Lewis Acids and Bases

The Brønsted-Lowry model defines acids as proton donors and bases as proton acceptors. The Lewis model defines acids as electron-pair acceptors and bases as electron-pair donors. Lewis theory is broader, classifying more compounds as acids, including those without ionizable protons. Brønsted-Lowry acid-base reactions represent a subcategory of Lewis reactions where the acid is specifically H+.

Boron trifluoride cannot be classified as a Brønsted-Lowry acid because it lacks hydrogen. However, the boron atom has only six electrons in its valence shell, creating an incomplete octet with an empty orbital. This empty orbital accepts electron pairs from Lewis bases like ammonia, allowing boron trifluoride to act as a Lewis acid.

A Lewis acid-base adduct is the product formed when a Lewis acid accepts an electron pair from a Lewis base, creating a coordinate covalent bond. In this bond, one atom provides both bonding electrons. For example, when ammonia donates an electron pair to a hydrogen ion, the resulting ammonium ion is a Lewis acid-base adduct.

Carbon dioxide can rearrange its electrons to act as a Lewis acid. An electron pair moves from the carbon-oxygen pi bond to the terminal oxygen, creating an empty orbital on the carbon atom. This empty orbital then accepts an electron pair from a water molecule, which acts as a Lewis base, forming carbonic acid.

Small metal cations like Al(III) act as Lewis acids by accepting electron pairs from Lewis bases. For example, Al(III) accepts lone pairs of electrons from water molecules, which donate electron pairs and act as Lewis bases. This interaction forms hexaaquaaluminum ions, demonstrating how metal cations participate in coordinate covalent bonding.

A coordinate covalent bond, also called a dative bond, forms when one atom provides both bonding electrons to the bond. This occurs in Lewis acid-base reactions when a Lewis base donates an electron pair to a Lewis acid. Examples include water combining with a hydrogen ion to form a hydronium ion.

The Lewis model allows scientists to classify a greater number of compounds as acids, including those without ionizable protons. Any compound with an empty orbital that can accept an electron pair qualifies as a Lewis acid. This broader definition encompasses nonmetal oxides, metal cations, and other electron-deficient species that the Brønsted-Lowry model cannot classify as acids.