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Lewis Acid

Lewis acid: electron pair acceptor; e.g., BF₃, AlCl₃, H⁺, Fe³⁺
Quick Reference
Formula / NotationLewis acid: electron pair acceptor; e.g., BF₃, AlCl₃, H⁺, Fe³⁺
Also Known AsLewis acid, electron acceptor, electrophile (related), HSAB acid

What is Lewis Acid?

A species that can accept an electron pair from a donor (Lewis base) to form a coordinate bond. Lewis acids are electrophiles. Examples include BF₃, AlCl₃, and metal cations. The Lewis definition of acids is broader than the Brønsted-Lowry definition.

Formula & Notation

Lewis acid: electron pair acceptor; e.g., BF₃, AlCl₃, H⁺, Fe³⁺

Other Names / Synonyms: Lewis acid, electron acceptor, electrophile (related), HSAB acid

Properties & Characteristics

A Lewis acid is any species that can accept an electron pair from a Lewis base to form a coordinate covalent (dative) bond. This definition (G.N. Lewis, 1923) extends beyond Brønsted acid-base theory to include reactions without proton transfer. Lewis acids include: electron-deficient compounds (BF₃, AlCl₃, SO₃), metal cations (Fe³⁺, Cu²⁺, Zn²⁺), and cations that accept electrons (H⁺ acting as Lewis acid). Hard/soft acid-base theory (HSAB) further classifies Lewis acids.

Uses & Applications

Lewis acid catalysis is ubiquitous in organic chemistry: Friedel-Crafts alkylation and acylation (AlCl₃ or BF₃ as catalyst), epoxide ring opening, Diels-Alder reactions (TiCl₄ as Lewis acid activator), zeolite acidity in petroleum cracking, and in polymerisation catalysis (Ziegler-Natta Lewis acid metal centres).

Safety Information

Many Lewis acids are corrosive and react vigorously with water: BF₃ hydrolyses to boric acid and HF; AlCl₃ + H₂O → Al(OH)₃ + HCl with evolution of heat and HCl gas. Use in anhydrous conditions with adequate fume hood ventilation. Strong Lewis acids (SbF₅, AsF₅) are extremely corrosive.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Lewis Acid
Formula Lewis acid: electron pair acceptor; e.g., BF₃, AlCl₃, H⁺, Fe³⁺
Synonyms Lewis acid, electron acceptor, electrophile (related), HSAB acid

Frequently Asked Questions

A species that can accept an electron pair from a donor (Lewis base) to form a coordinate bond. Lewis acids are electrophiles. Examples include BF₃, AlCl₃, and metal cations. The Lewis definition of acids is broader than the Brønsted-Lowry definition.

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