What are Lewis acids?

 Hello fam, yesterday, we began our study on the Lewis idea of acids. We said yesterday that acids in the Lewis sense must contain an atom that has an empty orbital. This orbital must be able to accommodate a lone pair of electrons. Recall the analogy of Mr. Smith yesterday? The entire compound of No. 38 Bronx street can be regarded as the atom as a whole. All the floors of the sky scrapper located on the property can be seen as the energy levels in the atom. The eight floor is the arena where we are most likely to find Mr. Smith. This could be the energy level where an electron occupies. The rooms on the floors where individual offices are located are the energy sublevels (orbitals) within that energy level.


What if all the rooms on the eight floor in the building at No. 38 Bronx street are not occupied? There just happens to be one room that is empty where we can gather and eat lunch if we like without disturbing the work of the organization that occupies the complex? That free room can be seen as an empty orbital.


Now back to the idea of a Lewis acid, let us look at boron trifluoride(BF3). First of all, let us look at the central atom in the compound which is boron. How do I know that boron is the central atom? Boron has the least subscript attached to it and it is the least electronegative atom in the compound. Note that the subscript of boron is 1 and this is why we didn’t write it explicitly. 


What is the electron configuration of boron? Here we go!

1s2 2s2 2px1 2py0 2pz0

I have written the electron configuration of boron in this way to buttress my point. I have shown that only one of the degenerate p orbitals is occupied.


Imagine that the 2px and the 2py orbitals are involved in the formation of BF3(since each orbital must hold a maximum of two electrons). We still have a free 2pz orbitals that can accommodate a pair of electrons. Do you remember that we said that there must be an atom in a Lewis acid that contains an empty orbital?


Stay with me for one moment let’s look at AlCl3. The electronic configuration of aluminum(the central atom) is; 1s2 2s2 2p6 3s2 3px1 3py0 3pz0. Does this sound more like the BF3 that we considered just a while ago? You can guess just as well as I can that the 3pz orbital would be empty on the aluminum central atom and this would make AlCl3 a good Lewis acid. Are we getting somewhere? Let me know via your feedback. See in in tomorrow’s class.


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