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Acids and bases are familiar concepts in chemistry, yet the term Lewis acid refers to a distinct class of reactants. While the Brønsted–Lowry framework focuses on proton transfer, G.N. Lewis expanded the definition to encompass any reaction where electron pairs are exchanged, thereby including non‑protonic processes.
Lewis acids accept electron pairs, whereas Lewis bases donate them. An electron‑deficient or positively charged species typically functions as a Lewis acid.
In 1923 Lewis demonstrated this principle using a hydrogen ion (H⁺) and the hydroxide ion (OH⁻). Whereas Brønsted–Lowry theory describes OH⁻ accepting a proton to form water, Lewis viewed the hydrogen ion as an electron acceptor that forms a covalent bond with the electron pair from hydroxide.
A Lewis acid is any chemical species capable of forming a covalent bond by accepting an electron pair from another species. Often, these acids possess vacant orbitals that can accommodate the incoming electrons.
Conversely, a Lewis base is a species that donates an electron pair to form a covalent bond. The relationship between Lewis acids and bases is analogous to the classic acid–base pairing but is defined by electron pair transfer rather than proton transfer.
Examples of Lewis acids include metal cations such as Al³⁺ and Fe³⁺, whose positive charge strongly attracts electron density.
A Lewis acid catalyst accelerates a chemical reaction by accepting electrons from a substrate, thereby increasing its reactivity. Importantly, the catalyst itself is not consumed in the reaction; it is regenerated at the end of the catalytic cycle.
AlCl₃ is a textbook Lewis acid. Aluminum has 17 valence electrons, leaving an empty p‑orbital that can accept an electron pair. This electron‑accepting ability enables AlCl₃ to facilitate numerous organic transformations.
Ammonia contains a lone pair of electrons on nitrogen, allowing it to donate that pair to an electron‑accepting species. When NH₃ reacts with HCl in aqueous solution, the nitrogen donates electrons to H⁺, forming NH₄⁺.
Both the Brønsted–Lowry and Lewis concepts describe related chemical behavior but from different perspectives—proton transfer versus electron pair transfer. Understanding both frameworks provides a comprehensive view of acid–base chemistry.
