Lewis theory of acids and bases was published in the same year as the Brønsted-Lowry Acid-Base Theory, but may be considered its generalization. Let’s recall the progression:
- In Arrhenius Theory, the acid is a molecule that produces upon dissociation, and a base is a molecule that produces upon dissociation.
- Bronsted-Lowry comes and says: there’s no dissociation as claimed by Arrhenius, there’s a proton transfer. If a molecule donates a proton, it’s an acid. If a molecule accepts a proton it’s a base. Note that any Arrhenius acid is a Bronsted-Lowry acid as well. A hydroxide ion is a Bronsted-Lowry base.
Lewis theory says: let’s extend the idea of proton transfer to just any electron transfer. Any molecule that donates a lone pair is a Lewis base, and any molecule that accepts the lone pair is a Lewis acid.
You may notice that any Bronsted-Lowry base, by definition, is a Lewis base. It has to have a lone pair to abstract a proton, so you could see the proton transfer in Bronsted-Lowry as an electron transfer between a Bronsted-Lowry base and , which would be considered a Lewis acid.
The Lewis Acid-Base theory is definitely an orgo territory and this note is given here only for completeness.