Lewis acids and bases

Lewis acids accept electron pairs, while Lewis bases donate them. Their interaction forms a coordinate covalent bond, broadening acid–base chemistry beyond proton transfer.

Coordinate covalent bond: A covalent bond in which both shared electrons are supplied by one atom. A Lewis base supplies both electrons in the bond formed with a Lewis acid.
Gilbert N. Lewis: An American physical chemist who developed the electron-pair theory of covalent bonding and a broader acid–base definition. His bonding theory underlies the names and electron-pair account used here.
Coordination complex: A central atom or ion bonded to surrounding ligands through coordinate bonds. Metal–ligand bonding is a direct application of Lewis base donation to metal acceptors.
Arrhenius acid–base theory: A theory defining acids as substances that increase aqueous hydrogen-ion concentration and bases as substances that increase hydroxide-ion concentration. Its aqueous-ion scope is much narrower than Lewis electron-pair chemistry.
Ammonia: A nitrogen hydride, NH₃, with a lone pair that can form bonds to electron-pair acceptors. Its nitrogen lone pair makes ammonia a familiar Lewis base.
Frontier molecular orbital theory: A model of reactivity based on interactions between molecules’ highest occupied and lowest unoccupied orbitals. The base’s occupied orbital donates into the acid’s available orbital.
Valence electron: An electron in an atom’s outer shell that can participate in chemical bonding. Lewis acid–base behavior tracks the availability and movement of valence electrons.
Friedel–Crafts reaction: An electrophilic aromatic substitution that adds an alkyl or acyl group to an aromatic ring. Aluminum chloride acts as a Lewis acid catalyst that activates the alkylating or acylating reagent.
Lux–Flood acid–base theory: An acid–base theory in which acids accept oxide ions and bases donate oxide ions. It classifies reactions by oxide-ion transfer rather than electron-pair acceptance.
Boron trifluoride: A trigonal planar molecule, BF₃, whose boron atom has an incomplete valence shell. Its electron-deficient boron center makes BF₃ a textbook Lewis acid.

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