Lewis Acids and Bases

Both acids and bases are important aspects when we study chemistry. Lewis acid/base motif is one of the most applicable theories, and it extends the denotation of acids and bases beyond H+ and OH- ions.

Lewis Acid

The acid-base theory of Brønsted has been used thoroughly in the history of acid and base. However, this theory is a bit restrictive and limited as it focuses mainly on the acids and bases that act as proton donors and acceptors. Some situations arise when this theory does not necessarily fit, especially when it comes to solids and gasses. In any species, if there is a Lewis acid, there are high chances of accepting a pair of electrons. A Lewis base is a type of species that can donate a pair of electrons to the acceptor of the same category. The reaction of Lewis acid/base forms a bond that is known as a coordinate covalent bond. 

Some of the Examples of Lewis Acids

Many reactions involve Lewis acids and bases. Some of the common examples are:

• Iron (Fe2+ and Fe3+). Cations of d block elements that portray states of high oxidation

• Copper (Cu2)

• Hydrogen (H+) ions with onium ions (H2O+)

• Cations of metals like Li+ and Mg2+

• Arsenic, Antimony and Phosphorus.

Remember, any deficiency in the electron π system will be acting as an acceptor of the electron pairs. For example, enones. 

Defining Lewis Base

A Lewis base is a donor of electron-pair. To make it simple, it is a substance through which a pair of electrons is donated to form a covalent bond. This states that a Lewis base is a nucleophile. The reaction of Lewis acid-base can be represented by the transfer of pairing electrons to an acid medium from a base. Hence, looking at the reaction, we can say that this pair of electrons is nonbonding to acids. Nonbonding electrons are loose changes that take place. For instance, considering a hydrogen ion, it can accept a pair of electrons due to the lack of electrons. 

Some of the Examples of Lewis Bases

All electron-pair donors that can form compounds with the binding of transitional elements can be termed Lewis bases. They are also known as ligands. Lewis bases modify the selectivity and activities of metal catalysts. Some of the common examples are:

• H2O

• Cl–

• I–

• CH3–

• NH3

• F–

• H–

• SbCl5

• SO42-

• C2H2

Chemical Reactions between Lewis Acids and Bases

When it comes to Lewis theory of acid-base reactions, bases are known for donating pairs of electrons, and on the other hand, acids continue to accept those pairs of electrons. Therefore, this indicates that a Lewis acid can be any substance, like an H+ ion, that accepts pairing electrons which are nonbonding. To make it simpler, it can be said that Lewis acid is an acceptor of electron-pair. On the contrary, an ion of OH- can be the best of a Lewis base as it donates a pair of nonbonding electrons. Therefore, a Lewis base is an electron-pair donor. One of the notable advantages of Lewis theory is the way it complements the model of reducing oxidation reactions. The reactions of oxidation reactions include transferring electrons from one atom to another. With this, a net change in the number of oxidations continues in one or more atoms. 

Application to Lewis Acid and Bases

An alternate theory to describe acids and bases was proposed by G.N Lewis. It states a general explanation of acids and bases based on bonding and structure. With the application and easy definition of acids and bases, lots of chemists can now predict a wider variety of acid-base reactions. The theory of Lewis was based on electrons rather than proton transfers.

The common examples of Lewis acids and bases are:

• Formation of ligands.

• Forming bonds with Lewis acids for the production of chelating agent.

• Modification of metallic catalyst.

Lewis acids play a crucial role in the form of catalysts. For example, a lone pair of electrons are accepted by AlCl3 that belongs to the chloride ion. This gives to the formation of AlCl4- in the alkylation process of Friedel Crafts.

Amphoterism

Till now, it must have been clear that acids and bases are two separate things. However, there are some substances that are both acid and base. One of the most common examples is water. It can play the role of either an acid or a base. This ability or quality of water makes this one amphoteric molecule. By donating its proton, water could act as the acid from the base. This helps to become the conjugate acid which is known as OH-. On the contrary, by accepting a proton from acid, it can also play the role of a base. This helps in becoming a conjugate base which is known as H3O+.

• H2O+NH3→NH+4+OH−(3)(3)H2O+NH3→NH4++OH− (When water acts as an acid)

• H2O+HCl→Cl−+H3O+(4)(4)H2O+HCl→Cl−+H3O+ (When water acts as a base)

The degree of molecular acts depends on the medium in which it has been placed. Taking the example of water, it does not act as an acid in an acidic medium or fails to act as a base in a related medium. Therefore, the medium in which the molecules are placed plays a key role and has an effect on the molecular properties.  Other molecules can also act as an acid or a base.

• Al(OH)3+3H+→Al3++3H2O(5)(5)Al(OH)3+3H+→Al3++3H2O (When Al(OH)3 acts as a Lewis base)

• Al(OH)3+OH−→Al(OH)−4(6)(6)Al(OH)3+OH−→Al(OH)4− (When Al(OH)3 acts as Lewis acid)

You should take note of the amphoteric properties of Al(OH)3 as it depends on the environment where the molecules have been placed.

Beyond Generic Acid-Base Chemistry

Apart from specific substances, if we consider electron pairs for treating acid-base reactions, the definition of Lewis can be applied to reactions that do not typically categorize under the explanation of acid-base reactions. Taking the example of silver, a cation of silver behaves in the form of Lewis acid with respect to ammonia. Here, ammonia plays the role of Lewis base.

Ag+(aq)+2NH3→[Ag(NH3)2]+Ag+(aq)+2NH3→[Ag(NH3)2]+

The result of the reaction shows the formation of diamminesilver(I). It is a complexion that is described by Lewis acid-base chemistry. However, it cannot be classified in terms of traditional definitions like Arrhenius and Bronsted-Lowry. In terms of organic chemistry, it is crucial to determine that Lewis bases are nucleophiles and Lewis acids are electrophiles. Almost all the reactions are considered as Lewis acid-base processes in organic chemistry.

Quick Summary

Sample Questions for Exam

1. How do you identify Lewis acid and Lewis base?

2. Define following terms

• Lewis Acid

• Lewis Base

• Amphoterism

3. Give some examples of Lewis acid

4. Give some examples of Lewis base

5. Explain the reaction between lewis acid and lewis base. 

6. What are the applications of lewis base?

7. What are the applications of lewis acid?

Did You know?

• Gilbert Lewis proposed this theory of Lewis acid and base, which proved to be a more general approach than Arrhenius and bronsted-lowry. 

• The corrosiveness of acid is not related to its strength.

• Acid and bases do not conduct electricity.

• There are acids present in the human body such as Hydrochloric acid HCl, which helps to digest food.