Лако-красочные материалы — производство

Pearson [30] proposed qualitative scales of acidity and basicity based on the numerical values of equilibrium constants for nucleophilic substitution reactions. Pearson noted that the stability of the acid-base adducts depended on the size and the charge of the adjacent acids and bases. Pearson identified hard and soft types of acids, and hard and soft types of bases.

Hard acids (or electrophiles) have a positive charge, are hard to reduce due to their high-energy lowest unoccupied molecular orbital (LUMO) and have a small size (e. g., H+).

Soft acids have a low-energy LUMO and are thus easy to reduce, do not necessarily have a positive charge and have a large size (e. g., I2, metals).

Hard bases (or nucleophiles) are difficult to oxidize for they have a low-energy highest occupied molecular orbital (HOMO), are usually negatively charged, and have a small size and a high pKa (e. g., O2—, ketones).

Soft bases are easy to oxidize due to their high-energy HOMO, do not necessarily have a negative charge, and have a large size and a small pKa (e. g., amines).

Pearson proposed the following expression to rationalize his HSAB concept:

log K — SASB + aAaB (2)

where S is a hardness factor, a is a softness parameter, and A and B stand for acid and base, respectively. Implicitly, Eq. (2) indicates that like species form stable adducts. In other words ‘‘hard acids prefer to bind to hard bases, and soft acids prefer to bind to soft bases.’’ Unfortunately, the HSAB theory remained of very limited utility since it failed to predict quantitatively the stability of the adducts. Drago [34] pointed out that in the HSAB literature, results are explained after the answer is known. Nevertheless, Lee [35] has related chemical hardness to the average energy gap of a solid and has proposed the following classification of solids:

Metals: soft and mostly acidic Semimetals: soft

Semiconductors: rather soft and mostly basic Most insulators including polymers: hard.

Lee reported that a metal-metal interaction could be viewed as an acid-base interaction. This is, for example, the case for the chemical interaction at the Cr/Cu interface which has been modeled as an acid-base interaction where Cr is a Lewis acid and Cu a Lewis base because it has more filled than empty orbitals [35]. The work of Lee has contributed considerably to the extension of the HSAB principles, established for liquid solutions, to solid-solid interactions.