Now we're going to move on to drawing and recognizing functional groups with hydrogen
in them.
These are going to be alcohols and amines.
The first one we want to focus on is really the granddaddy of all absorptions and that's
alcohol.
Alcohols, as we said, are one of the broadest absorptions that there is.
They absorb from 3,200 all the way to 3,600 in their absorption.
This thing is massive.
Remember that I told you that an alcohol almost looks like a parabola.
That OH is going to look huge.
The official name for this type of absorption is that it's strong and broad.
That should tell you – strong, it's moving all the way down to the floor and broad, it's
very, very wide.
Now on top of that, because of the fact that I'm always including an alkane component,
what other peak should we always be drawing?
We should always be drawing our sp3 CH bonds as well because all of these molecules are
going to have those.
But notice that I have C-C single bonds, which I'm excluding.
I also have CO single bond, which I'm excluding as well.
I don't have to worry about anything else, I'm just going to draw those two absorptions.
Let's go ahead.
I've got my fingerprint, who cares.
Now I move all the way to 2,900 and once again, I'm going to draw my sp3.
At this point, you guys should be pretty good at drawing these sp3.
Go crazy.
I expect different shapes coming up at this point.
You got your choppy sp3 peaks.
But what's going to happen?
Well when we get to 3,200, it gets real.
This alcohol is going to be huge, it's going to do something like this.
Boom.
Now in this spectrum, I actually went past 3,600, so I probably drew it a little bit
too big, but it's not a big deal.
The whole point is I just want you to know that it's a huge, massive peak that takes
up pretty much the entire 3,000 area.
And that can actually make it kind of challenging because alcohols are so large, in terms of
their absorption, that they tend to block out things that are behind them.
Remember that I told you, for example, what's an absorption that you can think of that happens
between 3,200 and 3,600?
Remember our triple bonds?
Our 3,300 alkyne.
So imagine that you have an alcohol, but you also have a terminal alkyne that has a sharp
peak around 3,300, you might barely see it.
In fact, what it might look like is you might have an alcohol that looks like this, it's
coming down and then it has this sharp little thing and then it keeps going.
Because you could almost think of an IR spectrum as a silhouette.
It's almost like a shadow of all the peaks, so you can't see multiple layers or multiple
dimensions here.
If there truly was a sp hybridized CH along with an alcohol, you might only see a little
peak like that popping out, just popping right out of the alcohol peak.
My whole point here is that alcohols can make reading the 3,000 region a little bit challenging
because they take up so much space and there's really nothing we can do about it, one of
the limitations of IR spectroscopy.
Now let's move on to amines.
What you'll notice is that I've got primary amines and secondary amines and they're going
to result differently.
Let's start off with primary amines and then move on to secondaries.
Then I'll explain why tertiaries are not on this page.
The general rule for amines is that you're going to draw as many peaks, same number of
absorptions, as hydrogens that you have in your amine.
A primary amine has how many hydrogens?
Two.
That means that we would expect it to have two absorptions.
In general, these absorptions are going to be weaker and sharper than alcohol.
If you guys recall, the range of amines was somewhere between 3,300 and 3,500.
That means that amines could completely get covered by an alcohol if it was present.
Thankfully, your class is not going to go into that much complexity where you have to
figure out what's behind the alcohol.
But just letting you know that there's a lot of overlap between these ranges.
And really the only way that you can tell difference is by the shape because it's weaker
and it's sharper.
Now you might be wondering why does it have these two different absorptions and I'm just
going to tell you quickly this is beyond the scope of this course.
You don't necessarily need to know this, but basically, there's two types of vibrations
that are going to be happening with your amine.
We talked about how there's stretching, wagging, rocking and all of those are under the umbrella
of vibrations.
When you have those two hydrogens present on the amine, you're going to have a type
of vibration called symmetrical stretching and you're going to have a type of vibration
called asymmetrical stretching.
I'll tell you where I'm going with this in a second.
Symmetrical looks like this that both of them are stretching exactly the same.
Asymmetrical is where one is stretching in a different direction than the other.
Because you have these two different vibrations that are kind of vibrating at different frequencies,
one is going to be higher than the other.
The symmetrical stretch is going to be right around 3,300.
The asymmetrical stretch is going to be a little higher.
It's going to be around 3,400.
That's why when you have two hydrogens present on the amine, we're going to expect double
peaks.
That's enough for now.
Let's go ahead and draw it and then I'll explain why this is important.
We're going to go ahead and do nothing, nothing, nothing.
We get to 2,900 and we draw our choppy sp3.
So much fun.
Then we get to around 3,300 and this is where we draw our amine peaks.
We're going to get a weaker, sharper peak around 3,300 and then another peak around
3,400 and that's so ugly I'm just going to do it again.
I'm sorry.
That looks like slime or something.
Let's just try this one more time.
That's a lot more like what I was trying to draw.
You've got this double peak where the lower peak, at the 3,300, has to do with the symmetrical
stretching and the higher peak is with the asymmetrical stretching.
Does that make sense so far?
Awesome.
Note that another bond that we didn't discuss was our C single bond N bond, but recall that
that would just go in your fingerprint so we don't have to draw it.
Excellent.
Why is this symmetrical and asymmetrical thing important?
Because look at secondary amines.
Secondary amines only have one hydrogen.
That means that when this thing is stretching it's only going to have one type of stretch
possible which is symmetrical because there's no asymmetry here.
There's no possibility of hydrogens going in different directions.
Since it only has one H, we expect it to have just one absorption.
What I haven't told you is what is going to be the range of that absorption.
Because it's symmetrical would you expect it to be lower, around 3,300, or higher, around
3,400?
What do you guys think?
Take a stab at it.
You can use the above notes.
Yeah, it's going to be a 3,300 because there's no reason for it to be boosted to that higher
frequency.
Hopefully, that difference is making sense.
One hydrogen, one type of stretch, one signal.
We're just going to go ahead and draw this again.
We've got our sp3 over here.
Then when we get to 3,300, we just have one lone, weak, sharp peak around 3,300.
As you can tell, these peaks, no matter what type of amine you have, they look far different
from alcohol and they should never be confused with alcohol.
In fact, if you confuse anything with alcohol, I'm going to be personally disappointed in
you because alcohol is probably the most easy to recognize absorption there is.
You can't really confuse anything with it.
Let me know if you have any questions.
Oh, wait.
One more thing.
Don't leave yet.
We did primary amine.
We did secondary amine.
Why is tertiary amine not on this chart?
Did I just run out of room?
Is that the next thing we're going to talk about?
Not necessarily.
It's because notice that a tertiary amine, if I were just to draw it down here, tertiary
amine, how many hydrogens does it have?
No hydrogens.
A tertiary amine, by definition, is that you have an N with only R groups around it.
So are you going to see an amine peak with a tertiary amine?
No, you're not.
So this is actually an amine that would not result in the functional group region because
we don't even have a hydrogen present.
That's why you're only going to see your double peaks for primary and your single peaks for
secondary and you're going to see zero peaks for tertiary.
All right.
That's it for this topic.
Let's move on.