Now that we know how to solve for the total number of signals in a proton NMR, it's time
to move on to our second piece of information and that's going to be the chemical shifts
of each signal.
The chemical shift indicates the exact electrochemical environment that each proton is experiencing.
Now, I know that sounds kind of complicated but there is a nice generalization we can
make.
What we say is that electronegative groups – remember electronegative, just something
that likes to pull electrons away, will pull the electrons away from the nuclei, deshielding
them.
Making them more naked so they're going to experience the NMR more.
The shifts, the chemical shifts, are going to increase or move downfield.
The number will get bigger, as the protons because more deshielded.
Before we get into some very specific shifts that you need to know, we're just going to
look at a general trend.
As a general trend, what you're going to see is that as our functional groups get more
and more electronegative, the bigger the numbers get.
Now remember, all of this is in reference to TMS, which is our reference molecule that
has a by definition shift of zero because that is our reference point.
Now we're saying, all of these things in order, they get more and more deshielded as you go
down the list.
What we see is that alkanes, this is just an alkane, are actually the most shielded
organic molecules because there's no electronegative things pulling electrons away.
It's very, very shielded.
These are going to shift at the lowest number around 1 to 2.
Then we have triple bonds.
Triple bonds are going to come next.
That's alkynes.
Alkynes are going to come next to about 2.5, still relatively shielded.
Then we have this very broad group called ZCH.
Now, what does Z stand for?
Z is just going to be something electronegative that's next to it.
Z could stand for an atom.
It could stand for a group that is electronegative.
I'm just putting EN for something electronegative.
Now this has a range anywhere from 2 to 4 because it really depends on what that Z is.
If it's something super electronegative like fluorine, then it's going to be equal to 4.
If it's something less electronegative, then a little bit less.
Then we've got alcohol and amine.
Now these have a very broad range of 1 to 5.
They're such a broad range, in fact, that I consider these useless in terms of chemical
shift because it's so variable that the difference between 1 through 5 is really huge on NMR.
Remember I told you that NMR really only goes to like 13, so 1 through 5 is like a third
of the entire spectrum.
That being said, it's not very helpful if I have a peak at 2 or I have a peak at 5,
it doesn't tell me much because I really know what it is.
So I'm really just not even focus on those shifts because they're pointless because they're
so wide.
They don't really give us a lot of information.
Then we have alkenes, which result higher than the two other hydrocarbons at 4.5 to
6.
I'm not going to be able to get into the theory of why alkenes are higher than alkynes, but
just know that it actually has to do with a very technical explanation dealing with
molecular orbitals.
And the shapes of the molecular orbitals around the double bond will actually deshield the
hydrogens more than the shapes of the orbitals on a triple bond, so that's why.
If you're really interested, Wikipedia it.
Benzenes are getting more deshielded.
If you think about it, benzene has a lot of electrons inside.
It's got those three triple bonds, the Clutch logo, right.
So there's a lot of electrons in there.
It's pulling electrons kind of towards itself.
So benzene is going to be higher, 6 to 8.
Now this is where things start to get really deshielded.
Aldehyde and carboxylic acid, one thing they have in common is that both of those H's,
I'm just going to draw the carboxylic acid, they're next to carbonyls.
Now remember, a carbonyl has a very strong dipole pulling away from it.
On top of that carboxylic acid has another strong dipole with the oxygen pulling away
from it.
All that's to say that - I didn't mean to draw it exactly there.
I meant to draw it here, that the oxygen is pulling electrons away from the H. All that
is to say that carboxylic acids and aldehydes are going to have the most deshielded or the
most naked protons because you can imagine that basically this hydrogen here, doesn't
have any electrons around it because all of the electrons are getting sucked up by the
carbonyl, sucked up by the oxygen.
That thing is like butt-naked.
Our highest peak possible is carboxylic acid at 13.
Now the specific chemical shifts that you need to know for your class, for your specific
classroom, is really going to be up to your professor.
There's absolutely no way for me to know on my end, exactly which shifts your professor
thinks is important and which ones they say you don't need to worry about.
In fact, double check, your professor might even give you a sheet on your exam that has
all of these shifts already written out for you, so you don't have to memorize them at
all.
But, as always, I'm going to go over them just in case.
Honestly, even if your professor told you, you don't need to memorize these shifts, I
still would recommend watching this part of the video because this can help you a lot
when we do structure termination later on in this course.
Let's go ahead and just go through some more specific ranges.
First of all, we're just going to start at the bottom.
Remember that our bottom is TMS.
Now we talked about alkanes being in the 1 to 2 range.
It turns out that alkanes actually follow a pattern that the more substituted the alkane,
the more R groups it has, the higher it's going to result.
That means that your professor may want you to be able to tell the difference between
a primary alkane and a tertiary.
Typically a tertiary one could come out at like 1.8, whereas a primary one might come
out at like 1.1.
It's little differences here and there.
Then a secondary would be somewhere in the middle, like, 1.4.
All of these ranges are definitely variable.
It doesn't have to be exactly that, but I'm just trying to illustrate how as you move
towards tertiary, you actually get a little bit more deshielded.
Now we move on to our ZCH range in which we said that it's anywhere between 2 and 4 and
it really depends on what Z is.
Well, that's actually really important.
We have to learn some specific Z's here.
I want you to know these.
First of all, the most electronegative things that we can put next to a hydrogen are either
fluorine or oxygen.
Both of these guys are actually going to result in a peak of just about 4.
If you ever see like an OCH, that's common in ether.
That usually results in right around 4, 3.9, 4, etcetera.
The next one I want you to be aware of is kind of the middle ground which is nitrogen
and iodine.
Now if you think about it, think about your periodic table, in terms of electronegativity,
it goes F, Cl, Br, I.
So you electronegativity goes up as you get closer to fluorine.
So that means that I would expect that iodine would be the least shifted alkyl halide and
that's exactly the case.
Iodine and nitrogen come out about 3, with the other alkyl halides, as you can see, Br
and Cl, coming out somewhere in between.
So we could theorize it's something like 3.2 and 3.5.
Just something in the middle.
Not for you to memorize, just for you guys to grasp and understand the general picture.
Now this part is actually the most important part I think in terms of your long-term knowledge.
Some Z's, additional Z's that we like to talk about are benzene rings, carbonyls and allyl
groups.
These are groups that are going to be able to pull electrons towards themselves.
I put that they're all roughly around 2.
If we just wanted to remember that they're all about 2, that's fine.
But they do kind of go in order.
For example, a benzene would typically be around 2.3, whereas a carbonyl would usually
be around 2.1, whereas an allyl will usually be about 1.9.
They're all in the range of 2, but there is kind of a hierarchy of benzene and carbonyl
being higher than allyl.
Just keep in mind that really it depends on what the Z is is going to tell you where it's
on this 2 to 4 spectrum.
Now going down to the triple bond.
Triple bond results anywhere from 2.5 to 3.
That whole range is kind of fair game for the triple bond.
What do we have for the double bond?
These ranges are really just the same as the ones I stated before, 4.5 to 6 for a double
bond.
For a benzene ring, anywhere from 6 to 8.
For aldehyde, anywhere from 9 to 10.
And for carboxylic acid, anywhere from 10 to 13.
There you have it.
Really all I did in this section is I went more into depth on the Z portion and more
into depth on the alkyl version because of the fact that it really kind of depends on
the types of Z's you have or the types of alkyl groups you have.
All right?
Awesome.
One last thing, I had the alcohols and amines at the bottom.
This just gives you a visual or how large that swath is and how really useless it is.
We don't really use this part of the graph.
We don't really worry about it because it doesn't tell us much information.
So now we're going to do a practice problem.
What I want you guys to do is use the definitions like an open book exam to look at this molecule
and tell me what you think the ranking would be in terms of shielded to deshielded.
It says order the following five protons in order from most deshielded so that means highest
number to most shielded, so that means to lowest number.
Go ahead and start at the highest parts per million, the highest chemical shift and then
we can go ahead and work our way down to the lowest one, just order the protons the way
you see fit and then I'll answer the question.