Aromatic stability
Aromatic Compounds and Huckel's Rule Aromatic Compounds and Huckel's Rule
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- We touched on this in the video on resonance, but I want
- to devote an entire video to aromatic compounds.
- And one of the big mysteries in chemistry is why they were
- named aromatic compounds.
- Because you know it clearly comes from the word aroma.
- So one would think maybe all of these compounds smell good,
- or smell bad, or don't smell at all.
- But actually it turns out that a lot of them don't
- even smell at all.
- So it's a little bit of a mystery.
- Some people think that they have some relation, the
- gentleman who named them, saw that they had some chemical
- relation to things that did smell, so that he named them
- aromatic compounds.
- So it is a bit of a mystery.
- In general, the most common aromatic compound-- and really
- this is like 99% of the time what you're going to see in
- chemistry class as an aromatic compound-- is either benzene
- or a molecule derived from benzene.
- Let me draw benzene right here.
- So benzene is normally drawn like this.
- So it's a six carbon ring.
- And it has three double bonds.
- It has three double bonds like that.
- And we learned in the resonance video, that this
- isn't actually the only configuration for benzene.
- It could just as likely be in this configuration.
- That this electron up here, this electron up here, might
- move there.
- This electron might move over there.
- And then this electron might move over there.
- Let me clear that up, so you don't make it too confusing.
- So it could just as likely be in this configuration.
- It could just as likely be in this configuration right here,
- where the double bonds are on the other bonds that aren't
- double bonded over here.
- So it could just as easily be over there.
- And we learned in the resonance video that the
- reality is that it's actually in between.
- And sometimes it's drawn like this.
- It's going back and forth between these configurations.
- But the reality-- and you'll sometimes see
- it drawn like this.
- You'll sometimes see it drawn like this, with just a circle
- in between, in the middle of the hexane ring, I guess you
- could say, or the benzene ring.
- Which is showing that the electrons, those pi electrons,
- those electrons that form the double bond are actually just,
- they're going back and forth between this.
- They're just spreading around the entire ring.
- And what makes aromatic compounds interesting is
- because these pi electrons can spread around the entire ring,
- it's actually a much more stable configuration, or a
- much more stable molecule, than what you would predict if
- you just looked at one of these two configurations.
- Another way that it's often drawn looks
- something like this.
- And I'm just doing it in yellow, just to have something
- in a different color.
- Sometimes you'll just see this, that you know, is
- someplace in between that and that.
- So you'll have a dotted line there, dotted line, dotted
- line, dotted line, dotted line, dotted line.
- This is the most common kind of short hand for showing both
- benzene and for showing that it's experiencing this
- resonance, that you have this conjugated
- system of pi electrons.
- And I'll help you visualize that in a second.
- But you also might see something like this.
- It can go back and forth, and everything in between, between
- these two configurations.
- Now just to visualize what's going on.
- Because you'll sometimes hear people talk about conjugated
- system of pi electrons.
- I want to actually think about, help a little bit more
- visualizing what the molecule might look like in three
- dimensions.
- So that is the six carbon ring right there.
- So it's carbon, carbon, carbon,
- carbon, carbon, carbon.
- And then each of these carbons is bonded to three other
- atoms. It's bonded to 1, 2 carbons.
- And it's also bonded to a hydrogen.
- So let me draw the hydrogens here.
- They're also bonded to hydrogen.
- So this one will have a hydrogen over here.
- This will have a hydrogen over here.
- That will have a hydrogen over there.
- That one has a hydrogen over there.
- Hydrogen and hydrogen right over there.
- So if we talk about hybridization, we have three
- hybridized orbitals.
- This is sp2 hybridization.
- And each of these have a leftover pi orbital that has
- not been hybridized.
- It's not directly bound to, or that does not have a sigma
- bond to another atom.
- So you have these pi orbitals.
- You know, these pi orbitals look like these dumbbells.
- So you have a pi orbital over here.
- You have a pi orbital here.
- Pi orbital over here.
- Pi orbital over there.
- You have another pi orbital over there.
- Pi orbital and pi orbital.
- And you know, pi orbital, I drew them like this, because
- if I drew them bigger than that, the
- diagram would get messy.
- But in general, whenever you have double bonds, like this
- double bond.
- Let's say that this carbon-- let me change colors, so we
- know what we're focused on.
- So let's say this sigma bond right over there, let's say
- that sigma bond right over there, is this sigma bond.
- Actually let me do another one, just so
- it's easier to see.
- Let's say that this sigma bond right over here is this sigma
- bond, between these two carbons.
- This double bond, the double bond in blue, this one, or in
- purple right over there, that's due to these
- overlapping pi orbitals.
- That carbon's pi orbitals and that carbon's pi orbitals
- overlapping.
- I haven't drawn them overlapping, but-- well,
- actually an electron can really show up.
- It's all probabilistic anyway.
- But I could draw them big enough, they would overlap,
- and these electrons would form that extra pi bond.
- What happens in a conjugated system of pi electrons.
- Let me write that down.
- It's just a very fancy word.
- Conjugated system of pi electrons.
- These guys, at we see, they could be
- bonded with each other.
- These guys could overlap.
- So there could be an overlap going on over here.
- Or we could flip into this configuration, where this guy
- would overlap with this guy over here.
- And the reality is that these pi electrons will actually be
- able to float around the ring.
- They'd actually be able to float around all of these pi
- orbitals right over there.
- They'd actually be able to float around the ring.
- So that's what people are really talking about when they
- talk about aromatic compounds, or aromaticity.
- That because of this, this is a more stable compound.
- The most typical one that you're going to see is a six
- carbon chain with three double bonds, benzene, or things
- derived from them.
- But there actually are more general ones.
- In general, you're going to see anything that has 4n plus
- 2 pi electrons in a cycle is going to have aromaticity.
- Or is going to be an aromatic compound.
- Let's just confirm this actually makes it.
- Each of these guys over here have one pi electron, even
- though this looks like two bonds.
- Remember, this is one pi orbital right over here.
- And you have one electron that's going to be in this
- entire pi orbital.
- So this has one pi electron, two, three, four, five, six.
- Another way to think about it, each of these double bonds
- involves two pi electrons.
- So one, two, three, four, five, six.
- So that follows what is called Huckel's rule.
- I think there's two dots above the u.
- Huckel's rule, maybe.
- That follows Huckel's rule.
- Because if you said n is equal to 1, 4 times 1
- is 4 plus 2 is 6.
- So 6 pi electrons work.
- If n was 2, you'd have 4 times 2 plus 2, which is 10.
- So if you had 10 pi electrons, it would also work.
- So a molecule that looks like this-- let me see
- if I can draw it.
- A molecule that looks like this would also work.
- So if you have one, two, three, four, five, six, seven,
- eight, nine, ten, ten carbons.
- And then you had five double bonds.
- So, one, two, three, four, five.
- Just like that.
- You could imagine, these guys could flip around.
- But this would also be an aromatic compound.
- This has 10 pi electrons.
- Each of the carbons have a pi electron.
- Or you could count the pi electrons on each
- end of these pi bonds.
- One, two, three, four, five, six, seven, eight, nine, ten.
- Now this is one thing that I always wonder.
- It's like OK, it worked with six.
- It worked with 10.
- But what about eight?
- It seems like with eight, maybe these electrons, these
- double bonds, could flip around just as easily.
- So what if I had, what if I had-- or even four.
- What if I had a molecule that looked like this?
- What if I had a molecule that looked like this?
- Or a molecule that looked like a stop sign-- so one, two,
- three, four, five, six, seven, eight-- that had double bonds
- that alternated like this.
- You might say, hey, maybe those also
- are aromatic compounds.
- Those also experience aromaticity.
- Because couldn't this guy jump around here, then that guy
- over there, and the electrons cycle around.
- Or this guy jump over, these electrons move there.
- Those move there.
- Those move there.
- And those move there.
- And it turns out that in these compounds, the electrons do
- not, the pi electrons do not stabilize the system.
- But this is actually less stable than if it
- was not in a cycle.
- So actually this is-- these right here, that don't follow
- the 4n plus 2.
- So 4n plus 2, you're talking about 6, 10, 14 pi electrons.
- Which usually means 14 carbons, or 10
- carbons, or six carbons.
- The ones that don't follow them but are still cyclical,
- and still have these alternating bonds, these are
- called anti-aromatic.
- And are actually very unstable.
- These are very, very unstable and are more likely to break
- into a non-cycle.
- Anyway, hopefully you found that vaguely useful.
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