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 a carbonyl group and methyl groups.

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. What we need to first do is we need to figure out how many proton signals will the following compound give me. We're going to take a look. We're going to say this is the only CH3 directly connected to a carbonyl. This carbonyl So it represents our first proton signal. We're going to say this is the only CH2 directly connected to the carbonyl, so that's our second signal. The only CH2 in between a CH2 and a CH3, so that's our 3rd signal. This would be our 4th signal. We have 4 proton NMR signals in this compound. Now that we figured that out, next thing we want to do is figure out what is our multiplicity. Which ones are doublets or singlets or triplets. First, if we look at our first signal, we're going to say these hydrogens represent 1. The neighboring carbon has no protons on it, so n is 0. This represents a singlet. Next, this CH two represents 1. Neighboring carbon is this one and this one. This one has 2 hydrogens on on it. The other one has none. So here this would be 3. So this is a triplet. We're going to say here this represents 1. This CH3 and this CH2, when you add up all the hydrogens together gives us 5, which is 6. So this is a sextet. Or we can say multiplet. Then finally, this, these 3 represent 1. The neighboring carpet has 2 hydrogens on it. This would be a triplet. So, we just figured out the number of signals. We just figured out the multiplicity. Now, we have to figure out what's the approximate chemical shift of each compound. Now, here, if we take a look at the first signal, it is a CH3 group. CH3 groups start off around 0.9 ppm. If you are next to a carbonyl, if you're alpha to a carbonyl, it's going to increase your chemical shift by 1 ppm. It's going to be around 1.9 PPM. Next, our second signal is the CH2 group. CH2 starts around 1.2 ppm. It is also alpha to carbonyl, so it is increased by 1. It's going to be around 2.2 ppm. The 3rd signal is also another CH2, which is going to start off at 1.2 ppm. If you're next to a carbonyl, it increases your chemical shift by 1 ppm. If you're beta to it, that means means you're 2 spaces away from it, you're going to feel 1 5th of the effect. So divide 1 by 5 will give you the 1 5th effect. So it's going to increase it by 0.2. And then here, last, we have a CH3 which is not alpha to the carbonyl, not beta to it. It's gamma to it. It's too far away. It's 3 spaces away from the carbonyl, so it's not going to feel its effect. It's just going to be at 0.9 ppm. These would be the approximate chemical shifts of each proton. Now, we come down here to the graph. Remember, proton MR spectrums normally are from 0 to 10 ppm or delta. We can expand them out up to 13 because if you have a carboxylic acid proton, it can go as high as 13. Here, we're just going to go from 0 to 10. Since most of our numbers seem to be around 1 or 2, we're going to make this 1 and we're going to make this 1 here 2, just to give us greater separation between the signals. Let me take myself out of the image and let's try our best to plot each of these. So the first signal, the first methyl group we looked at is a singlet that we said and we said it's around 1.9. Now, when we're plotting this, we also have to take into account another factor which is integration. Remember, the height of your peak is based on the number of protons giving us that signal. CH3 is a singlet so we got to make it go all the way up to 3. And there goes a singlet. Now, the CH2 next is a triplet that we said. It's around 2.2. It has 2 hydrogens on it, so we're going to make sure we draw it high enough. It's around 2.2. It's a triplet. Next, we have our 3rd signal which is also CH2. We said it's around 1.4. So, we're going to say it's around here. And then finally, the CH3 is around 0.9. It is a triplet as well. Oh, and actually, that CH2 is not a triplet. That CH 2 we said was a sextet. So we gotta correct that. Since it's a sextet, it's gonna look like this. Something around like that. And that CH3 is a triplet, so we got to draw it around 0.9. Its integration is 3 because there's 3 hydrogens giving us that signal. Again, not pretty. Not a great drawer. But this is the best approximation for the signal that we're expecting to see with their chemical shifts for the following compound. So remember, take into account first the number of proton signals given to us. Then we look at the multiplicity. Then we approximate the chemical shift. As we're plotting it on the NMR spectrum, you have to take into account the integration. That tells us the height of each signal. Doing that will guarantee you the best possible answer.

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

NMR Practice practice set