Draw the approximate positions that the following compound mig...

Question

Draw the approximate positions that the following compound might show in its 1H NMR absorptions?

1H NMR spectrum of a compound with hydroxyl groups, showing expected absorption positions.

Accepted Answer

Hey, guys. Let's take a look at the following practice question. Here it says, draw the approximate positions that the following compound might show in its proton NMR absorptions. Basically, what we need to first realize is when we're doing a proton NMR sketch, proton NMR usually goes from 0 parts per million or delta to around 10 parts per million or delta. That's the normal range. In some longer versions of NMR spectrums, we can go as high as 13. The reason for that is that carboxylic acid protons can go as high as 13 parts per million. We'll just do 0 to 13. Now, what we need to do here in order to draw the spectrum for this, we need to first figure out how many proton signals does this compound contain. And then second, what is the multiplicity of each one? And then finally, where exactly on the NMR spectrum will we find that particular signal? What we need to do first is let's look at how many different proton signals we have. We have CH2 here, CH2 here, CH2 here and CH2 here. Now, remember, this proton is unique because it's the only one connected directly to an oxygen. Now, here we have a CH2 connected to an Oh and connected to a CH2. This is our second sigma. Here is another CH2 that's in between 2 other CH2s, so it's its own unique signal. What we need to realize here is that this molecule is symmetrical. You can cut it in half and each half looks the same. This would be our 3rd signal again, second signal and first signal again. We figured out that there are 3 proton signals. Next, what we need to do is figure out what are the multiplicities of each of those protons. Here we're going to say remember Oh groups are always seen as singlets. This will be a singlet. Now, let's look at the CH2 next to it. Remember, we're following the n plus 1 rule. Here we're looking at these hydrogens here, which count as 1. The neighboring carbon is this carbon which has 2 hydrogens on it. That's 2 +1 equals 3, which is a triplet. That would be a triplet. Now, we're going to look at these protons here which count as 1. The neighboring carbon is this carbon. Remember, we can't count this carbon as a neighboring carbon different from me because both of these are the same exact signal. The only neighboring carbon we're looking at is this one which has 2 hydrogens on it. This would also be a triplet. You're going to have a triplet, a triplet and a singlet. Now, what we're going to say next is, where exactly would we find these guys on our graph? We're going to say here, an Oh group is usually between 1 to 5 PPMs or delta. Now, a CH2 group, which is called a methylene group, is around 1.2 ppm. But if we take a look at the first CH2 group, this CH2 2 group is right next to an Oh group. We're going to say since it's right next to that group, it's alpha to that group. So this CH 2 is alpha to an O from an alcohol. We're going to say if you're alpha to an alcohol oxygen or an ether oxygen, that's going to deshield you and increase your signal. And how much does it increase it by? By approximately 2.5 ppm. So We're going to say that this CH2 will be around 3.7 or so ppm. Again, remember that CH2 is alpha to that Oh. We also have another CH2. This CH2 over here, it is not alpha to that Oh. It is beta to it. It's 2 spaces away from the Oh group. We're going to say if you're beta to that oxygen, then you're going to feel 1 5th of its effect. What I mean by 1 5th is that CH2 initially is still 1.2. If you're alpha 208, you feel 2.5 increase in your PPM. If you're beta 208, you're going to feel 1 5th of that. What is 1 5th of 2.5? Divided by 5, that means it's going to increase your PPM by 0.5. So we should get a signal around 1.7 PPM. We're going to say that those protons on the first CH2 will be around 3.7. The other ones will be around 1.7. Since this is a hydrogen directly connected to an oxygen, we're going to say it's the most deshielded. We're going to say it's closer to the 5 ppm range. It's going to be the most downfield. If we look at this graph, we're going to say let's say that this here represents 1. Here, this represents 2. Here, this is 3, 4, 5. And the reason I'm spacing it out enough is that we can see each signal clearly. Let me take myself out of the image, guys. What we're going to do next is we've figured out the number of signals. We figured out the multiplicity. We figured out their chemical shifts. Last, we're going to put it on this graph. But we have to also account for integration. Remember, integration looks at the height of your peak. And the height of your peak is based on the number of hydrogens giving us that signal. So we're going to say the alcohol has only 1 well, actually more than 1. Let's take this into account. We're going to say the integration for this molecule is 2 alcohols, so that's 2. We have 2 CH2s that are equivalent, so that's 4. And then we have another 2 CH2 that are equivalent, so that's another 4. So the integration will be 2 to 4 to 4. If we wanted to reduce this to its simplest form, everyone is divisible alcohol hydrogen is the most deshielded. We'll put it closer to 5. We'll put it around here. We'll make the peak go up that high. It's a singlet so I'll show it like that. Next, we're going to look at the CH2, the second signal. It's going to be around 3.7. Remember it is a triplet so we need to draw it as a triplet. I'm going to show it like this. Remember, it's not an exact science how you should draw it. As long as you're within the right approximate range, if your professor can clearly see it as a triplet, then that's good. I'm a horrible drawer, but your professor should be able to see that as a single as a triplet. And then the last one is around 1.7. That's also a triplet. Do our best to draw it as best as we can. Hopefully you can draw it better than I can. So, that would be our proton NMR Spectral. So we went over a bunch of things. 1st, figure out the number of signals. 2nd, figure out the multiplicity. Are they singlets, doublets, triplets, whatever. Next, figure out doublets, triplets, whatever? Next, figure out their chemical shifts. What effect will an electronegative group have on the position of that signal? Then once you figure that out, you can plot it on your graph. But you also have to take into account integration. Integration is the number of hydrogens giving us that signal. It determines the height of my signal. Knowing all those things together helps to give you the best proton NMR.

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Draw the approximate positions that the following compound might show in its 1H NMR absorptions?

NMR Practice practice set