Lewis Acid and Base

To explain acid-base reactions, the first acid-base theory was given by Svante Arrhenius in 1884 in his work of establishing the presence of ions in an aqueous solution for which Arrhenius received the Nobel Prize in Chemistry in 1903. Arrhenius describes acid as a substance that contains hydrogen or produces H+ in an aqueous solution. While he described bases as that substance that contains hydroxyl groups or produces OH- in an aqueous solution. In 1923, Bronsted – Lowry's theory of acid-base was proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry independently. According to this theory, an acid is a substance that donates protons while a base is a substance that accepts protons. In the same year (1923) American Physical Chemist Gilbert N. Lewis proposed an alternate theory to describe acids and bases. His theory was widely accepted by chemists and scientists as by using Lewis acid and base concept they are now able to predict a wider variety of acid-base reactions. 

What is Lewis Acid? 

According to Lewis's theory of acid–base reactions, “a species that has vacant orbitals and can accept an electron pair is called Lewis acid.”

An electrophile is a Lewis acid as it is an electron-deficient group and can accept electrons easily. Acid uses its lowest unoccupied molecular orbital (LUMO) to form bonds with bases. 

Examples of Lewis Acids 

Few examples of Lewis acids are listed below –

• All cations are Lewis acids. For example, Cu+2, Zn+2, Fe+2, Fe3+ etc. are Lewis acids as they can accept electrons. 

• H+ ions can be considered as Lewis acids. 

• An atom or ion or molecule with an incomplete octet of electrons can act as a Lewis acid. For example, AlF3 (Aluminum fluoride).

• Molecules whose central atom can have more than 8 electrons in a valence shell and can accept electrons, can be called Lewis acid. For example, SiBr4 and SiF4.

• Molecules that have multiple bonds between atoms of different electronegativities act as Lewis acids. For example, CO2, SO2, SO3 etc. 

Applications of Lewis Acids

Lewis acids are used in many reactions as catalysts. In Friedel – Crafts alkylation reaction, AlCl3 accepts a chloride ion lone pair and forms AlCl4- and creates strongly acidic electrophilic carbonium ions. The reaction is given below –

RCl + AlCl3 🡪 R+ + AlCl4-

Apart from this Lewis acid's action can be seen in photochemical (CO2 reduction), electrochemical, hydrogenation, and Prins reactions.  

What is Lewis Base?

According to Lewis's theory of acid-base reactions, “a species that has a lone pair of electrons and can donate an electron pair is called Lewis base.”

A nucleophile is a Lewis base as it is an electron surplus group and can donate electrons easily. The base uses its highest occupied molecular orbital (HOMO) to form bonds with acids. 

Examples of Lewis Bases 

A few examples of Lewis bases are listed below –

• Any molecule, atom, or ion with a lone pair of electrons can act as Lewis bases. For example, NH3, H2O: etc. act as Lewis bases. 

• Anions can donate electrons, so they act as Lewis bases. For example, H-, F-, OH-, CN-etc. act as Lewis bases. 

• Pyridine and its derivatives act as Lewis bases.

• The pi-electron systems which are rich in electrons act as Lewis bases. For example, benzene, alkenes, alkynes, etc. can act as Lewis bases. 

• Complex anions such as sulfates also behave as Lewis bases. 

Applications of Lewis Bases 

Lewis bases are used to enhance the activity and selectivity of metal catalysts. They are used as ligands in coordination chemistry. Few Lewis bases are “multidentate” so they can form multiple bonds with Lewis acids. These Lewis bases are called chelating agents.  

Chemical Reactions between Lewis Acids and Bases 

Lewis acid accepts electrons from the Lewis base and the formation of Lewis adducts takes place. A dative bond is formed between them. Dative bond is also called a coordinate covalent bond or coordinate bond. A few examples of reactions between Lewis acids and bases are given below –

• The reaction of H+ (Lewis acid) and H2O (Lewis base) – Water molecule reacts with hydrogen ions and forms hydronium ions. Reaction is given below –

• The reaction of H+ (Lewis acid) and NH3 (Lewis base) – Ammonia combines with hydrogen ions and forms ammonium ions. Reaction is given below –

• Reaction of Boron trifluoride (Lewis acid) with fluoride ion (Lewis base) – Octet of Boron in Boron trifluoride (BF3) is incomplete so it acts as a very good Lewis acid and combines with fluoride ion and forms BF4-. Reaction is given below –

• The reaction of Aluminum trichloride (Lewis acid) with chloride ion (Lewis base)– It is a key step of Friedel – Crafts reaction. Aluminum trichloride combines with chloride ions and forms aluminum tetrachloride ions. The reaction is given below –

• The reaction of Ag+ (Lewis acid) and NH3 (Lewis base) – Ammonia donates a pair of electrons to a silver ion and forms dative bond and an acid – base adduct. Reaction is given below –

• Reaction of SO2 (Lewis acid) and O2- (Lewis base) – Non – metal oxides such as sulfur dioxide acts as a Lewis acid and accepts an electron pair from oxide ion, thus forming a dative bond and an acid – base adduct (oxyanion). Reaction is given below –

• Displacement Reactions of Lewis acids and bases – Many Lewis acid – base reactions are displacement reactions. In these reactions, Lewis base displaces another Lewis base from an acid base adduct. Reactions are given below –

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