Aromatic Compounds and Huckel's Rule

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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.