Lewis Acids and Bases

The following table gives the definitions for Acids and Bases using several common definitions. While the Lewis definitions seem different, they are consistent with the more specific types listed. This is due to a change in emphasis. In the Arrhenius and Bronsted-Lowry schemes, the emphasis is on the H⁺ ion. In the Lewis definition, the emphasis is on electron pairs (which carry a negative charge). In the first set of definitions, an atom (H⁺) moves. In the Lewis scheme, electron pairs form bonds, which (conceptually) "pulls" atoms into their new positions.

	Acid	Base
Arrhenius	H⁺ donor	OH^- donor
Bronsted-Lowry	H⁺ donor	H⁺ acceptor
Lewis	e^- acceptor	e^- donor

As an example, consider the following reaction between the borohydride ion and acetone:

BH₄^- + CH₃–C(=O)–CH₃ → BH₃ + CH₃–CH(O^-)–CH₃

In this reaction, the pair of electrons in a B–H bond attacks the central carbon and forms a new C–H bond. (To avoid having too many bonds, the pair of electrons in the C=O p-bond moves to the oxygen to become another lone pair). Thus, BH₄^- is the Lewis base and acetone is the Lewis acid.

Practice Problems

Select the Lewis acid and base (on the reactant side) for each of the following reactions. In general, you will need to draw Lewis structures to determine this. To view the answer, move the mouse over the "Answer" tag and look at the status bar (at the bottom of the screen).

NH₃ + BF₃ → H₃N–BF₃	Answer
CH₃–O^- + CH₃Cl → CH₃–O–CH₃ + Cl^-	Answer
(CH₃)₃O⁺ + H–O–H → CH₃–O–CH₃ + CH₃OH₂⁺	Answer
Cu⁺² + 4 NH₃ → Cu(NH₃)₄⁺²	Answer
CH₂=CH₂ + Br₂ → CH₂(Br)–CH₂⁺ + Br^-	Answer