Handbook of Solvents - George Wypych - ChemTech - Ventech!

572 S. Bistac, M. Brogly
First, it is not necessary that the donor and acceptor orbitals be localizable on a single
atom or between two atoms, as implied by Lewis dot structures. That is, the orbitals may be
multi-centered even in a relatively localized representation. Thus donor-acceptor interactions
involving delocalized electron systems (π-ring) 4-5 are naturally subsumed by the definitions.
Second, the HOMO or donor orbital on a base is likely to be either bonding or
non-bonding in character, the latter always being the case for mono-atomic species. The
LUMO or acceptor orbital on an acid is likely to be either anti-bonding or non-bonding in
character, the latter always being the case for mono-atomic species.
Third, all degrees of electron donation are possible, ranging from essentially zero in
the case of weak intermolecular forces and idealized ion associations to the complete transfer
of one or more electrons from the donor to the acceptor. This continuity can be represented
[10.2.1] by wave functions, 6 were the degree of donation increases as the ratio (a/b). 2
ψ = aψ + bψ
[10.2.1]
AB A B
where:
ψAB wave function of the acid-base one-to-one adduct
ψA ground state wave function of the acid
ψB ground state wave function of the base
a, b weighting coefficients
We have reported in Table 10.2.1 the possible adducts classified in terms of the bonding
properties of the donor and acceptor orbitals of the acid and base. Complete description
of the mechanisms involved during the following adducts formation can be found in Jensen 7
complete review.
Table 10.2.1. Possible acid-base adducts in terms of orbitals properties
Donor
orbital of
the base
Acceptor orbital of the acid
Non-bonding Anti-bonding
n σ* π*
Non-bonding n n⋅n n⋅σ* n⋅π*
Bonding
σ σ⋅n σ⋅σ* σ⋅π*
π π⋅n π⋅σ* π⋅π*
Even if the nature of an acid or a base is highly relative, a brief classification based on
bonds properties and orbitals symmetry can be proposed:
n donor: Lewis bases, complex and simple anions, carbanions, amines, oxides, sulfurs,
phosphines, sulfoxides, ketones, ethers, alcohols ....
π donor: unsaturated and aromatic hydrocarbons having electron donor substituents.....
σ donor: saturated hydrocarbons, CO, single links like C-C, C-H, polar links like NaCl,
BaO, silanes....
n acceptor: Lewis acids, simple cations....
π acceptor: N 2,SO 2,CO 2,BF 3, dienic and unsaturated and aromatic hydrocarbons having
electron acceptor substituents....