Aromaticity of Heterocycles

Concept: Concept: Heteroatoms

4m
Video Transcript

There's only one more type of molecule that we need to be able to predict aromaticity for. That’s heterocycles.
What is a heterocycle? That’s just going to be any ring that contains at least one heteroatom within it. Recall what a heteroatom is. That’s just going to be any non-carbon atom. That could be something like nitrogen or oxygen but also phosphorus or sulfur. These are all very common atoms we found within rings. When you that, it's called a heterocycle.
One of the best examples of a heterocycle that I could think of is pyridine. Pyridine was a base that we commonly used in Organic Chemistry 1. If you guys remember, it had this basic lone pair that could be used for reactions especially for acid-base type reactions. Later on in this section, we’re actually going to discuss why that lone pair is basic.
But for right now, we have to understand that heterocycles are going to present one extra complication to figuring out aromaticity which is that typically a heteroatom is going to have one or more lone pairs on it. The question is going to be “Do I count that lone pair towards the pi conjugated system?” For pyridine, would I go ahead and count this lone pair towards the total sum of electrons to determine Huckel’s Rule? Or would I ignore it?
It turns out that it's not a clean and simple rule. There’s actually a few steps that you have to go through to figure that out. It's not just that they either donate or that they don't donate. There's situations in the middle.
Let's look at what the rules are. Heteroatoms can choose to donate up to one lone pair each. That means for example, oxygen has two lone pairs but only one of them is able to be donated into the ring. Why would oxygen want to donate one of its lone pairs to the ring? Let's take a look.
One, the oxygen or whatever heteroatom already has to be sp3-hybridized to do this. That means that if it was sp2 or sp-hybridized, then that lone pair is definitely not getting donated. It’s only getting donated if the atom was sp3 to begin with.
But that's not the only thing. We have a second criteria. One, the heteroatom needs to be sp3. But two, you’re only going to donate if it helps to create aromaticity. Meaning that you're not going to donate a lone pair if it goes against Huckel’s Rule and if you wind up getting a number of pi electrons that makes it anti-aromatic or nonaromatic. You would only donate if it makes it aromatic.
We could just go back to this example of pyridine. I’ve already given you some clues. Why don’t you guys try to solve for question A, whether you think that pyridine is an aromatic compound or not? But also predict whether you think this lone pair will donate to the ring or won't donate. Basically, should this lone pair count towards Huckel’s Rule, or should you just ignore it?
Go ahead and try to use those tools and then I'll explain the logic behind it. It’s all your turn now.

Concept: Example 1: Determine heterocycle aromaticity

2m
Video Transcript

First of all, do you think a pyridine is aromatic? I gave that one away already so yeah, it’s aromatic. But how so? Why is it aromatic? Is that lone pair going to donate or not? Actually for two reasons it's not going to donate its lone pair. First of all, let's look at the hybridization of this nitrogen. What is that hybridization? That hybridization is sp2. I told you as explicitly that you're never going to donate a lone pair unless it's sp3. This one cannot donate so I’m not even going to consider it.
Second of all, even if it was sp3 and it donated, how many pi electrons would you then have? We already have 2, 4, 6. If I were to donate these electrons to the ring, I would get 8. For two reasons, that lone pair is just going to sit there and it's going to be highly accessible. It's not going to be involved with the ring at all. The answer was this would have 0 lone pairs donated.
Go ahead. Try to do the second problem. Try to use the same logic and predict what the aromaticity would be.

Concept: Example 2: Determine heterocycle aromaticity

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Concept: Example 3: Determine heterocycle aromaticity

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Concept: Example 4: Determine heterocycle aromaticity

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Concept: Example 5: Determine heterocycle aromaticity

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Concept: Example 6: Determine heterocycle aromaticity

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Concept: Example 7: Determine heterocycle aromaticity

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Concept: Example 8: Determine heterocycle aromaticity

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Concept: Example 9: Determine heterocycle aromaticity

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