Lewis and Brønsted Models of Acidity

The chemogenesis 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.

Introduction

In the Lowry-Brønsted model:

"A Brønsted acid is a proton, H⁺, donor and a Brønsted base is a proton, H⁺, 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 the Lewis model becomes:

"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.

Some statements:

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

Consider hydrogen chloride reacting with water:

HCl + H₂O → [H₃O]⁺ + Cl^–

HCl + :OH₂ → [H₃O]⁺ + Cl^–

The Brønsted description of this reaction says:

[Brønsted] acid + [Brønsted] base → conjugate [Brønsted] acid + conjugate [Brønsted] base

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, [H₃O]⁺, 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, K_a or pK_a. Therefore,

The term Brønsted base refers to proton affinity with respect to water, H₂O:
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.
In this web book, Lewis acids are coloured red and Lewis bases are coloured blue.

Jensen's Review

William (Bill) Jensen presented three papers in the ACS journal Chemistry, vol 47 (1974), Lewis Acid-Base Theory: Part I March pp 11-14; Part II April pp 13-18; Part III May pp 14-18.

The combined paper – which is both excellent and detailed – is available on Bill's webspace (and a clone of the file can be downloaded from this website).

Part III deals with Pearson's HSAB analysis, which we critique on the next page of this web book.

Due to the potential for confusion, throughout the Chemogenesis Web Book the word "acid" is always proceeded by the qualifiers "Lewis" or "Brønsted"... except where a compound is actually named the acid, for example: "sulfuric acid", "acetic acid", etc.

In these situations the names can be assumed to be: "sulfuric [Brønsted] acid" and "acetic [Brønsted] acid", etc.

Chemogenesis in 700 Seconds

HSAB Principle

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