A fundamental principle: all steps of all heterolytic reaction mechanisms are either Bronsted or Lewis acid-base reactions
- They involve either proton transfer (Bronsted), or unshared pair/empty orbital interactions (Lewis).
- When the interacting atomic orbitals are considered, the Bronsted reactions can be seen as simply a special case of the Lewis, in which the empty orbital is the antibonding orbital of the H-X bond.
In short, all heterolytic reactions are just examples of interactions between filled atomic or molecular orbitals and empty atomic or molecular orbitals - that is, Lewis acid-base reactions. Here is a diagram to explain this point:
The interaction of any two atomic or molecular orbitals, as you learned in general chemistry, produces two new orbitals.
- One of the new orbitals is higher in energy than the original ones (the antibonding orbital), and one is lower (the bonding orbital).
- When one of the initial orbitals is filled with a pair of electrons (a Lewis base), and the other is empty (a Lewis acid), we can place the two electrons into the lower energy of the two new orbitals.
- The "filled-empty" interaction therefore is stabilizing.
When we are dealing with interacting molecular orbitals, the two that interact are generally
- The highest energy occupied molecular orbital (HOMO) of one molecule,
- The lowest energy unoccupied molecular orbital (LUMO) of the other molecule.
- These orbitals are the pair that lie closest in energy of any pair of orbitals in the two molecules, which allows them to interact most strongly.
- These orbitals are sometimes called the frontier orbitals, because they lie at the outermost boundaries of the electrons of the molecules.
Here is the filled-empty interaction redrawn as a HOMO-LUMO interaction.
Let's look at some examples. First, a reaction that you would have categorized as a Lewis acid-base reaction when you were studying general chemistry:
NH3 has an unshared pair on nitrogen, occupying the HOMO (it is generally true that unshared pairs occupy HOMOs). BH3 has an empty valence orbital on B, since B is a Group II element. This is the LUMO.
Here are pictures of the two orbitals from AM1 semi-empirical molecular orbital calculations:
| NH3 HOMO | BH3 LUMO |
|---|---|
 |  |
The HOMO-LUMO energy diagram above describes the formation of a bond between N and B.
Now let's try a slightly more complex case. 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.
- Water is both a Bronsted base (capable of accepting a proton) and a Lewis base, with one of its unshared pairs (the HOMO).
- H-Cl is a Bronsted acid, capable of donating a proton, but it also is a Lewis acid, using the s* orbital of the H-Cl bond (the LUMO).
- Here are pictures of the relevant HOMO and LUMO, again from AM1 semi-empirical molecular orbital calculations:
H2O HOMO HCl LUMO 
- The interaction stabilizes the unshared pair of the oxygen, while simultaneously breaking the H-Cl bond because the interaction is with the antibonding orbital.
Here are the relevant orbitals:
| OH- HOMO | CH3-Cl LUMO |
|---|---|
 |  |
The interaction stabilizes the unshared pair of the oxygen, while simultaneously breaking the CH3-Cl bond because the interaction is with the antibonding orbital.
Other examples include the reaction of alkenes with H-X, where the HOMO is the p MO of the alkene and the LUMO is the H-X s* orbital:
and the capture of the carobcation in an SN1 reaction by nucleophile:
You should need no reminder that the carbocation is stabilized by a filled-empty interaction between the empty p orbital of the positive carbon and the s orbital of an adjacent C-H or C-C bond
In short, all heterolytic reactions proceed because the energy of a pair of electrons is lowered by the interaction of a filled atomic or molecular orbital with an empty one.
The same reasoning can be appllied to bimolecular pericyclic reactions like the Diels-Alder cycloaddition.
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