Lewis acid

A chemical species that accepts an electron pair to form a covalent bond. The definition includes electron-deficient molecules, cations, and metal centers.

Lewis base: A species that donates an electron pair to form a covalent bond. A Lewis acid accepts the pair that a Lewis base donates.
Brønsted–Lowry acid: A species that donates a proton in an acid–base reaction. Proton donation is narrower than Lewis acidity, which requires only electron-pair acceptance.
Friedel–Crafts reaction: An electrophilic aromatic substitution that attaches an alkyl or acyl group to an aromatic ring. Aluminum chloride commonly acts as a Lewis acid to generate the reactive electrophile.
Electrophile: An electron-poor species that reacts with an electron-rich site to form a bond. Electrophiles often behave as Lewis acids, though the terms emphasize different aspects of reactivity.
Lewis acid–base adduct: A compound formed when a Lewis acid accepts an electron pair from a Lewis base. Adduct formation is the bond-making event at the heart of the definition.
Brønsted–Lowry base: A species that accepts a proton in an acid–base reaction. Many proton-accepting bases also donate an electron pair, but the definitions are distinct.
Diels–Alder reaction: A cycloaddition between a conjugated diene and a dienophile that forms a six-membered ring. Lewis-acid coordination to the dienophile can accelerate the reaction and affect selectivity.
Oxidizing agent: A reactant that accepts electrons and is reduced during a redox reaction. Electron-pair acceptance forms a bond; accepting electrons in redox changes oxidation state.
Coordinate covalent bond: A covalent bond in which both shared electrons originate from the same atom. Lewis acid–base reactions often create this bond when the base donates its pair.
Arrhenius acid: A substance that increases hydronium-ion concentration in aqueous solution. This older definition covers aqueous acidity, while Lewis theory applies beyond water and protons.

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