Basicity of Aromatic Heterocycles

Concept: Concept: Heterocycles - Which lone pairs react?

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Video Transcript

So far, we haven't learned any reactions that aromatic compounds undergo because they're so crazy stable. They don't react with anything really. However, you might be asked to react an aromatic heterocycle with a strong acid.
In that case, this is going to be an acid-base reaction and nothing more. Nothing actually happens to the aromatic ring. We’re just doing an acid-base reaction with lone pairs on the outside of the ring. Let’s see how this works.
Heterocycles, as you have seen previously, often have multiple lone pairs that are available to react with acids. The question that we have to ask ourselves and the question you're going to be asking yourself in the exam is which lone pair do I react with? Because it can be very confusing. These lone pairs don't just say “Me, me, me!” You have to think about it. You have to conclude which lone pair is going to want to react with the acid.
Let's take this example. Here I have a molecule called imidazole. It has two nitrogens with lone pairs. It's got this nitrogen with the red lone pair and this nitrogen with the blue lone pair. I'm reacting this with a strong halohydric acid, HX. This could be HCl. It could be whatever. We know that this lone pair is going to be attracted to which atom? That one of the lone pairs would be attracted to which atom on the HX? You've got this incredibly strong dipole. You’ve got a partial negative and a partial positive.
These lone pairs our basic. They're going to be tempted to attack the H and do basically a proton grab. They’re going to be attracted to the proton. The question is will the H attach to the red N or will it attach to the blue N or will it attach to both?
How do we solve this question?

Concept: Example: Draw the acid/base reaction product

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