Lewis acid and base
/ LOO-iss /
The Bronsted-Lowry idea is about protons changing hands. The Lewis idea zooms out one level and watches the electrons instead. A Lewis base is anything that offers up a pair of electrons; a Lewis acid is anything hungry for that pair — an empty space waiting to be filled. When the base hands over its electron pair and the acid accepts it, a new bond forms. It is less a game of catch and more two hands clasping, with the base bringing both fingers.
Why bother with a second definition? Because many important reactions have no proton in them at all, yet behave exactly like acid-base chemistry. Boron trifluoride, BF3, has a boron atom with an empty orbital — no proton to give, but a clear gap waiting for electrons, so it is a Lewis acid. Ammonia, with its lone pair, is a Lewis base; the two snap together to form F3B-NH3. A metal cation like Al3+ pulling on a lone pair, a carbocation accepting electrons from a nucleophile — all of these are Lewis acid-base events. Every Bronsted acid-base reaction is also a Lewis one (the base donates a pair to the proton), but not every Lewis reaction involves a proton.
This broader lens is the quiet backbone of organic mechanism. Nucleophiles are essentially Lewis bases (electron-rich, pair-donors); electrophiles are essentially Lewis acids (electron-poor, pair-acceptors). Once you see a reaction as an electron pair flowing from a Lewis base to a Lewis acid, the curved arrows you draw, the catalysts you choose, and the products you predict all fall into one consistent story.
BF3 + :NH3 -> F3B-NH3. BF3 (empty orbital) is the Lewis acid; ammonia (lone pair) is the Lewis base. No proton moves.
Lewis acid-base chemistry is bond formation by electron-pair donation — broader than the proton picture.
Lewis and Bronsted are not rivals; they are two zoom levels on the same idea. A proton (H+) is itself a Lewis acid — an electron-pair acceptor — so the Bronsted picture is just the special case where the acceptor happens to be H+.