Now that we understand the frequencies of absorptions a lot better, it's time to move
on to the second part of what it means to really understand an absorption which is the
shape.
Remember that we need to know is it going to be broad, is it going to be sharp.
In this course, you may be asked to actually hand draw an IR spectrum from scratch.
At the very least, if your professor doesn't ask you to do that, they're going to want
you to be able to recognize what the peaks look like on an IR spectrum and be able to
understand the different shapes and identify what they are.
What we're going to do in this next section is I'm just going to walk you through drawing
every single important peak that you need to know and that's going to help you to familiarize
yourself with the shapes that are important.
Let's start off with the basic molecules which is just hydrocarbons.
We're going to start off with alkanes, which is just our number one go to simplest molecule
to draw.
I'm going to ask you guys to forgive my drawing here.
These are not meant to be perfect representations, but they should give you a pretty good idea
of what we're looking for.
We said that in a normal alkane how many different types of bonds are there?
Well there's two.
There's a C-C bond and there's an sp3 CH bond.
How many of these do we actually draw?
Just the second one.
We're only going to draw the sp3 CH.
Just so you guys know, sp3 CH, we know one thing about them, they result around 2,900.
But what is their shape?
Their shape is called choppy.
Notice that this doesn't follow the normal criteria of first word, second word.
I'm not saying sharp.
I'm not saying broad.
I'm just saying choppy.
The reason is because, typically, there's so many of these signals, of these absorptions
in a molecule, because think about it, there's hydrogens everywhere.
They're all going to overlap with each other and make kind of a clustered mess around 2,900.
Instead of giving it a specific shape, we just call it choppy.
Now you're wondering what is this going to look like, so let's go ahead and start drawing.
You might want to watch me do this one first and then just pause the video and copy it
down.
What I'm going to do is I'm really just going to neglect – as you can see this graph – my
1,500 starts here.
I'm never going to draw anything significant below there.
I'm just going to draw a line.
I'm literally just making this part up.
I'm not going to do anything until I get to right below 3,000.
This is my 3,000 mark.
Right before there, I'm going to start to draw something.
What I'm going to draw is a choppy peak.
When I get to 3,000, I abruptly stop and I continue.
Now notice what I just did there.
It's got a few little edges.
It's kind of slanted.
That's going to totally change based on the exact IR spectrum.
But, in general, that's what we're looking for.
We're looking for something that's a little bit more than half way down in terms of transmittance
and it's something that has a jagged edge.
You're going to see me draw these a lot during the course.
That was easy.
Let's move on to our second situation which is alkenes.
Notice that alkenes had more that we had to worry about because we still have that C-C,
which we're not going to draw.
But now we had two different peaks that we have to draw.
We had the C double bond C, which is here and here.
And we also have the sp2 CH.
That's in addition to the rest of the hydrocarbon that still has sp3 CH.
What I'm trying to say here is that we've got these two extra peaks that we have to
worry about that are complicating this more than just a regular alkane.
So how do we draw this?
Well, these are all going to have their own peaks that you need to know.
So 1,600 – I'm sorry.
I just gave it away.
I'm on automatic mode right now.
We just talked about how a C double bond C would result in a double bond region that's
going to be 1,600 and what is that going to look like?
That is going to be weak and sharp.
The peak at 1,600 is expected to be weak and sharp.
Then we would expect both of these to be choppy.
We know their ranges.
We talked about how sp3 is 2,900.
We also discussed how sp2 is 3,100.
Basically, we're going to have a lot of choppiness everywhere from between 2,900 and 3,100.
This time, what's going to be very distinct about this graph, is that it's going to go
past 3,000 and it's going to continue to be choppy after the 3,000 point.
Once again, probably better for you to just let me draw this and then you can copy it
down.
What we're going to do here is once again, blah, blah, blah, no one cares about1,500,
but wait.
This time when I cross 1,500, I immediately have to draw a weak, sharp peak.
That's going to represent my C-C double bond.
This is my C-C double bond.
That's my weak, sharp peak.
Now, once again, nothing really happens for a while.
I'm bored.
But I get to 2,900 and I have to start paying attention because now what I'm going to have
is I'm going to use the different colors.
I'm going to use red for the alkane, blue for the alkene.
I'm going to have my choppy peak once again.
Usually, if this was just an alkane, I would stop right here and that would be it.
Don't draw this.
Now because there's an alkene component, there's these sp2 hydrogens, that means that some
of this choppiness is going to continue past the 3,000 mark.
The little spikes that happen after 3,000 are for the sp2 component and the spikes before
are for the sp3 component.
Does that make sense?
So suddenly this graph just got a little bit more complicated than the one that we drew
before.
Cool so far?
Awesome.
Now let's move on to terminal alkynes.
Remember that I stated earlier that it's very important that you only use terminal alkynes
here because you must have a hydrogen on it to get that sp peak.
Let's look at this molecule.
This would be a typical alkyne that you might have to draw.
Let's identify, once again, like we've always done all the different bonds here.
We've got, once again, C,C, who cares.
I've also got my sp3 CH.
By the way, heads up, you're always going to be drawing that sp3 CH.
Get used to it.
These are around to stay.
Because remember that alkanes are the backbone of all organic molecules so what are the chances
that there's no alkane component.
Very little.
You're almost always going to draw an sp3 CH absorption.
We're X'ing out this one.
This one we're definitely going to draw.
But what else do we have?
Now we have a C triple bond C that we're adding and we've also got an sp CH bond, which is
the hydrogen that's right here.
This just got interesting.
We know that the peak at the sp3 is going to be 2,900.
We know it's going to be choppy.
But what do these other guys look like?
C-C triple bond if you had to guess what it's going to be, triple bond region.
That's going to be 2,200.
And once again, similar to our alkene, it's going to be weak and sharp.
It might be medium.
It's somewhere between weak and medium.
It really depends on the drawing.
But for the purposes of memorizing, I'm totally fine with you drawing it just like you would
draw an alkene because it really depends on the exact spectrum.
So 2,200 weak sharp.
Then we've got the sp CH which, because of what I told you guys about hybridization and
the wave numbers, this one's actually going to result the highest at 3,300 centimeters.
This one's actually going to be strong and sharp.
You might be wondering, Johnny, why is this hydrocarbon, why is this H showing up as sharp
when all the other H's that I've been drawing have been choppy?
The answer actually lies in the drawing.
Notice that the drawing I only have one.
There's only one H. So why would it be choppy?
There's only one absorption that's going to happen.
It's from that exact hydrogen.
That can't be choppy.
It's going to be just one lone sharp peak at 3,300.
Let's go ahead and give this a whirl.
I'm going to start off.
I completely ignore 1,500.
By the way, just so you know, I've been ignoring it, but it might as well look like this.
There might be stuff going on, but the whole point is that I don't care.
It could have spikes, it could not have spikes.
It doesn't really matter.
The point is that nothing is going to really happen until I get to 2,200.
When I get to 2,200, I'll give my weak, sharp peak.
That is going to represent my C triple bond C in the triple bond region.
From there I'm going to go ahead and draw my alkane, my CH that is sp3 hybridized.
As you can see, I pretty much always draw these peaks the same.
They're kind of choppy.
They're kind of irregular looking.
In this case, because there's kind of a gap between 2,900 and 3,300, they're not going
to be back-to-back.
So I actually would expect this to kind of descend all the way back down, but then I'm
going to have my strong and sharp peak coming from my alkyne.
This is my sp3 CH that's choppy, but now this is my strong and sharp sp hydrogen peak.
Notice that there's actually a separation here.
Now what might get even more confusing is if you had a molecule that not only had a
triple bond in it, but let's say it also had a double bond in it so now you had H's there.
Then what would happen?
Then that just means that anything that's sp2 would wind up going in here.
This would be your sp2 range so in that case there might be a little bit less separation.
But we've definitely gone over pretty much all the possibilities that can happen just
with these straight hydrocarbon chains.
I hope that's making sense so far.
We still have a lot of functional groups to go, but this is your basis.
This is kind of your fundamental groups and now we're going to start learning new functional
groups that you're going to add to this.
Let's move on to the next video.