1H NMR: Cumulative Practice

Concept: Practice 1: Drawing NMR Diagrams

9m
Video Transcript

Hey guys let's take a look at the following practice question so here's it says draw the approximate positions that the following compound might show in its proton NMR absorptions, so basically what we need to first realize is we're doing a proton NMR sketch, protons NMR usually goes from 0 parts per million or delta to around 10 parts per million or delta that's a normal range, in some longer versions of NMR spectrums we can go as high as 13 and the reason for that is that Carboxylic acid protons can go as high as 13 parts per million so we 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 is compound contain? And then second what is multiplicity of each one? And then finally where exactly on the NMR spectrum will we find that particular signal? So what we need to do first is let's look at how many different proton signals we have so 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 so this is our second signal, here is another CH2 that's in between two other CH2 so it's has its own unique signal and what we need to realize here is that this molecule is symmetrical, you can cut it in half and each half looks the same so this would be our third signal again, second signal and first single again so we figured out there are 3 protons signals. Next what we need to do is figure out what are the multiplicities of each of those protons? So here we're going to say remember OH groups are always seen as singlets so this will be a singlet, now let's look at the CH2 next to it remember we're following the N+1 rule so here we're looking at these hydrogens here which count as one, the neighboring carbon is this carbon which has two hydrogens on it so that's 2+1=3 which is a triplet so there will be a triplet. Now we're going to look at these protons here which count as one, the neighboring carbon is this carbon but remember we can't count this carbon as a neighboring carbon different for me because both of these are the same exact signal so the only neighboring carbon we're looking at is this one which has two hydrogens on it so this would also be a triplet so 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 group is right next to an OH group we're going to say that since it's right next to that group it's alpha to that group so this CH2 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 Deshields 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, so again remember that CH2 is Alpha to that OH we also have another CH2, this is CH2 over here it is not alpha to that OH it is beta to it its two spaces away from the OH group and we're going to say if you're beta to that oxygen then you're going to feel 1/5 of its effect what I mean by 1/5 is that CH2 initially is still 1.2 if you're Alpha to an OH you feel 2.5 increase in your PPM if you're beta to it you're going to feel 1/5 of that So what is 1/5 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 so we're going to say that those protons on the first CH2 will be around 3.7 the other 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 so we're going to say it's closer to the 5 ppm range so it's going to be the most downfield so 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're also going to 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 one let's take this into account we're going to say the integration for this molecule is 2 alcohol so that's 2, we have 2 CH2 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 by 2 so this becomes 1 to 2 to 2 so we're going to say that the alcohol hydrogen is the most deshielded we'll put it closer to 5 so put it around here we'll make that peak go up that high it's a singlet so I 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 so we're 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 but then that's good, I'm a horrible drawer but your professor should be able to see that as a triplet and then the last one is around 1.7 that's also a triple, try to do our best to draw it as best as we can hopefully you can draw better than I can so that would be our proton NMR spectrum so we went over a bunch of things, first figure out the number of signals, second figure out the multiplicity are they singlets, 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.

Concept: Practice 2: Drawing NMR Diagrams

7m
Video Transcript

Hey guys, let's take a look at the following practice question. So, here it says, draw the approximate positions that the following compound might show in its proton NMR absorptions. So, what we need to first do is we need to figure out how many protons signals will the following compound give me. So, 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, represents our first proton signal, we're going to say this the only CH2 directly connected to the carbonyl. So, that's our second signal, the only CH2 in between a CH2 and CH3. So, that's our third signal and this would be our fourth signal. So, we have for 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 one, the neighboring carbon has no protons on it. So, N is 0. So, this represents a singlet, next this CH2 represents one, neighboring carbon is this one and this one, this one has two hydrogens on it, the other one has none. So, here this would be three, so this is a triplet, we're going to say here this represents one, 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 and then finally these three represent one, the neighboring carbon has two hydrogen's on it. So, 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 look at the first signal it is a CH3 group, CH3 groups start off around 0.9 ppm. Now, if you are next to a carbonyl, if you are alpha to a carbonyl it's going to increase your chemical shift by 1 ppm. So, it's going to be around 1.9 ppm, next our second signal is a CH2 group, CH2 starts around 1.2 ppm, it is also alpha to a carbonyl so it is increased by 1, it's going to be around 2 to 2.2 ppm, the third signal is also another CH2, which is going to start off at 1.2 ppm. Now, if you're next to a carbonyl it increases your chemical shift by 1 ppm, if your beta to it means you're two spaces away from it, you're going to feel one fifth of the effect. So, divide 1 by 5, will give you the 1-5 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, its gamma to it, it's too far away, three spaces away from the carbonyl. So, it's not going to feel its effect. So, skin is just going to be at 0.9 ppm. So, these would be the approximate chemical shifts of each proton.

Now, we come down here to the graph. Remember, proton NMR 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. So, here we're just going to go from 0 to 10 and since most of our numbers seem to be around 1 or 2, we're going to make this one, I'm going to make this one here too, just to give us greater separation between the signals, let me take myself out of the image and let's try best to plot each of these, so the first signal, the first methyl group, we looked at is a singlet that we set and we set 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 then it goes to singlet. Now, the CH2 next is a triplet that we said, it's around 2.2, it has two hydrogen's on it so we got to make sure we draw it high enough it's around 2.22 triplet, next we have our third 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, and actually that's CH2 is not a triplet, it should be a sextet. So, we've got to correct that, since it's a sextet it's going to look like this, like that and then that CH3 is a triplet. So, we got to draw it around 0.9, its integration is 3 because there's 3 hydrogen's giving us that signal, again not pretty I'm 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, a number of proton signals given to us then we look at the multiplicity then we approximate the chemical shift and 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.

1H NMR: Cumulative Practice Additional Practice Problems

The reaction between compound 1 and NaOCHMe2 in Me2CHOH as a solvent produces a mixture of two products 2 and 3, all with identical molecular formula and with a relative yield of 50% and 50% respectively. The 1NMR of these compounds with relative integrations of signals is shown below. What are the structures of compounds 2 and 3?

b. With what reagent and solvent would you treat compound 1 if you want to obtain higher yield of compound 2? Explain your answer with 1-2 sentences

c.  With what reagent and solvent would you treat compound 1 if you want to obtain higher yield of compound 3? Explain your answer with 1-2 sentences

d.  Based on the data above, what product(s) and in what relative ratio(s) would you obtain by treating compound 4 ((t-But)2CHCH(Br)CH3) with NaOCHMe2 in Me2CHOH as the solvent under the same conditions as above? Explain your answer with 1-2 sentences.

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Predict the 1H NMR of compound A in CDCl3. Show clearly the relative intensity, chemical shift and splitting pattern of each peak. Indicate in the spectrum, using the appropriate number shown for the carbons, where the protons resonate. Note: add as reference peaks the TMS and residual CHCl3.

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A compound C4H9Br gave the following 1HNMR spectrum: triplet, δ 1.0 (3H); doublet, δ 1.7; multiplet, δ 1.8; multiplet, δ 4.1 (1H) Which is a reasonable structure for the compound?

A) CHCH2CHBrCH3

B) CH3CH2CH2CH2Br

C) (CH3)2CHCH2Br

D) (CH3)3CBr

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A compound C5H10O gave the following spectral data:

1H NMR spectrum                              IR spectrum

doublet, δ 1.10                                  strong peak

singlet, δ 2.10                                   near 1720 cm    -1

septet, δ 2.50

Which is a reasonable structure for the compound?

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An unknown compound W has the molecular formula C 4H8O2. Based on the following 1H NMR spectrum, what is the structure of compound W? 
 

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The molecular formula, IR & NMR are provided for the following unknown compounds. For each problem, provide the structural information from the IR & NMR and propose a structure for the unknown.

 

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Which of the following atoms has a quantum spin number?

A) 2H

B) 12C

C) 15N

D) 16O

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Compound A can be converted into compound B by the reaction shown below. Considering the two 1H NMR spectra provided, draw the structures corresponding to compounds Aand B in the box.

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The 1H NMR spectrum of Compound C (C10H14O) is shown below. Draw the structure of the compound C in the box provided. Mark down your thought process. Even if the final structure is wrong, partial credit might be given if your analysis and approach make sense.

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The 1H NMR spectra for the following molecule is provided. In the fragments box below, propose which fragments are elucidated by the spectra and use arrows to match those fragments to each signal on the spectra, respectively. 

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Answer the following questions about product identity based on IR & NMR spectroscopy.

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Which structure is consistent with the following 1H NMR spectrum?

a ethyl acetate

b 2-propanone

c 3,3-dimethyl-2-pentanone

d ethyl 2,2-dimethylpropionate

e none of theses

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Which of the compound listed below has a structure consistent with the following   1H NMR spectrum?

a. 4-bromoanisole
b. Anisole
c. 4-bromoaniline
d. None of them
e. Phenol

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What  1H NMR spectral data is expected for the compound shown?

a. 3.8 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)                        c 3.3 (3H, s), 2.6 (3H, septet), 1.0 (6H, d)

b. 3.8 (1H, septet), 9.3 (3H, s), 1.0 (6H, d)                        d 2.6 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)

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

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

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There are four constitutional isomers of dichloropropane. The NMRs of 3 of these are shown below. Match each isomer with its NMR and provide an interpretation of each spectrum. (i.e. draw the structure on the NMR and  label protons , with their splitting.)

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Predict the structure of the compound that gives rise to the following  1H NMR spectra.

Molecular Formula C5H9CIO

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Sketch the 1H NMR spectrum and assign the peaks in each of the following compound.

 

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Which compound belongs to following 1H NMR?

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Which compound  shows  three signals (all singlets) in 1H NMR spectrum?

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Assign the 1H NMR to the appropriate structure out of the following possibilities. Choose the letter corresponding to the appropriate molecule  for this question. 

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Assign the 1H NMR to the appropriate structure out of the following possibilities. Choose the letter corresponding to the appropriate molecule  for this question. 

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Assign the 1H NMR to the appropriate structure out of the following possibilities. Choose the letter corresponding to the appropriate molecule  for this question. 

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The 1H NMR spectrum of a compound with formula C 7H14O gives two signals. Which of these structures is a possible one for this compound?

A) 2-Heptanone

B) 3-Heptanone

C) 2,4-Dimethyl-3-pentanone

D) 2,2-Dimethyl-3-pentanone

E) Two of the above

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How would you distinguish between these compounds via  1H NMR?

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Predict the structure of the compound that gives rise to the following  1H NMR spectra: Molecular Formula: C9H11Cl

 

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 Predict the structure of the compound that gives rise to the following  1H NMR spectra: Molecular Formula: C4H8O

 

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