The Lewis and Brønsted Models of Acidity
analysis uses the concept of Lewis acids and Lewis bases extensively, and
the Brønsted model of acidity is employed as well. It is essential
that the reader is quite clear about the similarities and differences
between the two approaches. This page reviews these issues.
In the Lowry-Brønsted
model: "A Brønsted acid is a proton donor, and a Brønsted base is a proton acceptor (abstractor)".
In the Lewis model: "A
Lewis acid is an electron-pair acceptor, and Lewis base is an electron-pair donor ".
Or, in the language of frontier molecular orbital (FMO) theory: "A Lewis acid interacts by its lowest unoccupied molecular orbital or LUMO, and a Lewis base interacts via its highest occupied molecular orbital or HOMO."
The two theories can be reconciled
by recognising that the proton, H+, is a unique and versatile
Lewis acid that is the agent of Brønsted acidity.
The Lewis model
is more general than the more commonly used Brønsted model.
- All Brønsted
acids are proton/Lewis base complexes.
- The transfer
of H+ between Lewis bases equates with Brønsted acidity.
- While a
Brønsted acid is an H+ donor, the proton, H+,
is a Lewis acid.
- All Lewis
bases can be protonated. It follows that the ability of a species to
complex a proton defines that species as being both a Brønsted
base and a Lewis base.
- Any species
able to complex with a Lewis base is a Lewis acid.
A Model Reaction
chloride reacting with water:
The Brønsted description of this reaction says:
- Hydrogen chloride, HCl, is the proton donor [Brønsted] acid and water, :OH2, is the proton-accepting [Brønsted] base.
- The oxonium ion, [H3O]+, is the conjugate [Brønsted] acid and the chloride ion, Cl, is the conjugate [Brønsted] base.
The Lewis description of this reaction says:
- The chloride ion, Cl, and water, :OH2, are both Lewis bases and they compete with each other to complex the proton Lewis acid, H+.
- The water Lewis base "wins" and the proton transfers from chloride ion to water. Thus, hydrogen chloride is an H+/Cl complex that transfers H+ to water to give the oxonium ion, [H3O]+.
- The oxonium ion is an H+/water complex, H+/:OH2
In the Brønsted
analysis, all proton acceptors (Brønsted bases) are standardized against the aqueous Brønsted base, water, :OH2. The measure
is expressed as the reaction equilibrium constant, Ka
or pKa. Therefore,
- The term
Brønsted base refers to proton affinity with respect to
- pH is a measure
of hydrogen ion concentration in water. For good reasons, see here,
pH is defined as "minus the log10 of the
hydrogen ion concentration":
pH = log10[H+]
- The term
Lewis base is more general and refers to the propensity to complex with a
Lewis acid. Lewis bases can present as nucleophiles, ligands, spectator anions or electron rich π-systems, as well as proton abstractors. There is
no general scale of Lewis acid or Lewis base behaviour,
although periodicity can be observed as discussed
in this webbook.
Due to the potential for confusion, throughout the chemogenesis web book the word "acid" is always proceeded by the qualifier "Lewis" or "Brønsted".
|The Five Reaction Chemistries
© Mark R. Leach 1999-
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