We're going to begin talking about
delocalized electrons and resonance.
This is a fundamentally important topic that you
really want to make sure you understand completely,
because we're going to be using resonance
to explain things this semester well into
Organic II.
And, what you want to understand about
delocalized electrons is that, you know,
it's really just as the name states,
instead of electrons being localized,
you're stuck on a single atom.
If electrons are delocalized,
they're spread over multiple atoms.
Let's take a look at a carbocation structure.
This particular one where we have a positive charge
next to a double bond is called an "allylic
carbocation."
[pauses]
And, I don't want you to think so much about
the charge.
More so, I want you to think about the pi bond—
the electrons in the pi bond.
So, when we're talking about delocalized electrons,
we're really looking at pi and lone pair electrons.
And, in the case of this allylic carbocation,
these electrons in the pi bond,
they're not stuck in just this single location.
Instead, we can draw another structure—
and, I'm gonna represent these using this
double-headed arrow which is called a "resonance
arrow."
[pauses]
And, what we find is that we get another structure
where the pi bond is on the other side.
And, then, since we, kind of, put electrons here
where the positive charge was,
that leaves a void over here,
so we'll put the positive charge right there.
In reality, neither of these structures are
really accurate.
And, neither structure exists.
So, it's not like we have two separate structures.
If you had this molecule,
what you have is a single hybrid structure
that meshes these two together.
Since neither structure is entirely accurate, really,
what we have is the pi electrons
and the positive charge being
spread among the three atoms.
And, what this gives us is a resonance hybrid.
So, we'll say the true structure . . .
is a resonance hybrid.
[pauses]
Now, what that looks like,
we don't mess with the sigma bonds.
And, I'm going to give you rules for
resonance here in just a bit.
I'm gonna draw the carbon-carbon sigma bonds
just like we have in the original structures.
But, in reality, those pi electrons which I have in blue
are spread over all three carbon atoms,
as is the positive charge.
It's kind of spread over the three carbon
atoms, as well.
So, this would be our resonance hybrid structure.
It's kind of a mash of the two.
Here's one more case where we have this carboxylate
anion.
And, I'm going to show you all the details
of how we move these electrons here in a bit.
But, I what I want you to think about is,
in delocalized systems,
it's the lone pair and the pi electrons being
delocalized over multiple atoms.
So, what's effectively happening here in this resonance
is this red lone pair
can be shared as a pi bond between the oxygen and the carbon.
This blue pi bond
can reside up on this oxygen as a lone pair.
[pauses]
Once we've done that,
now I have a lone pair on this top oxygen,
a pi bond to this bottom oxygen.
Still a valid structure,
but now the oxygen on the top
has the negative charge.
So, we can see this negative charge,
and the electrons here are delocalized over
three atoms.
