Linear free energy relationships INDEX

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

If relative HBA basicities of electron donor solvents were correctly represented by the Koppel-Paju B scale on the one hand, and the present j8 scale on the other, it would logically follow that solvent B and 8 values must be proportional to one another. Unfortunately, however, no such direct correlation is observed. Indeed, the nonproportionality between the B and indexes is so marked and so readily evident as to indicate that one or the other of these measures of hydrogen bond basicity must be incorrect in concept, as must be, to a greater or lesser extent, the many linear free energy relationships based thereon in the recent chemical literature. 

Nieboer E, McBryde WAE (1970b) Free-energy relationships in coordination chemistry. 11. Requirements for linear relationships. Can J Chem 48 2565-2573 Nieboer E, McBryde WAE (1973) Free-energy relationships in coordination chemistry. 111. A comprehensive index to complex stabUity. Can J Chem 51 2512-2524 Nieboer E, MaxweU Rl, Rossetto FE, Stafford AR, Stetsko PI (1986) Concepts in nickel carcinogenesis. In Xavier AV (ed) Frontiers in bioinorganic Chemistry. VCH, Weinheim, pp S142-S151... 

---