I'm going to teach you guys this rule through a really interesting example that might actually come up in your homework. So 8 annulene is also called cyclooctatetraene or cot for short. That's what I like to call it. I like to call it cot, cyclooctatetraene. Okay? And remember that this is your taco molecule, right? This is the molecule that hates being anti-aromatic, so it folds on itself so that it becomes non-aromatic. Well, here's the crazy thing. What happens if I ionize it to give it a net charge of 2 negative? What if I add 2 negative charges to the cyclooctatetraene?
Well, it turns out, first of all, what's that called? That would be called an eight annulene dianion. As you can see here, it has 2 negative charges. What would be the number of pi electrons in this molecule now? Let's count them up. It would be 2, 4, 6 but now wait, anions count as 2 so that would be 8, 10, right? So you have 10 pi electrons. Is that a good number or a bad number? That's a Huckel's rule number. This has an aromatic number of electrons. But remember, to be aromatic, you need to be planar. What did we say about the taco? It's not planar.
But wait, here's the confusing part, guys. Because molecules love being aromatic, they're going to do whatever possible to be aromatic and they're also going to do anything possible to not be anti-aromatic. That's the theme of this pattern here. It turns out that if you have a 2 negative charge on your molecule, it's actually going to flatten out again to become planar. This molecule actually is aromatic. I know that sucks in terms of memorizing. But you have to think about it, maybe less in terms of memorizing and more in terms of motivation. These molecules are motivated to be aromatic. So they're going to do anything they can. And they're unmotivated to be anti-aromatic, so they're going to do anything they can to fold out of the plane so they don't have to deal with that.
What are these rules that I keep talking about? Here it is. This only pertains to all cis-annulenes by the way. What would be an all cis-annulene? It just means that all of your double bonds have single bonds that are facing towards the ring. Both of these would be examples of all cis. Really, if you want an example of not all cis, it would be something like molecule B or molecule D. As you can see, there are some trans bonds here. That's a trans, and that's a trans bond. These larger rings would not apply to my rule and these are just going to be special cases that I'm going to tell you about.
But my rule specifically applies to stuff like cyclooctatetraene or this big thing that I have in molecule A. So let's go ahead and take a look. If you have 4n+2 pi electrons, is that a good number or a bad number? It's a good number, right? These molecules are going to want to do anything possible to be aromatic.
It turns out if they have 9 carbons or less, they will become planar to be aromatic, so they will be planar. However, if they have 10 carbons or more and they're all cis, then they're going to be too strained to make a planar structure because those bond angles are going to wind up getting more and more shallow. They're not going to be able to meet the 120 degree bond angle and it's going to be too stretched out. This will be too strained, and it will actually become non-aromatic. It's kind of a sad situation. It wants to be aromatic but it can't meet the right bond angles. It's just too big to stay as a plane, and it's going to start to twist. That's if you have 4n+2. This is the good numbers, right?
Well, what if you have the bad numbers for n? Then think about the motivation here. It's trying its hardest to not be anti-aromatic, right? It's trying to avoid anti-aromaticity. It's going to do whatever it can possible to fold out of the plane. That's exactly what happens if you have 8 carbons or more. If you have 8 carbons or more like cyclooctatetraene, 8 annulene, you're going to be non-aromatic because you're going to fold. Just like we did, I'm just going to put here fold taco. I'm literally writing taco there, so you can remember it's going to fold like a taco on itself.
But what happens if you have 7 carbons or less? Well then you're in a tough situation. If it has 7 atoms or less in the ring, then it's too small to fold. So it's going to have to be anti-aromatic. I'm going to put here too small, twofold. That means it's going to be anti-aromatic. That's the rule.
Another note here, guys. This is just a note of guidance here. It turns out that I put together these rules over years of tutoring and year-end doing a lot of research online and trying to figure out what most professors consider to be anti-aromatic, non-aromatic. But this is actually controversial. I'm going to go ahead and write here in a little bracket. I'm going to put controversial because some professors, remember I told you this has to do with bond lengths and X-ray crystallography. Maybe your professor is like a professional in X-ray crystallography and they have their own idea of what some of these molecules. So obviously go with your own professor's judgment. If they decide that they want a 7-membered ring to be non-aromatic, then go with it. Just go with the flow. But these rules should apply definitely to your textbooks and should apply to most sources that you would find online or most professors what they think. Okay? But don't argue with your professor about this. In general, don't argue with your professor because they're the ones that give you a grade, not me.
For the following molecules, go ahead and use the rules that we talked about to figure out what they would be. As you can see, only compound A and C actually can use these rules. Meaning that B and D, I will address separately as different types of molecules. Go ahead and do A and then, you know, we'll just take these questions one at a time.