Drago’s parameter

Jensen [9] indicated that there is no evidence that Drago s parameters reflect the relative electrostatic and covalent contributions to the bonding in resulting adducts. They were not correlated with either a physical property (dipole moment, ionization potential) or with a quantum-mechanically calculated index. Drago s approach is a purely empirical method of calculating enthalpy of formation for molecular adducts. Fowkes applied the Lewis E C equation [19] and has attempted to determine E and C parameters for both polymers and surfaces. However, Jensen [9] indicated the potential problem connected... 

Fowkes and co-workers have used test acids (e.g., phenol and chloroform) and bases (e.g., pyridine and ethyl acetate) to determine Drago s parameters for polymers and metal oxides using essentially calorimetric heats and infrared (IR) measurements [13,27,37,38]. 

Table 2 Drago s Parameters for Some Commonly Used Acids and Bases... 

Table 10.2.2. Drago s parameters of some common solvents [after reference 18]...

However, the sequence of C/E values is not in accordance with practical experience with the solvents. Drago s parameters and his equation also could not be used to describe Pearson s hard-soft concept. 

A self-consistent set of E and C values is now available for 33 acids and48 bases, allowing AH prediction for over 1584 adducts. It is assumed that the conditions under which measurements are made (gas phase or poorly coordinating solvents) give rather constant entropy contribution and that most of adducts are of one-to-one stoichiometry. Table 10.2.2 gathers Drago s parameters, given in (kcaEmol) of some common solvents. 

The parameters and Ca are associated with the Lewis acid, and Eg and Cb with the base. a and b are interpreted as measures of electrostatic interaction, and Ca and Cb as measures of covalent interaction. Drago has criticized the DN approach as being based upon a single model process, and this objection applies also to the — A/y fBFs) model. Drago s criticism is correct, yet we should be careful not to reject a simple concept provided its limits are appreciated. Indeed, many very useful chemical quantities are subject to this criticism for example, p o values are measures of acid strength with reference to the base water. 

We can now examine in further detail the E, C, R, and T parameters Drago s system We have seen that a o indicate tendencies to form... 

Table 3 Drago s E and C Parameters (kcal mol for Some Polymers and Fillers... 

Conversely, the AN seale ean be viewed as a scale of hardness for adds since EtsPO is a hard reference base. Nevertheless, the merit of Gutmann s approach Kes in the fact that his scales provide both addic and basic parameters for amphoteric species, which is not the case with Drago s E and C classifications. 

On the basis of Drago s concept other two parameter empirical models were suggested [e.g. Sh 70a, Ko 75,77a, La 81a, b, Ma 79, Ri 80] for the generalization of different type of weak interactions (not only solvation) in different fields of chemistry. 

Drago s concept has the merit that two specific factors governing the solvent effect are written together in one common equation. As seen above, characterization of the solvent effect by means of this model succeeded well in certain systems, and less well in others. It appeared that replacement of the two-parameter approach by several parameters might lead to a more generally valid solution. For a joint description of non-specific and specific effects, Koppel and Palm [Ko 72] proposed the introduction of the following four-parameter equation ... 

A further distinction that should be kept in mind is between those parameters that clearly pertain to solvents, such as those derived from bulk properties (refractive index, electrical permittivity, rate and equilibrium constants of reactions in the solvent, UV/visible or IR frequency shifts of solutes), and those that relate to molecules in solntion, such as those derived from enthalpies of reactions in dilute solution in an inert solvent. The donor number DN and Drago s E, E, C, Cg acidity and basicity parameters for Lewis acids and bases are examples of the latter kind. 

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