Kinetic control definition

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. 

Accordingly, isotopic equilibration for Cr and Se species is expected to be much slower than for the aqueous Fe(III)-Fe(II) couple, which reaches equilibrium within minutes in laboratory experiments (Beard and Johnson 2004). Additionally, Cr(III) and Se(0) are highly insoluble and their residence times in solution are small, which further decreases the likelihood of isotopic equilibration. In the synthesis below, isotopic fractionations are assumed to be kinetically controlled unless otherwise stated. However, definitive assessments of this assumption have not been done, and future studies may find that equilibrium fractionation is attained for some reactions or rmder certain conditions. 

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. 

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). 

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. 

As noted above, the first definition of "aromaticity" was in terms of substitution rather than addition. This is certainly true for many benzene derivatives. However, it must be used with some care since thiophene is by most criteria about as "aromatic" as benzene, but when treated with chlorine or bromine it gives an addition product. The latter is, however, the kinetically controlled product, for when heated or treated with base it loses hydrogen halide and gives the 2-halothiophene.20 Compounds such as anthracene and phenanthrene, which are recognized as having considerable resonance stabilization, also undergo addition reactions. 

In the description of crystals and crystal structures the two terms/om and habit have very specific and very different meanings. Form refers to the internal crystal structure and etymologically is the descendant of the Greek morph. Hence, polymorph refers to a number of different crystal modifications or different crystal structures, and the naming of different structures as Form F or a Form follows directly from this definition and usage. As we have seen above, the difference in crystal structure is very much, although not exclusively, a function of thermodynamics. Certainly, only the structures which are thermodynamically accessible can ever exist, but there often is a question of thermodynamic vs kinetic control over which particular structure may be obtained under any particular set of crystal growth conditions. 

Here Xa, Ya are strictly equilibrium mole fractions for component A in the adsorbed phase and adsorbate (fluid) phase, respectively as are Xb, Fb for component B. For equilibrium-based adsorptive separation process, the adsorbent selectivity is the same as the separation factor as defined in Eq. (1). Apparently, this definition is not applicable to other processes based on kinetic and steric effects. In a kinetically controlled adsorption process, the adsorbent selectivity depends on both equilibrium and kinetic effects. A simplified definition for adsorbent separation factor is given by Ruthven et al. ... 

Serine and cysteine peptidases are not perfect acyltransferases. Therefore, it is useful to have a method for the prediction of the outcome of the kinetically controlled peptide synthesis. In order to get a simple efficiency parameter we decided to introduce the partition value p11061 analogous with the definition of the Michaelis constant according to Eq. (3), where P2 = Ac-OH, P3 = Ac-N, and N = HN. 

In the light of these examples, we hereby provide a different definition of stereospecificity and nonstereospecificity - one that is applicable only to kinetically-controlled transformations. 

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. 

In the compound 579 stereospecificity has been observed in product formation. The compound 581, and not 582, is a kinetically controllable product in this rearrangement. It is also shown clearly that there is a definite upper limit for the... 

We should emphasize here that Markov-grown polarity is not the result of a kinetically controlled growth process (i.e., fast grow), although we assume kinetic stability for the grown-in state of polarity. This means that guest molecules do not reverse their dipolar direction if definitively included in a channel. 

In synthesizing polymers in vivo and in vitro, molecular homogenous ( monodisperse ) polymers (i.e., those in which every macromolecule has the same molar mass or molecular weight ) occur only under quite specific conditions. The overwhelming majority of polymer syntheses proceed more or less randomly, and the resulting macromolecular substances have more or less broad molar mass distributions. The kind of molar mass distribution obtained depends on the nature of the polymerization, which may be either thermodynamically or kinetically controlled. Each kind of distribution is characterized by a definite relationship between the mole fraction x and the degree of polymerization X. Consequently, it is possible in many cases to deduce the kind of polymerization involved from the type of distribution function obtained. 

The above rationalization is confirmed experimentally by the addition of methanol in mild acid to enol ether 71 (Fig. 3.20). Under these kinetically controlled conditions, a mixture of 74a and 75 was obtained in 76 24. This ratio corresponds to the energy difference of 0.70 kcal/mol for the transition state favoring the formation of 72a which is in complete agreement with the AMI [46] and 6.31G [47] calculation. This value is in agreement with that found by van Eikeren [45]. In addition, these calculations also show that the transition states are definitely late transition states and resemble the geometry of the oxocarbenium ion. 

The potentials for the reduction of water ([H+] = 10 moldm ) to H2, and for the reduction of O2 to H2O (the reverse of the oxidation of H2O to O2) are of particular significance in aqueous solution chemistry. They provide general guidance (subject to the limitations of thermodynamic versus kinetic control) concerning the nature of chemical species that can exist under aqueous conditions. For reduction process 8.25, " = 0 V (by definition). 

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... 

Reaction of most enolates with MeOTf or Me0S02F is always likely to be kinetically controlled. There does not appear to have been a definitive study, but 0-alkylation is the normal outcome. O-Alkylation of a bicyclodecatrienone by Me0S02p is enhanced by the use of polar solvents like HMPA. O-Alkylation of enolates of appropriate cyclohexadienones by MeOTf has been used to generate various 3ai/-indenes. A ketene acetal is formed by exclusive 0-alkylation of the sodium enolate of isopropyl bis(pentachlorophenyl)acetate by MeOTf. ... 

The experimental data mentioned in this section and in more detail in other reviews, substantiates the general theoretical framework for protein folding and function given by the single funnel hypothesis. However, in spite of all the studies, theoretical and experimental, that seem to support it, there is not, as yet, a definitive proof for the single funnel hypothesis and in the next sections an alternative theoretical framework, based on a kinetic control of folding, will be put forward. 

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