Solvent-free systems hydrogenation rate

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. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Several solvent polarity scales vere proposed to quantify the polar effects of solvents on physical properties and reactivity parameters in solution, such as rate of solvolyses, energy of electronic transitions, solvent induced shifts in IR, or NMR spectroscopy. Most of the polarity scales vere derived by an empirical approach based on the principles of the linear free energies relationships applied to a chosen reference property and system vhere hydrogen bonding effects are assumed negligible [Reichardt,1965, 1990 Kamlet, Abboud et al., 1981, 1983]. 

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