Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.2 Effect of polymer/solvent acid-base interactions 571
10.2.1.1 The nature of acid-base molecular interactions
10.2.1.1.1 The original Lewis definitions
The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless
they are always useful and have broadened their meaning and applications, covering
concepts such as bond-formation, central atom-ligand interactions, electrophilicnucleophilic
reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor
reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory
of the electron-pair bond, 1 which he had first proposed in 1916. 2 In this small volume which
had since become a classic, Lewis independently proposed both the proton and generalized
solvent-system definitions of acids and bases. He wrote:
“An acid is a substance which gives off the cation or combines
with the anion of the solvent; a base is a substance which gives
off the anion or combines with the cation of the solvent”.
The important point that Lewis revealed is that though the acid-base properties of species
are obviously modified by the presence or absence of a given solvent, their ultimate cause
should reside in the molecular structure of the acid or base itself, and in light of the electronic
theory of matter, not in a common constituent such as H + or OH - , but in an analogous
electronic structure. He states that a basic substance is one which has a lone pair of electrons
which may be used to complete the stable group of another atom (the acid) and that an acid
substance is one which can employ a lone pair from another molecule (the base) in completing
the stable group of one of its own atoms. Moreover by tying his definitions to the concept
of chemical bond, Lewis linked their usefulness to contemporary views on the nature of
the chemical bond itself. Hence, the shared electron-pair bond model explains the existence
of both non-polar bond and polar link from the same premises. As the electrochemical natures
of two atoms sharing an electron pair began to differ more and more, the pair should
become more and more unequally shared, eventually becoming the sole property of the
more electronegative atom and resulting in the formation of ions. Ionic and non-polar bonds
appear as logical extremes of a continuum of intermediate bond type. Differences in the
continuum are only attributable to variations in the electron-pair donation, i.e., in the way in
which the charges are localized within the molecule. As a consequence the distinctions between
salts, acids and bases, coordination compounds and organic compounds are not of
fundamental nature.
10.2.1.1.2 Molecular Orbital (MO) approach to acid-base reactions
Translated into the idiom of molecular orbital theory, 3 the acid-base definitions should be
read as follows:
- A base is a species, which employs a doubly occupied orbital in initiating a reaction.
- An acid is a species, which employs an empty orbital in initiating a reaction.
The term species may mean a discrete molecule, a simple or complex ion or even a
solid exhibiting non-molecularity in one or more dimensions (graphite as an example). Free
atoms seldom act as Lewis acids and bases. They usually have one or more unpaired electrons
and their reactions are more accurately classified as free radical. The donor orbital is
usually the highest occupied molecular orbital HOMO, and the acceptor orbital is usually
the lowest unoccupied molecular orbital or LUMO. The molecular orbital definitions have a
number of important consequences: