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I came across the compound 3-methylpentan-2-ol. Looking at its proton NMR spectra, it seems there is a lot of splitting for the signal at 4 ppm.

3-methylpentan-2-ol NMR spectra

I'm assuming that the 4 ppm represents the proton in the hydroxyl group (since it clearly is much more deshielded than the other protons), but this would mean it only has one neighbouring hydrogen and hence should exist as a doublet.

I just wanted to clarify, is there any reason why the 4 ppm peak has so much splitting despite only having one neighbour? Or otherwise, I have also considered, could I be incorrect in assuming the 4 ppm peak represents the hydroxyl group, and if so why?

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  • $\begingroup$ You are not correct in assuming the proton at 4 (actually closer to 3.6 - it all counts) is the proton on the -OH. So a question back to you - which proton is it? $\endgroup$
    – long
    Commented Mar 12, 2020 at 5:40
  • $\begingroup$ Why wouldn't the proton at 4 be due to OH, as wouldn't it be the most deshielded due to oxygen? But anyways, to answer your question, would it be the proton attached to the C-OH group? $\endgroup$
    – math_learner
    Commented Mar 12, 2020 at 6:09
  • $\begingroup$ Peaks from [labile protons in NMR ](chemistry.stackexchange.com/…) are frequently not observed, and have variable chemical shifts, unless great care is taken in sample preparation. This sample was runin CDCl3, and the -OH peak exchanges with H2O, and comes at ~1.6ppm. So, yes, the peak at 3.6 is from the CH(OH). So, can you account for the amount of splitting now? This is actually a very tricky spectrum if you are starting out with NMR. $\endgroup$
    – long
    Commented Mar 13, 2020 at 1:25

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This question has a few different parts to it.

Part 1 - what's that peak at 3.6ppm? Well, it's not the -OH peak. The peak at 3.6 is from the CH(OH).

Part 2 - where is the -OH peak if that isn't it at 3.6ppm? Peaks from labile protons are frequently not observed, and have variable chemical shifts, unless great care is taken in sample preparation. This sample was run in CDCl3, and the -OH peak exchanges with H2O, and comes at ~1.6ppm.

Part 3 - can we account for the splitting of the peak at 3.6ppm now we know what it is? enter image description here The CH(OH) proton should couple to proton on the adjacent carbon (split to a doublet) and also the protons of the methyl group (split to a quartet). It won't couple to the -OH for the same reason we don't readily observe the -OH; it is rapidly exchanging with water. So, we expect that the peak at 3.6ppm should be a doublet of quartets. Looks more complicated than that to me.

Part 4 - It looks more complicated that a doublet of quartets to me. What's happening there? Of course, 3-methylpentan-2-ol has 2 stereocentres, and so we have here a mixture of diastereomers in solution in the sample. Both diastereotopic CH(OH) protons have very similar chemical shifts, and so there are in fact 2 sets of doublets or quartets. It's also why the rest of the spectrum looks such a mess.

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