Lewis Acids and Bases

Group 13 elements like boron and aluminum have three valence electrons, forming trivalent compounds with a sextet of electrons and a vacant orbital. These electron-deficient compounds cannot achieve a stable octet independently, so they accept electron pairs from other species to complete their valence shells. This electron-pair acceptance defines them as Lewis acids.

Lewis theory expands acid-base chemistry beyond proton transfer. While Brønsted-Lowry acids donate protons, Lewis acids accept electron pairs. All Brønsted-Lowry acids are protic acids, but Lewis acids can be protic or aprotic. For example, hydrochloric acid is both a Brønsted-Lowry acid and a Lewis acid because its hydrogen accepts electrons from ammonia while losing shared electrons to chlorine.

Aluminum chloride accepts the lone pair of electrons from ammonia's nitrogen atom. The electron pair transfers from ammonia (the Lewis base) to aluminum chloride (the Lewis acid), completing aluminum's octet. This forms a Lewis acid-base adduct where the oppositely charged species attract and bond together.

Boron trifluoride has a significant positive charge on the boron atom due to its vacant orbital, while ammonia's nitrogen carries a negative charge from its nonbonding electron pair. These opposite charges cause them to attract. The lone pair from ammonia fills boron's valence shell, giving it an octet and forming a stable Lewis acid-base adduct.

A Lewis acid-base adduct is the product formed when a Lewis acid accepts an electron pair from a Lewis base. In the adduct, the acid's octet is completed, and formal charges develop on both species to conserve net charge. For example, in the boron trifluoride-ammonia adduct, boron carries a formal negative charge and nitrogen carries a formal positive charge.

Electron-deficient compounds like those from group 13 elements start with fewer than eight valence electrons and cannot satisfy the octet rule alone. They achieve stable octets by accepting electron pairs from Lewis bases in chemical reactions. This electron-pair acceptance is the defining characteristic of Lewis acids and explains why these compounds are reactive.

Yes. Hydrochloric acid demonstrates this dual nature: it is a protic acid because it donates a proton to ammonia, and it is also a Lewis acid because its hydrogen atom accepts an electron pair from ammonia while losing shared electrons to chlorine. Lewis theory encompasses both electron-pair transfer and proton-transfer mechanisms.