Lewis Acid-Base Theory
Skills to Develop
- Define Lewis acids and Lewis bases
Previously we said that an acid produces H + when dissolved in water, and a base produces OH – when dissolved in water. Then the acid and base (meaning H + and OH – ) can react (without redox) to make water. This is a pretty good definition, but it is kind of small.
Lewis explained that in many reactions that form new bonds, both electrons in the new bond come from 1 atom (or 1 reactant) only, instead of 1 electron coming from each. He called all these reactions acid-base reactions . The picture shows water forming from the elements, in a redox process, and water forming from hydrogen ion and hydroxide ion, in an acid base process. It also shows how the tetrafluoroborate ion, BF 4 – can form from boron trifluoride and fluoride ion.
In general, if we can draw a good Lewis structure by making two molecules or ions share an electron pair, it's likely that the reaction can happen. For instance, BH 3 can react with NH 3 , because N has an extra lone pair, and B only has 6 electrons and 3 connected atoms. So a Lewis acid is something than can fit 2 more electrons from a different molecule. It can share another molecule's lone pair. A Lewis base is any molecule or ion with a lone pair to share. It's easy to see what can be a Lewis base just by drawing a Lewis structure. Lewis acids are usually cations, like H + or Al 3 + . Boron is a famous Lewis base because it often makes electron-deficient compounds, like BH 3 , in which it only has 6 electrons. Try drawing Lewis structures for an acid-forming anhydride combination reaction . Is the anhydride a Lewis acid or base?