Acids and Bases: Lewis Theory
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

Molecular Lewis Acids

There are molecular compounds (such as carbon dioxide and sulfur dioxide) that are able to neutralize basic oxides and hydroxides. These neutralization reactions can not be described using either the Arrhenius or Brønsted theories since they do not involve protons. The chemical equation for the reaction of carbon dioxide neutralizing the strong base calcium oxide is shown below:

CaO(s) + CO2(g) CaCO3(s)

The Lewis Theory provides the best description for reactions such this. Carbon dioxide is a polar molecule whose positive center is on the carbon atom:

This positive center is able to attract (and accept) the lone electron pairs present on the oxide ion (O2-). Thus, carbon dioxide is acting as a Lewis acid and the oxide ion is acting as a Lewis base.

In the following reaction, is BH3 acting as a Lewis acid or base? (Hint: Draw the Lewis structures for both reactants)

(CH3)3N + BH3 (CH3)3N __ BH3
Lewis acid
Lewis base

Good! The Lewis structure of BH3 shows boron having only three bonds and no lone pairs of electrons allowing it to accept electrons from a donor. BH3 is acting as a Lewis acid, accepting a pair of electrons from (CH3)3N to form a bond.

(CH3)3N has a lone pair of electrons on the nitrogen. Is this characteristic of a Lewis acid or base?