To explain acid - base reactions, first acid – base theory was given by Svante Arrhenius in 1884 in his work of establishing the presence of ions in aqueous solution for which Arrhenius received Nobel Prize in Chemistry in 1903. Arrhenius described acids as that substance which contain hydrogen or produces H+ in aqueous solution. While he described bases as that substance which contain hydroxyl group or produces OH- in aqueous solution. In 1923, Bronsted – Lowry theory of acid – base was proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry independently. According to this theory an acid is that substance which donates proton while a base is that substance which accepts proton. 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 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 incomplete octet of electrons can act as a Lewis acid. For example, AlF3 (Aluminium fluoride).
Molecules whose central atom can have more than 8 electrons in valence shell and can accept electrons, can be called a 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 ion. Reaction is given below –
RCl + AlCl3 🡪 R+ + AlCl4-
Apart from this Lewis acids action can be seen in photochemical (CO2 reduction), electrochemical, hydrogenation and Prins reactions.
What is Lewis Base?
According to Lewis 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. Base uses its highest occupied molecular orbital (HOMO) to form bonds with acids.
Examples of Lewis Bases
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, and alkynes etc. can act as Lewis bases.
Complex anions such as sulfates also behave as Lewis base.
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 Lewis base and formation of Lewis adduct takes place. A dative bond is formed between them. Dative bond is also called coordinate covalent bond or coordinate bond. Few examples of reactions between Lewis acids and bases are given below –
Reaction of H+ (Lewis acid) and H2O (Lewis base) – Water molecule reacts with hydrogen ion and forms hydronium ion. Reaction is given below –
Reaction of H+ (Lewis acid) and NH3 (Lewis base) – Ammonia combines with hydrogen ion and forms ammonium ion. 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 –
Reaction of Aluminium trichloride (Lewis acid) with chloride ion (Lewis base)– It is a key step of Friedel – Crafts reaction. Aluminium trichloride combines with chloride ion and forms aluminium tetrachloride ion. Reaction is given below –
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 forms 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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