1H NMR: Chemical Shifts

Concept: Concept: 1H NMR Chemical Shifts

12m
Video Transcript

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. 

Concept: Example: Ranking Shielded Protons

5m
Video Transcript

So, first I'm just going to go over the rankings in order and then I'll go over each individual chemical shift just to reinforce what we just learned. So, our highest number, our most deshielded is going to be H5, okay? Because H5 is on a double bond so that has the highest range as you can see above, okay? So, it's going to go from H5, then what's the one with the next highest number, okay? That's actually going to be H2 because H2 is directly attached to a carbon that has fluorine the most electronegative atom on it, so that would be an example of z CH, okay? So, I'm going to come in next. So, H5 was double bond, H2 is Z CH, okay? So, then after that what I'm going to get is H3, okay? Because H3 is on a triple bond and remember that triple bond results a little bit lower than a fluorine would result then we would get is H4 because if you notice H4 is actually allyl, okay? So, this would be an example of Z CH but it's allyl, okay? Now, I'm just going to erase this and move it up a little bit Z CH. Now, allyl is actually on the lower side of the Z CH but it still counts, okay? So, that's Allyl and then finally we would end up with H1 at the bottom because it's nothing, all it is is just CH, it's just an alkane, okay? So, hopefully that makes sense so far. Now, let's go over the exact chemical shifts.

Now, something that I neglected to mention in the previous example when I showed you guys the spectrum is that chemical shifts are actually denoted by a Greek symbol and this Greek symbol is the lower case Delta, okay? So, the lower case Delta is the same thing as saying parts per million, okay? So, I'm just going to put all of the different parts per million here, okay? What I would expect for h5 is that it's somewhere between 4.5 and 6, we'll just give it a shift of five, okay? What we would expect for h2 is that it's on the top range of z ch because as a fluorine present, so we are going to say that around four then for h3 since it's on a triple bond it's somewhere between 2.5 and 3, we'll just give it a chemical shift of 3, make it easy, okay? Now, for h4 because it's a low, this is going to be around 2. Remember, I said that usually they're a little bit below two. So, you know what even though I could make it easy I could say 2, I'm just going to be a little bit more accurate I'll say that it's like 1.9 because usually it's right below 2, okay? And then finally, we have h1, which it's secondary. So, it's actually probably not going to be 1.0, it's probably going to be a little bit more in the middle, probably something a little bit more like 1.4, okay? So, I got a little bit tricky at the end, sorry if I'm blocking that a little bit, I'm not trying to trick you guys or anything, just trying to show you guys kind of the way this works, the general trend, and that a lot of times you are going to see little fluctuations with these values and the biggest point isn't to argue over 3.9 or 4, it's to memorize and understand kind of the general idea of what's going on, okay? One more note about this, you know, these values that I taught you, it might be in your best interest to learn these values because the sheet that you get on your exam in case your professors just giving you these shifts, right? There are some professors that really don't care for you to memorize them, they just give them to you, you might not understand them the same way that you understand them here, the sheet that you get, okay? So, sometimes it's just better to learn it anyway and then you can go into your exam more confident, okay? So, I'll leave that one up to you guys but anyway that being said, let's go ahead and move on to the next topic.

Problem: Which of the labeled protons absorbs energy most upfield in the 1H NMR? 

4m

Problem: Which of the labeled hydrogens will be most de-shielded? 

4m

Problem: Which compound possesses a hydrogen with the highest chemical shift for its 1H NMR signal? 

2m

1H NMR: Chemical Shifts Additional Practice Problems

Consider all of the spectroscopic information given below. The molecular formula is C10H12O. Provide a structural formula which is consistent with all of the given information; give the index of hydrogen (H2) deficiency.

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Which hydrogen of 1-chloropent-2-ene shows the largest chemical (downfield) shift in its NMR spectrum?

a. the H on C-1

b. the H on either C-2 or C-3

c. the H on C-4

d. the H on C-5

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Which of the labeled peaks would allow the distinction of an aldehyde from a ketone based on this spectrum?

a. 1

b. 2

c. 3

d. 4

e. 5

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Which of the following compounds will display a singlet, a triplet and a quartet in its 1H NMR spectrum?

a.  2-chloro-4-methylpentane 

b. 3-chloro-2-methylpentane 

c. 3-chloropentane 

d. 1-chloro-2,2-dimethylbutane

e. 3-chloro-3-methylpentane

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Order the following protons from lowest to highest chemical shift value.

a Ha < Hc < Hb < Hd
b Ha < Hc < Hd < H
c Hc < Ha < Hd < H
d Hc < Ha < Hb < H
e Hc < Hd < Ha < Hb

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Circle the molecule that corresponds to the NMR spectrum shown below.

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Which of the labeled protons would absorb furthest downfield in an NMR spectrum?

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Consider the following compound whose protons are labeled as a, b, c, d and e. Its   1H NMR spectrum is also shown below. Circle the proton that shows a quintet at δ3.55 ppm.

 

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In the  1H NMR spectrum of 1-fluorobutane, the most deshielded hydrogrens are those bound to

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A computer program capable of predicting chemical shifts was utilized for the structure below. Evaluate the results by answering the following questions.

a. Which protons were miscalculated? 

b. Are you predicting that the experimental chemical shift will be in higher or lower field?

c. Why?

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Which of the following is a correct prediction of the chemical shifts for the signals in the 1H NMR spectrum for the following compound?

a) I=3.9 ppm, II=2.2 ppm, III=2.4 ppm

b) I=3.7 ppm, II=2.7 ppm, III=1.9 ppm

c) I=3.7 ppm, II=2.2 ppm, III=1.9 ppm

d) I=3.9 ppm, II=2.7 ppm, III=1.9 ppm

e) none of these

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What is the most likely chemical shift for the CH 3OCH3 in 1H NMR?

a) 1.0

b) 1.5

c) 4.0

d) 5.5

e) 10

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What is the most likely chemical shift for the aldehyde C-H in  1H NMR?

a) 1.0

b) 1.5

c) 4.0

d) 5.5

e) 10

 

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