system. For inert gas having a low content of organics, assuming that it can be considered as an ideal gas, these few parameters allow a complete definition of the thermodynamic parameters of the gas entering the reactor. 

Why does heat flow from a warm body into a cold one Why doesn t it ever flow in the reverse direction We can see that differences in temperature control the direction of flow of heat, but this observation raises still another question What is temperature Reflection on these questions, and on the interconversion of heat and work, led to the discovery of the second law of thermodynamics and to the definition of a new thermodynamic function, the entropy S. 

Cyclodehydration. This reagent is clearly the most satisfactory Lewis acid for cyclization of 1 to the tricyclic 2, a model for tricyclic diterpenes. The by-product 3 is also converted to 2 by further treatment with the same reagent. Stannic chloride is almost as effective, but PPA is definitely inferior. The trans product, 2, is the product of kinetic control but is also more stable thermodynamically than cis-2. 

There are many types of corrosion, as would be expected from its general definition. It has been traditional (4) to divide the study of corrosion into two areas the study of low temperature corrosion by aqueous or other solutions, controlled by electrochemical processes (wet) and the study of gaseous corrosion at high temperatures, controlled by thermodynamics and diffusion processes (dry). In addition to the obvious differences, the two areas have many phenomena in common. 

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