Selectivity thermodynamic

High thermodynamic selectivity (7) demands that the initially formed cis olefin be displaced rapidly relative to its saturation or to its isomerization. As the reaction nears completion and the acetylene concentration diminishes, its effectiveness in displacing olefin will diminish and selectivity will fall. Displacement by acetylene is also impeded through depletion of acetylene in the vicinity of the catalyst owing to intra- or interpartile diffusion resistance (53a). A change in a reaction parameter thus can have different influences... 

Equation (1) shows another important advantage of CE. As already mentioned, the separation selectivity in chromatographic techniques very closely relates to the thermodynamic selectivity of recognition (ar), in that the thermodynamic selectivity of recognition, which is the ratio of the binding constants, is the upper limit of separation selectivity. In contrast, the separation selectivity (as) in CE may be expressed by the equation... 

The most important point to be noted is that as is not limited by the ratio K2/K1 and can reach infinitily high values in CE (3,4). Although Eq. (2) does not explicitly contain the binding constants of enantiomers with chiral selector, A/x and /xav are dependent on these thermodynamic quantities. Capillary electrophoresis offers several possibilities for almost unlimited enhancement of separation selectivity without any significant change of thermodynamic selectivity or recognition (3-5). 

In [LJ-control maps the substitution of one ligand by another one results in a change of the range of existence of the manifold intermediates. This change can be expressed by the ligand-property imluced shift of the titration curves identified by the relative position of their inflection points Lq s on the log (lL o/[Ni)Q) scale. These characteristic shifts provide information on the thermodynamic selectivity governed by the association processes only. This type of analysis is designated by . 

The possible factors involved in the biological selectivity towards metal ions have been considered by Frausto da Silva and Williams3 and by Kustin et al.4 In terms of thermodynamic selectivity a useful formalism for the uptake of any metal ion from a multimetal system is the quotient A Cm, where Km is a relative stability constant and Cm is the concentration of the metal ion. However, as these authors point out,3 a combination of both thermodynamic and kinetic properties must be considered. An appreciation of kinetic factors is often absent in this field, but must be of prime consideration in chelate exchange reactions and in the final irreversible step of metal ion insertion to form the metalloenzymes. 

The kinetic and thermodynamic selectivity factors are quantities which are functions of the chemistry of the system. When an active catalyst has been selected for a particular reaction (often by a judicious combination of theory and experiment) we ensure that the kinetic and thermodynamic factors are such that they favour the formation of desired product. Many commercial processes, however, employ porous catalysts since this is the best means of increasing the extent of surface at which the reaction occurs. Chemical engineers are therefore interested in the effect which the porous nature of the catalyst has on the selectivity of the chemical process. 

The high isomer selectivity observed for ZSM-5 can be further enhanced by imposing kinetic diffusion-al effects upon the thermodynamic selectivity, thereby magnifying the preference for p-xylene sorption (1 ). ... 

As we have seen already, there are two kinds of selectivity thermodynamic selectivity and kinetic selectivity (Chapter 2). Let us first consider how we could use thermodynamic selectivity for amperometric sensors. The electrode and the solution form a double-layer capacitor. The minimum energy of this capacitor occurs at... 

With the elimination of bias in the experimental setup, a range of ion-ophores must be re-characterized, as their reported selectivity coefficients can differ significantly from the unbiased ones [19]. It is very important to obtain true thermodynamic selectivity coefficients as already discussed above. From the three proposed methods that were interchangeably used in the past, a modified SSM has emerged as the most convenient one. Here, the calibration curves for all ions are determined in separate titrations. The modification from the traditional method involves removing the experimental bias by one of the following proposed methods ... 

Similarly, a difference between the kinetic and thermodynamic selectivities was observed by Taguchi and coworkers while generating zirconaaziridines by... 

Because of the different degree of steric congestion in the interstices between the NO, Cp, and PPhs ligands (NO/PPhs most congested > Cp/PPhs > NO/Cp least congested), the chiral (Re)+ fragment is able to discriminate between both faces of prochiral alkenes and aldehydes with high kinetic and thermodynamic selectivities. For example, the thermodynamic... 

Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

---