Hey guys, so now let's talk about a new concept called conjugation.
Conjugation exists when three or more atoms with the ability to resonate are next to each
other or back-to-back.
Another way that you can think of it is that they're orbitals are overlapping.
Now this idea of resonating or resonance is an old concept from organic chemistry one
that you guys should all be relatively familiar with.
We've all drawn a resonance structure at this point.
So you might be wondering, “Johnny, what's the difference between resonance and conjugation?”
Essentially, there isn't really a difference.
They're two names for the same idea.
Whereas, to resonate, resonate is a verb.
If you resonate something, that's an action.
Well, conjugation or conjugated would be the adjective that describes that you can do that.
I don't want to get too much into grammar, but basically just saying that something that
is conjugated has the ability to resonate.
So they're really a similar word for the same idea.
Now, what does this mean?
Well, conjugation provides a highway or an electron highway for electrons to delocalize
from one side of a molecule to another.
And we all know that delocalization provides stability for molecules.
That's something that we learned a long time ago about resonance.
It turns out that these conjugated molecules, because they have the extra stability, they're
going to display unique chemical reactivity that we're going to spend a few topics talking
about.
Now, in another note that's pretty much unrelated to everything I just said, there's an important
side not for you to know which is that the higher the level of conjugation in a molecule,
the higher the UV wavelength is going to be in UV-Vis spectrometer.
Now why am I mentioning this?
The higher the UV wavelength.
If you guys remember, wavelength looks like that.
Now the reason I'm mentioning that is because I'm really not going to spend any time talking
about UV-Vis spec, but this is the only meaningful application that you really need to know about
it for organic chemistry one and two, which is that as your conjugated compounds have
more and more conjugation, meaning that more and more atoms can resonate together, the
higher the wavelengths tend to be for this UV-Vis spectrometer.
This could be a multiple choice question or it could be a free-response question that
you get asked.
So that's just something that I wanted to throw in there.
Now let's actually talk about the properties of the types of molecules that are conjugated.
Well, we just said that three atoms with the ability to resonate have to be back to back,
so what type of atoms are the ones that can resonate?
Well, we all know that pi bonds can resonate.
So we're going to put here one, pi bonds.
Now a pi bond just doesn't have to be a double bond, it could also be a triple bond because
we know that triple bonds actually have two pi bonds in them.
So double bonds and triple bonds are definitely capable of resonating.
Now the other ones that are capable of resonating would be ones that have orbitals that are
free to accept or donate electrons.
That would be, for example, if you have basically a lone pair or an anion.
I'm going to put here anion or lone pair, really depending on what the formal charges
of that molecule.
In terms of resonating, the idea of having a lone pair or a negatively charged anion,
really they resonate the exact way.
The whole deal of having a negative charge just has to do with what's the formal charge
of that specific atom.
So for the context of conjugation, we're going to treat these exactly the same.
So that's what happens if you have two electrons in your orbital.
But we know that you don't have to always put two.
Another idea is what if I just put one electron in the orbital.
What's that called?
That's called a radical.
Radicals are also capable of resonance or conjugation.
So radicals can also conjugate.
And the last idea would be how about if you put zero electrons in there?
Then that would be a positively charged atom.
So that would be a cation.
So these are all of the types of atoms that I want you to think about when we talk about
atoms that can resonate.
We're just saying that here I actually have five atoms listed because I have the three
different charges.
I have the anion, the radical, the cation.
Then I also have the two atoms from the pi bond.
All I'm saying is that you need some combination of these three atoms in a row that will provide
for conjugation to take place.
What we're going to do is we're going to do this practice problem and you have to identify
which of the following molecules exist in a conjugated state.
So go ahead and use what I talked abut earlier above as a reference and figure out which
of these molecules are conjugated and which ones are not conjugated.
So go ahead and do that now.