In 1923, G. N. Lewis (yes, the Lewis structure guy)
|G. N. Lewis|
suggested a way of describing a number of reactions that did not fit the Bronsted definition of acid-base reactions, yet seemed to have some unifying structural features.
This definition allows us to write acid-base reactions like these:
NOTE several things about these reactions:
has pointed out that:
are a little confusing viewed as the acid (FeCl3) being a species lacking an octet - Fe3+ is 4s2, 3d4; and three Cl- add six electrons to give a neutral species.
He suggested that all of these reactions be viewed as involving the interaction of a filled atomic or molecular orbital on the base, and an empty atomic or molecular orbital on the acid, regardless of the octet rule.
Here is this idea pictured in orbital terms:
Look at the formation of a molecule of hydrogen. Here are two H atoms, each with an electron, forming a molecule:
Each atom brings an electron.
This is absolutely general:
Now suppose we bring together a proton, H+, with no electrons, and a hydride ion, H-, with two electrons. The proton is a Lewis acid, and the hydride ion is a Lewis base!
Again, the process occurs because the energy of a pair of electrons is lowered.
In general, the HOMO and the LUMO will not move up or down by the same amount, and the energy diagram will look more like this:
Every step of every reaction we write can be described as a filled-empty orbital interaction! Each step proceeds because a pair of electrons moves to lower energy.
Finally, let's demonstrate that all acid base reactions are really Lewis acid-base reactions.
Here's a typical Bronsted acid-base reaction:
The curly arrows track which bonds are made, and which are broken, but they do not indicate what orbitals are involved.
Here are pictures of the relevant HOMO and LUMO, from AM1 semi-empirical molecular orbital calculations:
|H2O HOMO||HCl LUMO (antibonding)|
The interaction stabilizes the unshared pair of the oxygen, while simultaneously breaking the H-Cl bond because the interaction is with the antibonding orbital.