Lewis’ Definition of Acids

Gilbert Lewis, who developed Dot Structures as a way to show the bonded structure of molecules, also had something to say about acids and bases. His definition is the most comprehensive definition, but also the most cumbersome to work with.

The best way to understand it is to start with a reaction. We’ll look at the reaction between HCl and water:

HCl + H2O → H3O+1 + Cl-1

Of course, Lewis doesn’t see the reaction that way. What he “sees” is this:

If we then apply the Brønsted/Lowry definition of acids and bases we can see that an H+1 (a proton) was transferred from the HCl to the water, making hydronium (H3O+1). 

What interested Lewis however was how this worked. Specifically, where did the bond indicated in the diagram below come from?

As you can see by comparing the Lewis Dot Structure for the water on the left and the hydronium on the right, that bond must have been formed using the electron pair that had been on the top of the water. 

Please recognize that this is weird! When we think about bonds forming, we imagine each atom bringing in one electron and the two nuclei then pulling on both electrons which ties the atoms together. Here, one atom (the oxygen) had 2 electrons (the lone pair) and the other atom (actually the H+1 ion) brought NO electrons with it. The bond was formed because the H⁺¹ ion (remember, just a proton) pulled so hard on the oxygen’s electrons that it forced a bond to be formed.

Lewis coined a special term for this type of bond (where one atom brings both electrons to the bond). He called it a Coordinate Covalent Bond. It’s important to realize that once this bond has formed it is exactly the same as any other covalent bond, the name is ONLY about the formation of the bond. That means that when this reaction runs backwards, any one of the O-H bonds may break.

As a result of this perspective, Lewis pictures the oxygen atom and its electron pair as the ‘active” part of this process. He therefore defines acids and bases from that perspective. According to Lewis:

A Base is an electron pair donor

and

An Acid is an electron pair receiver

This approach to acids and bases does several things for us. 

First, if a base is something that can donate a pair of electrons we can then see potential bases in a multitude of compounds. In fact, any compound that has an oxygen atom or a nitrogen atom on the outside will have some potential to act as a base by forming a bond with the lone pairs of electrons present. 

Secondly, we can understand on a deeper level what occurs in certain special synthesis reactions, often involving Boron.

Take a look at this reaction:

BF3 + PI3 → BF3PI3 

This is not a common reaction and it was probably not obvious what the product would be. However , if we think of this reaction as a Lewis Acid/Base reaction, it is much easier to see. 

Here are the Lewis Dot Structures for BF3 and PI3.

The boron compound is unstable, as the central atom does not have an octet. The phosphorus has a lone pair of electrons. What occurs is that the boron atom pulls on the electron pair and forms a coordinate covalent bond, resulting in the Lewis Dot Structure shown here.

Pros and Cons of the Lewis definition:

Pro: The most comprehensive definition (it includes any reactions the other definitions describe and more)

Pro: a deep explanation of how the reaction occurs

Pro: helps us predict the basic behavior of certain compounds

Con: requires the drawing of Lewis Dot Structures which is time consuming

Con: Does not add anything of consequence for most acid/base reactions