1923

Acids and Bases

Thomas Martin Lowry (1874–1936), Gilbert Newton Lewis (1875–1946), Johannes Nicolaus Brønsted (1879–1947)

The year 1923 saw the publication of two general theories about the nature of acids and bases. Danish chemist Johannes Nicolaus Brønsted and English chemist Thomas Martin Lowry, working independently, both defined acids as compounds that gave up hydrogen atoms and bases as compounds that accepted them. Now known as the Brønsted-Lowry acid-base theory, this definition was more general than that formulated by Swedish scientist Svante Arrhenius, who had thought of everything in terms of hydrogen ions and hydroxide ions in aqueous solutions. But American chemist Gilbert Newton Lewis flipped things around the same year in a paper that extended the concept beyond what anyone had imagined. Instead of positively charged hydrogen ions being the currency of exchange, he looked at acids as being acceptors of negatively charged electron pairs and bases as donors of them.

This idea came from his definition of a chemical bond, which was the sharing of two electrons between two atoms. A covalent bond, in Lewis terms, was what you had when each atom contributed an electron on equal terms, and a “coordinate” or “dative” bond was when one of the atoms donated both electrons to the new bond. The prototype Lewis acid is boron trifluoride—with three fluorines bound to it, the boron atom in the middle is parched for electrons and will vacuum up electron pairs wherever it can find them, tightly coordinating on atoms (like oxygen or nitrogen) that have them to spare.

This meant that “Lewis acids” could behave similarly to more traditional acids in many reactions, despite having no hydrogen atoms to donate. (Strong ones aren’t much good in aqueous solution, though, since they go berserk sticking to all the water molecules and become mostly inactivated.) Lewis-acid based reagents are used in catalytic reforming in the oil industry, the Friedel-Crafts reaction, Ziegler-Natta catalysis for plastics, and more. Interestingly, while many electron-deficient metal compounds are good Lewis acids, they all seem to have slightly different personalities, with different affinities for oxygen and other atoms. A wide range of them can be tried to see which ones make a reaction go most quickly and cleanly.

Lewis in his lab at Berkeley, where he was working when he died in 1946. He is widely considered one of the most influential chemists never to have won the Nobel Prize.