Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.2 Effect of polymer/solvent acid-base interactions 573
σacceptor: Brönsted acids, boranes and alkanes having strong acceptor substituents, such as,
CHCl 3, halogens....
To summarize how acid-base reactions
do work on the basis of molecular
orbitals perturbation theory, we have reported
on Figure 10.2.1, the relative energies
(as perturbed by the field of the other
reactant) of the frontier orbitals HOMO and
LUMO of a hypothetical species A and of
the frontier orbitals of several hypothetical
reaction partners B, C, D, E and F. This figure
is intended to represent possible variations
of donor-acceptor properties in the
broadest possible context i.e. not only those
Figure 10.2.1. How do donor-acceptor molecular orbital species encountered in aqueous solution but
interactions work?
also those stabilized by non-aqueous environments.
According to Figure 10.2.1, with respect to B, complete electron transfer from B to A
will be favorable and A will act as an oxidizing agent. With respect to C, the A(LUMO) -
C(HOMO) perturbation will be favorable and A will act as an acid. With respect to D, the
A(HOMO) - D(LUMO) perturbation will be favorable and A will act as a base. Lastly, with
respect to E, complete electron transfer from A to E will be favorable and A will act as a reducing
agent. For F species, the frontiers orbitals are quite degenerated with those of A.
Here neither species is clearly the donor nor acceptor and species may display both behavior
simultaneously (case of multisite interactions encountered in concerted organic
cycloaddition reactions).
10.2.1.1.3 The case of hydrogen bonding
In addition to the general discussion of Lewis acid-base interactions, hydrogen bonding represents
a special case. According to Pauling, hydrogen bonding is partly covalent and partly
ionic (polar). Nevertheless, it is obvious that electrostatic and charge transfer interactions
are predominant for hydrogen bonds. In most cases the principal charge transfer contribution
is derived from the proton acceptor - proton donor charge transfer complex through the
σ-type interactions. Moreover hydrogen bonds complexes are linear and the angle between
the H bond and the molecular axis of the proton acceptor are mainly 0° or 60°.
To conclude this first theoretical part, the chemical phenomena subsumed by the category
of acid-base reaction, are the following:
• Systems defined by the Arrhenius description, solvent system, Lux-Flood and
proton acid-base definitions
• Traditional coordination chemistry and “non-classical” complexes
• Solvation, solvolysis and ionic dissociation phenomena in both aqueous and
non-aqueous solutions
• Electrophilic and nucleophilic reactions in organic and organometallic chemistry
• Charge transfer complexes, molecular addition compounds, weak intermolecular
forces, hydrogen bonds
• Molten salt phenomena, salt formation