Acid-Base Theories

From WikidChem

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Slide 34

We can ignore most mixing of orbitals that does not affect overall energy or overall electron distribution. This is in the following three cases: when both of the molecular orbitals will both be vacant or filled, there is poor energy match, or there is poor overlap. When both MOs are filled, the higher energy from one orbital and the lower energy from the other effectively cancel each other out. And when both are vacant, there are no electrons to worry about. When there is poor energy match, the energy difference is so much that it is unlikely for bonding to occur. When there is poor overlap, there is not enough constructive interference between orbitals to actually create a bond, which is also seen in Dunitz's experiment where the overlap of the "missing bond" is so insignificant X-ray diffraction cannot actually determine its existence.

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Slide 35

As said previously, we can ignore most cases of mixing. However, when there is mixing between a unusually high HOMO and an unusually low LUMO, there is reactivity.

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Slide 36

This slide shows the different acid/base theories over time and how they have become more generalized as more is discovered about chemistry. The first theory was proposed by Antoine Lavoisier in 1789, where an acid is defined as an oxidized substance and a base is defined as the substance to be oxidized. The next theory was advanced by Arrhenius and his theory defines an acid as the source of H⁺ and a base as the source of OH^-. Brønsted and Lowry advanced the next theory where an acid is a H⁺ donor and a base is an H⁺ acceptor. At the same time, Lewis presented the theory that defined an acid as an electron-pair acceptor (called an electrophile) and a base as an electron-pair donor (called a nucleophile). The most recent and most general of the theories listed is the HOMO/LUMO theory which defines an acid as having an unusually low LUMO and a base as having an unusually high HOMO.