spectroscopy quiz: on aromaticity (huckel's rule)

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lotus0618
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spectroscopy quiz: on aromaticity (huckel's rule)

Post by lotus0618 » Wed Jan 15, 2020 4:40 pm

So i came across this question on the quiz, can you please show me how to identify which electrons are pi electrons (aside from the obvious pi electrons in the pi bond of a conjugated ring). For instance, how am I supposed to know if the lone pairs on this ring count towards aromaticity or not. Ex: the picture (attached with this question)-why are there 6 electrons in this? but pyridine's lone pair is considered to be pi electrons? thank you!
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NS_Tutor_Mathias
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 12:43 am

Right, lone pairs can participate in pi bonds, and the oxygen on furan is therefore going to be sp2 hybridized and its lone pair will be participating in resonance. They'll pretty much always count >if< they can be used to satisfy Hueckel's rule. I don't think the MCAT features a lot of cases where you'll need to overthink this - just keep an eye out mostly for lone pairs in rings, and if they can turn it aromatic, they will.

The most common exception would be cases where the atom whose lone pair you care about is already sp2 hybridized. But that is honestly too rare of a case to worry about, so don't.
lotus0618
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by lotus0618 » Thu Jan 16, 2020 4:21 pm

Wait, I still don't understand why there aren't 8 electrons instead of 6 electrons in a furan...
So long as the lone pair can participate in the resonance, we will consider them to be pi electrons? If this is the case, why don't we consider the lone pair on N of Pyridine to be pi electrons and thus Pyridine would satisfy the Huckel's rule?
NS_Tutor_Mathias
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 7:11 pm

The system just trends towards low energy states. So you don't have to worry about 'extra' lone pairs gumming up the works. It just grabs the ones that are actually helpful and has those participate in pi bonds.

So for example, purine is an aromatic heterocycle:
https://en.wikipedia.org/wiki/Purine

But it has waaaaaaaaaaay too many lone pairs by your reasoning. The answer to that is easy: Take only the ones you need to fulfill Hueckel's rule. On the flipside, you can of course not decide to count or not-count already existing pi bonds. Those must be counted.
lotus0618
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by lotus0618 » Thu Jan 16, 2020 8:25 pm

I know this concept is supposed to be easy but it's harder for me to understand your explanation via texts here. Based on what you said, then all conjugate rings could satisfy the Huckel's rule then cause I can decide if i want to count the electron pairs or not. With that furan molecule, we decide to consider only on extra lone pair to make a total of 6 instead of 8 cause that would give us an integer for n. Is this what you meant?
NS_Tutor_Mathias
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 11:00 pm

lotus0618 wrote:
Thu Jan 16, 2020 8:25 pm
I know this concept is supposed to be easy but it's harder for me to understand your explanation via texts here. Based on what you said, then all conjugate rings could satisfy the Huckel's rule then cause I can decide if i want to count the electron pairs or not. With that furan molecule, we decide to consider only on extra lone pair to make a total of 6 instead of 8 cause that would give us an integer for n. Is this what you meant?
Pretty much. Not sure what you mean with 'conjugate rings' (a conjugated ring system is the definition of aromaticity, yes), but you are totally on the right track. Aromatic situations are more energically favorable, so they'll happen whenever they're easily possible. But having excess lone pairs doesn't preclude aromaticity. Those extra lone pairs just don't participate in the conjugated system, that is all.

If visual problems help you work through this, look at hypoxanthine below - the aromatic polyheterocycle. Notice how many excess lone pairs it has, versus just the 10 electrons that must be participating in resonance. Edit: Looking at it for an extra second, the lone pairs from the extra sp2 hybridized nitrogens cannot participate in resonance anyway. That is entirely my bad. Hypoxanthine turns out to be fairly straight-forward, since each nitrogen with only a lone pair (no pi bonds) will be contributing it's lone pair, and the others with already-existing pi bonds will contribute those.

Also take a look at the two states of imidazole below. What does that suggest about the aromaticity of histidine at low and at high ph?
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NS_Tutor_Mathias
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Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 11:12 pm

As an added bonus, check these out for an idea of under which conditions a ring system with pi bonds might NOT be aromatic.
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