[combatwombat]

I realize there are a lot pro's on this board as well as of other students who, like me, are still struggling with NMR problems. I was wondering if any of the pro's (or students who are really good at NMR) could give me some tips on how they learned to do well with NMR/spectroscopy problems.

I have a feeling that the spectrscopy problems in my course are a little on the tougher end. They typically involve NMR (usually 1H, 13C, or both), IR, as well as some clues about the molecule's reactivity. E.g. in this question, we are given the hint that a polymer was reacted with alcohol and acid to give the compound that gives these spectroscopic data.

General advice (i.e. not necessarily specific to this problem) would be much appreciated!

[azmanam]

"a polymer was formed by reacting an alcohol and an acid" ... or ... "a polymer was reacted with an alcohol and an acid."

Anyway, here's how I always tutor people struggling with NMR:

NMR is just a big jigsaw puzzle... except you don't know what the pieces look like.  The NMR describes what the pieces look like.  you have to figure out what the pieces are... then put the puzzle together.

So how do you determine what puzzle pieces you have?  There are 3 main pieces of information you derive from looking at an NMR spectrum: chemical shift, integration, and multiplicity.  for each signal in an NMR, you need to address those three pieces of information in order to draw one puzzle piece.

Take this NMR for example (C₄H₈O₂):
http://www.sigmaaldrich.com/spectra/fnmr/FNMR001634.PDF
There are three signals in the NMR.  We'll go left to right.

First, a quartet just above 4 ppm.  Just above 4 ppm tells us something about the electronic environment of the proton(s) which give rise to that signal.  We know that as protons become more deshielded, the signal moves downfield.  Protons around 4 usually mean the proton(s) are next to an electronegative element: O, Cl, F, ...  We only have O in the given formula, so the protons giving rise to the signal around 4 ppm are probably attached to a carbon atom attached to an oxygen.  The puzzle piece so far is C-O. 

The integration is derived from measuring the ratio of the heights of the integration curves given on the spectrum.  I'll save you the trouble and let you know that, from left to right, the integration values are 2:3:3.  So the signal at 4 ppm has an integration of 2.  That means there are 2 equivalent protons that resonate (give rise to a signal) at 4 ppm.  The puzzle piece now is -H₂C-O. (I've bolded the protons we're discussing at this time)

The multiplicity is a quartet.  Multiplicity arises from protons on adjacent carbon atoms.  n+1 rule tells us that a quartet means that the protons resonating at 4 ppm are on a carbon adjacent to a carbon with 3 protons of its own.  The puzzle piece is completed at H₃C-H₂C-O.




Next signal is a singlet just above 2 ppm.  Same analysis.  2 ppm tells us there's some electron withdrawing ability of adjacent groups, but not a strong one... not really enough information from the chemical shift.  Integration is 3. this is a big clue that the protons resonating at 2 ppm are probably on a methyl group.  Multiplicity is a singlet, which means the protons are on a carbon atom adjacent to something without any protons of its own.  So the puzzle piece for the middle signal is H₃C-.




Final signal is a triplet just above 1 ppm.  1 ppm tells us it's probably alkyl, i.e. just part of a hydrocarbon chain.  Integration of 3 is also a clue it might be a methyl group, and the multiplicity is a triplet - which means the protons are on a carbon atom adjacent to a carbon atom with 2 protons of its own.  puzzle piece = H₃C-H₂C.




Now we need to start putting the puzzle pieces together.  I hope you see the overlap in puzzle pieces 1 and 3.  They both have a CH₃-CH₂ subunit.  That means the pieces probably overlap and the two puzzle pieces can be combined.  Now we take those two puzzle pieces and make them one: H₃C-H₂C-O.  We also still have the 2nd piece: H₃C-.



At this point, there's no obvious way to connect the puzzle piece is there?  Well, there's at least one more piece of information we have (and in this case, two).  The molecular formula was given as C₄H₈O₂.  We have two puzzle pieces, which together account for C₃H₈O₁.  That means we still have one carbon atom and one oxygen atom unaccounted for.  We also have the ¹³C spectrum (above the ¹H).  Note the signal around 170.  this is characteristic of a C=O double bond.  This nicely accounts for the remaining elements and allows us to combine all the puzzle pieces.

See, C=O is missing 2 substituents on C.  H₃C- needs to form one more bond to C and H₃C-H₂C-O needs to form one more bond on O.  They can both be tethered through the carbonyl to finally complete the puzzle and derive the structure for ethyl acetate:  H₃C-C(O)-O-CH₂-CH₃.

Take home message was: 1) build puzzle pieces addressing chemical shift, integration, and multiplicity.  2) Look for overlap among puzzle pieces and combine puzzle pieces where appropriate.  3) Subtract known puzzle pieces from molecular formula (if given) to figure out 'whats missing.'

Hope that helps.

[combatwombat]

I should have said "A polymer is treated with acid and alcohol..."

Great walkthrough, you should have taught NMR in my class...

As for the problem you went through, what makes you choose ethyl acetate over methyl propanoate? Is the deshielding of the 2H quadruplet all you have to go on? (i.e., methyl propanoate would have it's 2H quadruplet more upfield, where the 2H singlet is?)

[azmanam]

what makes you choose ethyl acetate over methyl propanoate?

chemical shift at 4.  the furthest downfield is most deshielded, i.e. next to oxygen.  methyl propanoate would have a singlet integrating to 3 at 4 ppm and a quartet integrating to 2 at 2 ppm. 

great question.

Is the deshielding of the 2H quadruplet all you have to go on?

no, there's also the singlet at 2 ppm (which is not at 4 ppm).  One thing I forgot to mention is as you build your puzzle pieces and start to put them together, you can (and should) always go back and check your work.  Look at your puzzle piece and go in the opposite direction.  Look at the puzzle piece and say 'what signal would I expect to see for those protons?'  for the methylene protons (the CH₂ protons), I'd say, 'well, there's 2 protons, so the integration is 2.  They're next to a methyl group, so the multiplicity is a quartet.  And they're next to an oxygen atom, so they'd be around 4 or so.'

[azmanam]

and, yes.  Your spec questions are really hard. (at least this one is).  i finally figured it out, though.  I'll try to walk you through it best I can.  Have you given this one a start?

[Squirmy]

When you first learn NMR, you're given general approaches to the problems...Azmanam's example is a very good one.

Once you've gotten the general stuff down (general ranges for chemical shifts, integration, splitting), your interpretation skills will likely only improve with practice. I've had more years of practice than I'd like to admit, but I still pick up things here and there. For me that keeps it fun

I like the problem you posted. Combined spectral questions are pretty pointless if you don't have to use more than one spectrum. In this case, I found the MS and IR quite useful. The "polymer" starting material was somewhat helpful, but it's a subtle hint.

I'd also be happy to help if you throw in your 2 cents first.

[Icey_cold]

I have qns regarding the 1H NMR. how to u tell the different between a meso acetonide from chiral acetonides using 1H NMR?

[MissPhosgene]

Think about symmetry. The only way to really learn how to solve NMRs is to practice solving problems on your own.

[McCoy]

I realize there are a lot pro's on this board as well as of other students who, like me, are still struggling with NMR problems. I was wondering if any of the pro's (or students who are really good at NMR) could give me some tips on how they learned to do well with NMR/spectroscopy problems.

I have a feeling that the spectrscopy problems in my course are a little on the tougher end. They typically involve NMR (usually 1H, 13C, or both), IR, as well as some clues about the molecule's reactivity. E.g. in this question, we are given the hint that a polymer was reacted with alcohol and acid to give the compound that gives these spectroscopic data.

General advice (i.e. not necessarily specific to this problem) would be much appreciated!

NMR is very easy...I think you are a first student (taking chemistry class for the "first time at uni"). All that I can say is get a good introductory book on spectroscopy. I suggest Pavia, Lambert. Study them for a month and you'll be amazed how easy this thing is.

[coolakul]

I would recommend a couple of websites to learn the basics of nmr, they really helped me out a lot and will help you learn how to solve problems like this in a jiffy. In fact, in real life we do not have the luxury of getting as much structural data as what you have got in the above problem. We would probably just take a proton or a proton and carbon and try to predict the structure based on the reaction. If that doesn't work we will try to get the mass spectrum. The two websites I would recommend are:
1. NMRCentral.com - this website covers the basics of nuclear magnetic resonance and discusses why chemical shifts occur etc.
2. NMR at MSU - This page covers a lot of examples of sample spectra and it gives the solutions to the examples as well so that you may practice.
Going through these two websites really helped a lot for me personally.

[fledarmus]

I took Org 1 when NMR's were just starting to be used for compound identification. My professor was one of the leaders in the field at that time and had a unique way of teaching it that has stood me in good stead to this day. He taught us to predict the NMR spectra of compounds at the same time that he taught us nomenclature. For example, we learned that methane was a single carbon attached to four hydrogens that were all equivalent, and would show a singlet integrating to four protons in the NMR. Ethane was two carbons linked together, each attached to three hydrogens, and all six hydrogens were equivalent, still showing a singlet in the NMR. Propane had three carbons, but the protons on the middle carbon were not equivalent to the protons on the end carbons, and you would now see a splitting pattern where the methyl groups, being adjacent to two protons, would be split into a triplet integrating to six protons, while the mehylene protons, adjacent to six protons, would show a septuplet... and so on. Every new functional group that we learned how to name, we also learned its effect on the NMR spectrum.

I've found that this practice predicting spectra has been a solid foundation for interpreting them.