Reactivity linear free energy relationship method

This study is a comprehensive review of data reported on the effect of the composition of the reaction mixture on the hydrogenation of olefinic reactants in the liquid phase. It is mainly based on papers published by the authors, which deal with the effect of the structure of the reacting compounds on their reactivity and adsorptivity on hydrogenation catalysts, and with the effect of solvents on hydrogenation in the liquid phase. The majority of these studies were carried out with a view to quantify the particular effects, with the utilization of the LFER (linear free energy relationship) method. On the one hand, new possibilities for the application of these relationships appeared, but on the other, a number of limiting factors were found, connected predominantly with the considerably complex character of the systems involved in catalytic hydrogenation in the liquid phase. 

Another method for studying solvent effects is the extrathermodynamic approach that we described in Chapter 7 for the study of structure-reactivity relationships. For example, we might seek a correlation between og(,kA/l ) for a reaction A carried out in a series of solvents and log(/ R/A R) for a reference or model reaction carried out in the same series of solvents. A linear plot of og(k/iJk ) against log(/ R/ linear free energy relationship (LFER). Such plots have in fact been made. As with structure-reactivity relationships, these solvent-reactivity relationships can be useful to us, but they have limitations. 

There has been a decisive evolution in the treatment of steric effects in heteroaromatic chemistry. The quantitative estimation of the role of steric strain in reactivity was first made mostly with the help of linear free energy relationships. This method remains easy and helpful, but the basic observation is that the description of a substituent by only one parameter, whatever its empirical or geometrical origin, will describe the total bulk of the substituent and not its conformationally dependent shape. A better knowledge of static and dynamic stereochemistry has helped greatly in understanding not only intramolecular but also intermolecular steric effects associated with rates and equilibria. Quantum and molecular mechanics calculations will certainly be used in the future to a greater extent. 

There have been many attempts to develop linear free energy relationships for nucleophilicity so that rate constants of substitution reactions could be predicted quantitatively and so that the effects on reactivity of changing reaction conditions could be ascribed to particular aspects of the intermediates involved. This has not been easy to accomplish, however, because at least seventeen different factors have been suggested as contributors to nucleophilic reactivity. ° ° The following discussion will highlight some of the more prominent methods that have been proposed. Further details are available in reviews. ... 

However equally important is the availability of physically grounded models that can provide understanding of chemical reactivity. In the past, theories of reactivity have been based on empirical structure-reactivity relationships (viz. linear free energy relationships) or qualitative theoretical concepts (viz. Woodward-Hoffmann approach or the frontier orbital method)[4-ll]. However, there is a different approach, which is potentially more fruitful. In this approach one uses physically grounded models that can be obtained from the best state of the art methodology of quantum chemistry. These physically grounded models must be both quantitative and qualitative. On the one hand, any model used should reproduce the numerically computed quantities exactly, on the... 

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