Lewis Acid and Lewis Base: Examples, Applications, Reactions and Important Questions

Lewis acids and Lewis bases are named after an American Chemist Gilbert Newton Lewis. He postulated the Lewis theory to explain the Lewis acid-base reactions. According to this theory, electron-pair acceptors ‘Lewis Acid and electron pair donors 'Lewis Base' react to form a coordinate covalent bond referred to as a Lewis adduct. 

Keyterms: Lewis Acid, Lewis base, Covalent Bond, Electron pair, Electrophile, Ion, Hydogen, Ammonia

---

Lewis Acid 

[Click Here for Sample Questions]

The chemical species with empty orbitals that can accept a pair of electrons from the Lewis bases are called Lewis acids. They contain an unoccupied p-orbital and have a trigonal planar shape. The electrophiles are Lewis acids as they tend to accept a pair of electrons. Lewis acids use the Lowest Unoccupied Molecular Orbital (LUMO) to form a bond with the Lewis base.

Applications of Lewis Acid

• Lewis acids are used as catalysts in a variety of reactions. In Friedel – Crafts alkylation reaction, AlCl₃ accepts a lone pair of chloride ions and forms AlCl₄^-. This produces a highly acidic electrophilic carbonium ion. The reaction for the same is given below:

XCl + AlCl₃ → X⁺ + AlCl₄^–

• Lewis acids are active in photochemical, electrochemical, hydrogenation, and Prins reactions.
• Lewis Acids are widely used to carry out many cationic and pseudo-cationic reactions.

Also Read:

---

Lewis base

[Click Here for Sample Questions]

A Lewis base is a species that has a lone pair of electrons and can donate a pair of electrons to form a covalent bond. The most strongly located HOMO (Highest Molecular Occurrence Orbital) atomic or molecular chemical species serve as Lewis bases. These chemical species can donate a pair of electrons to a certain Lewis acid to produce an adduct.

Anything like the OH^- ion that can transfer a few electrons that are not bonding are the Lewis base. Thus, an electron pair donor is a Lewis base. Ammonia, alkyl amines and other typical amines are the most common Lewis bases. Lewis

bases are usually anionic, and their base strength usually depends on the pK_a of the parent acid. 

As Lewis bases are electronically rich, they can be categorized as nucleophiles and so contribute electron pairs. 

Lewis base: Examples

• The compounds in which Oxygen, Sulphur, Selenium and Tellurium exhibit an oxidation state of -2 are often Lewis bases. Water and ketones are examples of such chemicals.
• Pyridine and the derivative of pyridine is the most common example of Lewis base as they are capable of acting as an electron-pair donor.
• The strong conjugate Lewis bases have weak Lewis acids. Besides that, because of its ability to donate electron-pairs, some chemical species with lone pairs of electrons such as CH₃^- and OH^- are also recognized as the Lewis bases.
• CN^-, CH₃COO^-, H₂O are some other examples of Lewis Bases.
• The simple anions which have an electron pair can also act as Lewis bases by donating the electrons. For example, H^- and F^-.
• The complex anions such as sulfates also behave as Lewis Base.

Applications of Lewis bases

• Lewis bases have tremendous applications in the modification of the selectivity and activities of metallic catalysts.
• Asymmetric catalysis is a key aspect of enantioselective synthesis in the manufacturing of medicinal products. Chiral Lewis bases frequently are used to confer chirality on catalysts in order to enable asymmetric catalysis.
• Few Lewis bases are ‘multi-dented’; so that they can form bonds with Lewis acids. These Lewis bases are known as chelates.

---

The reaction between Lewis acids and Lewis bases

[Click Here for Sample Questions]

The formation of an adduct takes place when a Lewis acid takes an electron from a Lewis base, and this results in the formation of a covalent bond or coordinate bond between the two reactants.

Let Lewis acid be A and Lewis base be B

Lewis acid, A is an electron acceptor while Lewis base, B is an electron donor. A coordinate covalent bond exists between A and B. 

 

Given below are some examples of the reactions between Lewis acid and Lewis base to yield a Lewis adduct. 

• Lewis base H₂O water and Lewis acid H⁺ hydrogen ion react to form Lewis adduct hydronium ion H₃O⁺.

The oxygen atom tends to donate an electron to the hydrogen ion to form the coordinate covalent bond.

• Lewis base NH₃ ammonia and Lewis acid H⁺ hydrogen ion react to form Lewis adduct ammonium ion NH₄⁺.

The nitrogen atom tends to donate an electron to the hydrogen ion to form the coordinate covalent bond.

• Lewis base NH₃ ammonia and Lewis acid Ag⁺ silver ion react to form Lewis adduct with the chemical formula Ag(NH₃)₂⁺ 

The nitrogen atom tends to donate an electron to the silver ion to form the coordinate covalent bond.

• Lewis base F^- fluoride ion and Lewis acid BF₃ Boron trifluoride react to form Lewis adduct with the chemical formula BF₄^- tetrafluoroborate ion. 

The fluoride ion tends to donate an electron to the Boron trifluoride to form the coordinate covalent bond.

• Lewis base NH₃ ammonium ion and Lewis acid BF₃ Boron trifluoride react to form Lewis adduct with the chemical formula NH₃BF₃ ammonia boron trifluoride. 

The NH₃ ammonium ion tends to donate an electron to the BF₃ Boron trifluoride to form the coordinate covalent bond.   

• Lewis base H₂O and Lewis acid sulphur dioxide SO₂ reacts to form a Lewis adduct with a chemical formula H₂SO₃ that is, Sulfurous acid. 

The oxygen atom tends to donate an electron to the sulphur atom in SO₂ while the hydrogen atom tends to donate an electron to the oxygen atom in SO₂ to form the coordinate covalent bond.      

---

Things to Remember

• The chemical species with empty orbitals that can accept a pair of electrons from the Lewis bases are called Lewis acids. 
• A Lewis base is a species that has a lone pair of electrons and can donate a pair of electrons to form a covalent bond.
• Lewis acids are used as catalysts in a variety of reactions.
• Lewis bases are used in the modification of the selectivity and activities of metallic catalysts.
• When a Lewis acid takes an electron from a Lewis base, a covalent bond or coordinate bond between the two reactants and the formation of an adduct takes place.

Also Read: