What is the definition of acid according to Lewis?

According to the glorious revelation of IUPAC, whose Gold Book is the canonic reference standard for the definitions of terms in modern chemistry :

A molecular entity or chemical species capable of donating a hydron (proton) (see Brønsted acid) or capable of forming a covalent bond with an electron pair (see Lewis acid).

2. base:

A chemical species or molecular entity having an available pair of electrons capable of forming a covalent bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of some other species (see Lewis base).

Thus, IUPAC have adopted the satisfactory (though proton-infatuated) Brønsted-Lowry definition for the majority of situations, simply because it is convenient for daily use. In situations where this convenient model does not apply, we are directed to Gilbert N. Lewis' definition. Thus, acids and bases have been divided into Brønsted molecules and Lewis molecules. The two models are complimentary, and coexist easily in the complex minds of chemists, as they are useful for different situations.

The Lewis definition of acids and bases

Gilbert Newton Lewis

(1875 - 1946)

Gilbert N. Lewis, otherwise credited with the characterisation of such fundamentals as the covalent bond and the name "photon", is responsible for the theoretical framework which defines acids and bases without resorting to the discussion of proton traffic. Recognising the limitations of contemporary definitions, he wrote:

"When we discuss aqueous solutions of substances which do not contain hydroxyl [ion], it is simplest to define a base as a substance which adds hydrogen ion. . . . Since hydrogen is a constituent of most of our electrolytic solvents, the definition of an acid or base as a substance which gives up or takes up hydrogen ion would be more general than the one we used before, but it would not be universal."

Thus, acknowledging the value of proton-based definition in routine transactions of chemistry, Lewis was looking for a more general definition which would retain its validity in a variety of nonstandard situations, in the absence of solvents, and in the absence of protons.

In his 1924 work, he went on to offer this definition:

"We are inclined to think of substances as possessing acid or basic properties, without having a particular solvent in mind. It seems to me that with complete generality we may say that a basic substance is one which has a lone pair of electrons which may be used to complete the stable group of another atom, and that an acid is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms."

IUPAC, over 80 years later, have not made any substantial changes to any of this. The Gold Book holds forth with the following definitions of Lewis' concepts:

A molecular entity (and the corresponding chemical species ) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base.

2. Lewis base:

A molecular entity (and the corresponding chemical species) able to provide a pair of electrons and
thus capable of coordination to a Lewis acid, thereby producing a Lewis adduct.

The "Lewis Adduct" these definitions keep referring to is actually just the combination of a Lewis base and a Lewis acid in a weird state of non-covalent attraction. The textbooks describe this "adduct" as some sort of transitional state which occupies a space somewhere between true ionic and true covalent bonding, but is more accurately neither.

To refer to such a strange bond, chemists usually resort to a single central dot, eg. Me3B·NH3.

Thus, in the Lewis definition, the reaction between an acid A and a base B would look like this:

A + :B → A-·B+

Here, the (:) semicolon represents the pair of electrons which are available for "donation" by the base.

The adduct is composed of an anionic A- which has received electrons, and a cationic B+ which has donated them. In this definition, the hydrogen ion itself is actually the acid- as a proton, it is "a molecular entity that is an electron-pair acceptor".

So what the hell happened to the H+

It is perhaps best not to abandon the use of the hydrogen ion. It is a useful concept, even though - as Peter Stewart put - it is a symbol for a metaphor. The convenience of this symbol, in spite of its gross electrochemical inaccuracy, has made it a convention of scientific literature. This specific issue is explored in some detail in a chapter dedicated to the use of H+ as a narrative device. In short, it's ok to continue to talk about hydrogen ions, provided one is aware that the concept is deeply flawed.

In the exploration of these concepts, one can tumble down many deep rabbit holes. If one wishes to pursue acid-base chemistry in greater detail, one may wish to begin with the enormous, detailed 6th edition of Advanced Organic Chemistry by Smith and March. Additionally, one may wish to pay homage to Prof. Martin Chaplin's site "Water Structure and Science", which is a beautiful temple dedicated to the marvel and wonder of H2O.

Lewis acid base concept or theory explains the acids and bases properties in terms of electronic structure with the formation of the coordinate covalent chemical bond in chemistry.

According to American chemist Gilbert Newton Lewis,

1. An acid is any species (molecule, radical, or ion) that can accept an electron pair to form a coordinate covalent bond.
2. A base is any species that can donate an electron pair to the coordinate covalent bond.

In learning chemistry, the Lewis acids and bases concept does not restrict to the concentration of hydrogen ions or the pH scale of the solution. It is based on the electronic configuration or structure of atoms, ions, or molecular species.

Lewis acid base theory

In an acid base chemical equilibrium reaction, a base leads to form a coordinate link to the acid. An acid must have a vacant orbital in which the electron pair donated by a base can be accommodated. A Lewis base is a substance that has at least one unshared (lone) pair of electrons.

A (Lewis acid) + :B (Lewis base) → A : B (Adduct)

According to Lewis theory, the neutralization reaction is simply for the formation of the co-ordinate bond between an acid and a base. The neutralization product is called acid-base complex or adduct. It is either non-ionizable or may undergo ionization depending on its stability.

Lewis acids and bases examples

In the Lewis acid-base neutralization reaction, an acid act as an electron acceptor, and the base is considered an electron-pair donor. In the above reactions, we provide some examples of Lewis acids and bases with acid base complexes. The stability of such complexes can be explained by the hard soft acid base theory.

Arrhenius acid base theory

Arrhenius defined, that acids when dissolved in water dissociate into hydrogen ions and anions. Therefore, hydrogen ion (H+) is released from nitric acid, and sulfuric acid acts as an acid.

The hydrogen ion has an affinity to receive an electron pair from the base. Thus all the Arrhenius acids are also acids under the Lewis definition.

Bronsted Lowry acid base

According to the protonic definition by Bronsted-Lowry, a base is any molecule or ion, which accepts a proton. According to Lewis definition, an acid is a molecule that has an atom capable of sharing a pair of electrons with a base.

During the neutralization reaction, a coordinate covalent bond is formed between the donor atom of a base and the proton or hydrogen ion of an acid. Hence all Bronsted-Lowry bases are bases in the Lewis sense also.

Question: What is the conjugate acid base pair of bisulfate?

Answer: The molecular formula of bisulfate ion is HSO3−. It may lose one hydrogen ion to give the conjugate base SO3−2. Again it may use hydrogen ion to give the conjugate acid, H2SO3−.

Solvent system concept

The solvent system concept recognizes that ammonium chloride should behave as an acid in liquid ammonia, alcohol, or acetic acid solution. But an ammoniated hydrogen ion can accept a lone pair of an electron from the base. Therefore, an ammonium ion in a liquid ammonia solution is an acid according to Lewis’s sense.

According to the solvent system concept, an amide ion is a base in liquid ammonia. The amide ion is capable of donating a lone pair to an acid. Hence amide ion is a base according to Lewis theory.

Question: What type of chemical bonding is formed, when sulfur trioxide reacts with water?

Answer: Sulfur trioxide (SO3) like boron trifluoride (BF3) has less than an octet and will be termed as an acid according to Lewis concept. In water oxygen atoms contain lone pairs to donate the Lewis acid sulfur trioxide.

Classification of Lewis acid and base

A wide variety of chemical compounds are recognized as an acid or a base according to Lewis theory. Lewis theory can be classified acids and bases into the following categories,

• Incomplete octet of the central atom
• Elements having vacant d orbital
• Cations of an atom

Incomplete octet of the central atom

Electron deficient molecules such as alkyl and halides of beryllium, boron, and aluminum are examples of these classes of acids. Some reactions of these types of Lewis acids and bases are,

Question: What happens when BF3 reacts with NH3?

Answer: In ammonia, the central nitrogen atom has a lone pair of electrons. This lone pair coordinates with the empty orbital of an atom. Therefore, ammonia is a base according to Lewis.

The compounds with less than an octet for the central atom are Lewis acids. In BF3, the central boron atom contains six electrons. Therefore, it behaves as an acid.

Elements having vacant d orbital

The central atom of the halides such as SiX4, GeX4, TiCl4, SnX4, PX3, PF5, SF4, SeF4, and TeCl4 has vacant d-subshell. These chemical substances can accept an electron charge pair from the Lewis base to accommodate in their vacant d-subshell.

The central atom of these compounds forms adducts with halide ions, oxygen, nitrogen, and sulfur-containing organic hydrocarbon. These substances are thus Lewis acids.

These halides are vigorously hydrolyzed by water to form an oxy acid or oxide of the central atom. The hydrolytic reaction takes place through the formation of an unstable Lewis acid base adduct (intermediate) with water.

Problem: Why PCl3 can act both as Lewis acid and Lewis base?

Solution: PCl3 has a lone pair of electrons in phosphorus. This lone pair may coordinate with a metal ion thus the compound has properties of Lewis base.
Ni + 4 PCl3 ⇄ [Ni(PCl3)4]0

Again the quantum shell of phosphorus has provision for d sub-orbital which can receive back donated electrons from electron reach low oxidizing agents or metal ion. In this latter case, the tri-covalent phosphorus compound serves as a Lewis acid.

Cation of an atom

Theoretically, all simple cations (silver, copper, cobalt, and lithium-ion) are potential Lewis acids. Therefore, the reactions of these cations by Lewis acids base theory given below,

Lewis acids and bases strength

The Lewis acid strength or coordinating ability of the simple cations is increased with an increase in the positive charge carried by the cation.

• With an increase in the charge on the nucleus or increasing the atomic number of the metals in any period of the periodic table, acid strength is increasing.
• It decreases by decreasing ionic radius the number of shielding electron shells.
• Hydrogen bonding also affects acid-base strength.

The Lewis acid strength of simple cations increases when we move from left to right in a period or from bottom to top in a group of the periodic table.

• Fe+2 < Fe+3 (positive charge increases)
• K+ < Na+ (on moving from bottom to top in a group)
• Li+ < Be+2 (on moving from left to right in a period)

Example of Lewis acid

Molecules having multiple electric polarization between atoms of dissimilar electronegativity like carbon dioxide, sulfur dioxide, sulfur trioxide, borax, hydrogen peroxide, and boric acid are examples of Lewis acids.

In these compounds, the oxygen atom is more electronegative than the central atom. As a result, π-electron density is displaced away from the central atom.

These are electron deficient chemical compounds able to accept the electron pair from Lewis bases. They form a dative bond with very low bond energy.

Acidic nature of oxides

Acidic oxides react with water to give oxoacids in redox reactions. The higher the oxidation number and higher the electronegativity the greater the central chemical element will force to react with water to give the oxoacid.

Acidic nature of oxides: Na2O < MgO < As2O3 < N2O5 and oxidation number of oxide-forming element = +1, +2, +3, +5

Utility of Lewis acid base concept

• Lewis acids bases concept includes those chemical reactions in which no hydrogen atom or ion is involved.
• The concept is more general than the Bronsted – Lowry concept or protonic concept.
• The acid base properties are not dependent on the presence of one particular element or on the presence or absence of a solvent.
• It explains the long accepted basic properties of metallic crystalline solid oxides and the acidic properties of non-metallic oxides.
• The Lewis concept or theory explains many acid base reactions in the gas phase in high temperatures and non-solvent reactions as the neutralization process.