Solvent-free systems adsorption coefficients

In a competitive hydrogenation of two substrates in a solvent-free system the relative reactivity is again given by Eq. (8). On the other hand, however, the properties of the bulk phase vary with each change in the substrate, and this change may of course variously affect the rate constants and the adsorption coefficients. Moreover, the rate constant of A cannot be directly measured in the presence of an unsaturated compound B. As a consequence, substitution of /cah and knu into Eq. (8) instead of and k, yields the values Ka/Kb subjected to the same inaccuracy. In this case, therefore, the re-... 

Cerveny et al. (100 report an investigation of the hydrogenation of 12 olefinic substrates in the liquid state with 5% Pt on silica gel as catalyst under usual conditions and without solvents. The reaction rates related to 2,3-dimethyl-2-butene and the relative adsorption coefficients obtained in systems of various pairs of substrates and by recalculation using Eq. (25) to 2,3-dimethyl-2-butene are given in Table IV. Comparison between the measured reaction rates and the rates of hydrogenation in solvents (71 has revealed that relations existing between the rates of hydrogenation in a solvent-free system approximately correspond to those determined in... 

Relative Hydrogenation Rates and Adsorption Coefficients of Substrates on Pt in Solvent-Free Systems... 

The measured data also were used (700) in a quantitative representation of the effect of structure on the reactivity and adsorptivity of substrates by means of the Taft-Pavelich equation (22). The adsorption data suffered from a larger scatter than the rate data. No substrate or substituent could be detected that would fail to satisfy completely the correlation equations. In the correlation of the initial reaction rates and relative adsorption coefficients the parameter p was negative, while the parameter S was positive. In correlations of the reaction rates obtained by the hydrogenation of a similar series of substrates on the same catalyst in a number of solvents, the parameters p and had the same sign as in the hydrogenation in solvent-free systems, while in the correlation of the adsorption coefficients the signs of the parameters p and in systems with solvents were opposite to those in solvent-free systems. This clearly indicates that solvents considerably affect the influence of the structure of substrates on their reactivity. 

Because of a support-free partition system, HSCCC provides an important advantage over other chromatographic methods by eliminating various complications, such as adsorptive loss and deactivation of samples, as well as contamination from the solid support. As shown by our examples, HSCCC can isolate various components from a complex mixture of natural products by carefully selecting the two-phase solvent system to optimize the partition coefficient (K) of the target component(s). The HSCCC system can also be applied to microanalytical-scale separations without excessive dilution of samples. We believe that HSCCC is an ideal method for the separation and purification of natural products. 

In some adsorption systems, as an additional method, pulsed field gradient (PFG) diffusion measurements were applied to dilute colloidal dispersions, these methods are described in detail elsewhere [18, 19]. Briefly, by this method the self-diffusion coefficients for all spectrally resolvable liquid resonances can be obtained. Usually the method is applied to protons, and can in coated colloid dispersions monitor mobile species such as the solvent or free surfactants. 

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