According to Lewis's concept, a neutralization reaction is fundamentally the formation of a chemical bond between a Lewis acid and a Lewis base, resulting in an addition compound.
Understanding Lewis Acids and Bases
Before diving into neutralization, it's essential to grasp Lewis's definitions of acids and bases, which are broader than other concepts like Brønsted-Lowry:
- Lewis Acid: An electron pair acceptor. These are typically species with incomplete valence shells (like BF₃), positive ions (like H⁺, Al³⁺), or molecules with polar multiple bonds that can accept electron pairs (like CO₂).
- Lewis Base: An electron pair donor. These are usually species with lone pairs of electrons (like NH₃, H₂O, OH⁻, Cl⁻) or molecules with pi bonds.
Lewis Neutralization Explained
Based on these definitions, a Lewis neutralization reaction involves the combination of a Lewis acid and a Lewis base.
The core principle, as highlighted by the reference, is that:
The reaction between an acid and a base (neutralization) results in the formation of an addition compound, in which the electron pair that constitutes the chemical bond comes from only one reactant.
Here's a breakdown:
- Formation of an Addition Compound: The product of a Lewis neutralization is often called an adduct. This is a single species formed by the direct combination of the acid and the base.
- Electron Pair Donation: The Lewis base donates a lone pair of electrons.
- Electron Pair Acceptance: The Lewis acid accepts this electron pair.
- Coordinate Covalent Bond: A coordinate covalent bond (or dative bond) is formed between the acid and the base, where both electrons for the bond come from the base.
This process is distinct from Brønsted-Lowry neutralization, which involves proton transfer (H⁺). Lewis neutralization focuses purely on electron pair movement and the resulting bond formation.
Examples of Lewis Neutralization
Let's look at a few examples to illustrate this concept:
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Reaction between Boron Trifluoride (BF₃) and Ammonia (NH₃):
- BF₃ acts as a Lewis acid (boron has an incomplete octet, it accepts an electron pair).
- NH₃ acts as a Lewis base (nitrogen has a lone pair to donate).
- They form an adduct: BF₃ + NH₃ → F₃B-NH₃ (where the bond between B and N is a coordinate covalent bond).
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Formation of a Hydronium Ion (H₃O⁺):
- A proton (H⁺) acts as a Lewis acid (it has an empty orbital and accepts an electron pair).
- Water (H₂O) acts as a Lewis base (oxygen has lone pairs).
- They form the hydronium ion: H⁺ + H₂O → H₃O⁺
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Reaction between Aluminum Chloride (AlCl₃) and Chloride Ion (Cl⁻):
- AlCl₃ acts as a Lewis acid (aluminum has an incomplete octet).
- Cl⁻ acts as a Lewis base (it has lone pairs).
- They form the tetrachloroaluminate ion: AlCl₃ + Cl⁻ → [AlCl₄]⁻
In each example, the Lewis base contributes the electron pair to form the new bond, resulting in a single adduct species.
Lewis neutralization provides a powerful framework for understanding reactions that don't involve proton transfer but still exhibit acid-base characteristics based on electron pair interactions.
