Aromaticity of Hydrocarbons

Concept: Concept: Aromaticity of Hydrocarbons

5m
Video Transcript

Hey guys! Now that we know those four tests of aromaticity and now that we're experts on counting up pi electrons, it’s time to put that information together to figure out if a molecule is aromatic or not.
Remember what those four tests of aromaticity were. We had the whole “It has to be a ring” thing, cyclic. We had fully conjugated. We had that it has to be planar.
But remember there was that last rule that was a little confusing, the 4n+2 pi electrons, Huckel’s Rule.
Now we know how to count up pi electrons but 4n+2 is still kind of confusing. What does that mean? The whole reason that we have this idea of 4n+2 is because someone realized that 4n+2 would be an easy shorthand to memorize these magical numbers that make a molecule extra stable and therefore aromatic.
The way it works is that n is equal to any integer, any whole number. When you make n to equal an any integer, then what you do is you wind up getting these numbers that are the super stable numbers. That would be if n equals 0, then that would be 2. If n equals 1, then that would be 4 times 1 plus 2, which would equal to 6. If n equals2, then that will be 2 times 4 plus 2, which equals 10, and so on and so forth. We get these numbers that go to 2, 6, 10, 14, etc. forever. These numbers are the Huckel’s Rule numbers. Some of my students just prefer to memorize the numbers instead of 4n+2 because they think it’s easier that way. I'm going to leave that to you. If you want to just memorize 2, 6, 10, 14 instead of 4n+2, if that’s easier for you, go for it. All I care about is that you use the right numbers on your exam.
You might be wondering “Why are these numbers so great? Why are they so stable? They look like normal numbers to me.” That is the topic for a different video. In another video, we're going to discuss why these numbers actually contribute to stability and why they make the molecule so badass. But for right now, just memorize it.
Then remember that we had this other category which let's say that you meet the first three tests. But we get a 4n number of pi electrons. These are different numbers. These are going to be the multiples of 4. These are going to be numbers like 4, 8, 12, 16. These are magical numbers as well but they’re magical in a bad way. They suck. They make the molecule super unstable. In fact, it's really hard to even synthesize these molecules in the lab because they are so unstable. These molecules are going to be what we call anti-aromatic. Remember, these are molecules that are much less stable than normal. Remember that these are called since they follow Breslow’s Rule because Breslow’s Rule said that you have the first three tests met. You’re still cyclic. You’re still fully conjugated. You’re still planar. But you have the wrong number of pi electrons. In fact, you have a 4n number of pi electrons.
Now let’s talk with this third category of non-aromatic. Recall that I stated that non-aromatic molecules are simply molecules that fail one or more of the tests. If you’re not a ring, you’re automatically non-aromatic. One thing I want to point out is that some pi electrons can actually count towards failing the rule. If you have an odd number of pi electrons. That would literally be any odd number.
For example, the number 7. The number 7 doesn't fall into any of these types of electrons. It’s not 4n+2. It's not 4n. It’s simply left out. We never discussed it. If you have an odd number of pi electrons, you're also said to fail Huckel’s Rule. That means that you would be automatically non-aromatic. Just keep that in mind that you can even fail the test of aromaticity by the number of pi electrons that you have.
You might be wondering, “When would you get an odd number of pi electrons?” When you have radicals because radicals count as only one. Now I have a ton of practice for you guys. We're just going to do one at a time. Go ahead and look at the first one. I know it’s a little bit too easy. But let me know if you think that first molecule is aromatic or not based on the four tests of aromaticity and your ability to count up 4n+2 pi electrons. Go.

Concept: Example 1: Determine the aromaticity

1m
Video Transcript

Is this molecule aromatic? Of course it is. But let's obviously go through the four tests to verify it.
One, is it a ring? Yes.
Two, is it fully conjugated? Yup.
Three, is it planar? You bet.
Four, does it have a 4n+2 number of pi electrons? It has six pi electrons so that would be one of those magical Huckel's Rule numbers,
This is aromatic.
Awesome. Now we know how to prove something that you just vaguely knew before. Move on to the second compound and tell me what type of aromaticity it has.