9.1.1 Lewis Concept of Acids and Bases

This topic explains the Lewis concept of acids and bases. It describes how Lewis acids and bases are defined in terms of electron pair acceptance and donation.

Definition

Lewis Acid: A substance that can accept a pair of electrons to form a coordinate covalent bond.

Lewis Base: A substance that can donate a pair of electrons to form a coordinate covalent bond.

Main Idea

Certain substances like SO₂, CO₂, CaO, BF₃, etc., behave as acids or bases although they do not have the ability to donate or accept protons. The nature of such substances cannot be explained by Arrhenius theory or the Brønsted-Lowry theory. In 1923, G.N. Lewis proposed an acid-base theory that focuses on reaction. This concept is more general than either the Arrhenius theory or the Brønsted-Lowry theory.

Core Concept

• Lewis acid: Electron pair acceptor.
• Lewis base: Electron pair donor.
• Coordinate covalent bond: Formed between Lewis acid and Lewis base.
• Lewis acid: Central atom has incomplete octet.
• Lewis base: Central atom has complete octet with lone pair(s).

Detailed Explanation

What is the Lewis Concept?

• Certain substances like SO₂, CO₂, CaO, BF₃, etc., behave as acids or bases although they do not have the ability to donate or accept protons.
• The nature of such substances cannot be explained by Arrhenius theory or the Brønsted-Lowry theory.
• In 1923, G.N. Lewis proposed an acid-base theory that focuses on reaction.
• This concept is more general than either the Arrhenius theory or the Brønsted-Lowry theory.

Definitions

• A Lewis acid is a substance that can accept a pair of electrons to form a coordinate covalent bond.
• A Lewis base is a substance that can donate a pair of electrons to form a coordinate covalent bond.
• In a Lewis acid-base reaction, a coordinate covalent bond is formed between the acid and the base.

Example: Reaction of HCl with H₂O

HCl + H₂O ⇌ H₃O⁺ + Cl^-

• Electron pair acceptor: H⁺ (from HCl)
• Electron pair donor: H₂O (oxygen has lone pairs)

Questions: (i) Which species is donating an electron pair? → H₂O (ii) Which species is accepting an electron pair? → H⁺ (iii) Which species is a Lewis acid? → H⁺ (iv) Which species is a Lewis base? → H₂O

Example: Reaction of NH₃ with HCl

NH₃ + HCl → NH₄⁺ + Cl^-

• Nitrogen atom in ammonia donates an electron pair to the H-atom in HCl.
• Lewis acid: HCl (H⁺)
• Lewis base: NH₃

Requirements for Lewis Acid and Base

• The Lewis structure demands that the central atom of a Lewis acid has a deficiency of an electron pair and can accommodate an unshared electron pair.
• On the other hand, the central atom of a Lewis base has a complete octet possessing one or more unshared electron pairs.
• Hence, a base has the ability to donate an unshared electron pair.

Example 9.1: Classifying Substances as Lewis Acids or Lewis Bases

Identify the Lewis acid and Lewis base in the following reactions.

Reaction: NH₃ + BF₃ → NH₃BF₃

Problem Solving Strategy:

• Draw electronic structures of both the species.
• Look for the species that has a lone pair of electrons or negative charge. Such a species has a complete octet, so it does not need any electron. It can, however, donate an electron pair. This species is a Lewis base.
• Look for the species that can accommodate an electron pair. Such a species has an incomplete octet. So this species can accept an electron pair. This species is a Lewis acid.

Solution:

• NH₃ has a lone pair on N-atom. So it is an electron pair donor. NH₃ is a Lewis base.
• Boron in BF₃ has an incomplete octet. It has six electrons (3 electron pairs). So it needs an electron pair to complete its octet. Hence BF₃ is an electron pair acceptor or Lewis acid.

Concept Assessment Exercise 9.1

Identify the Lewis acid and the Lewis base in the following examples.

• Cl^- + AlCl₃ → [AlCl₄]^-
• H⁺ + OH^- → H₂O

Intermediate Level

Lewis Acids and Bases

Example 9.1

Reaction: NH₃ + BF₃ → NH₃BF₃

• NH₃: Lone pair on N → Lewis base.
• BF₃: Incomplete octet on B → Lewis acid.

Concept Assessment Exercise 9.1

• Cl⁻: Lewis base (lone pairs)
• AlCl₃: Lewis acid (incomplete octet)

• OH⁻: Lewis base (lone pairs)
• H⁺: Lewis acid (empty orbital)

Important Points

• 1. Lewis acid: Electron pair acceptor.
• 2. Lewis base: Electron pair donor.
• 3. Coordinate covalent bond formed.
• 4. Lewis acid: Incomplete octet.
• 5. Lewis base: Complete octet with lone pair(s).
• 6. Examples: NH₃ (base), BF₃ (acid).
• 7. Concept Assessment 9.1: Cl⁻/AlCl₃, OH⁻/H⁺.

Remember

• - Lewis acid: Electron pair acceptor
• - Lewis base: Electron pair donor
• - Coordinate covalent bond: Formed between them
• - Lewis acid: Incomplete octet
• - Lewis base: Complete octet with lone pair(s)
• - Examples: NH₃ (base), BF₃ (acid)

Common Mistakes

• 1. Confusing Lewis acid with Brønsted acid: Lewis acid = electron pair acceptor; Brønsted acid = proton donor.
• 2. Forgetting that Lewis bases have lone pairs: Essential for donation.
• 3. Thinking all acids contain H: Lewis acids do not need H.
• 4. Mixing up electron pair donor and acceptor: Base = donor; Acid = acceptor.

Quick Revision

• Lewis acid: Electron pair acceptor.
• Lewis base: Electron pair donor.
• Coordinate covalent bond formed.
• Lewis acid: Incomplete octet.
• Lewis base: Complete octet with lone pair(s).
• Examples: NH₃ (base), BF₃ (acid).
• Concept Assessment 9.1: Cl⁻/AlCl₃, OH⁻/H⁺.

60-Second Revision

• Lewis acid: Electron pair acceptor
• Lewis base: Electron pair donor
• Coordinate covalent bond: Formed between them
• Lewis acid: Incomplete octet
• Lewis base: Complete octet with lone pair(s)
• Examples: NH₃ (base), BF₃ (acid)

QUALITY VALIDATION

• Topic: 9.1.1 Lewis Concept of Acids and Bases
• Chapter: 9 (Acids-Bases Chemistry)

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