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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:



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, Ka or pKa. Therefore,

pH   =   –log10[H+]


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.


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Chemogenesis in 500 Seconds HSAB Principle

© Mark R. Leach 1999 –


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