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#1 2011-03-05 19:03:49

orgopete
Administrator

Unknown (cyclohexanol) from IR and NMR spectra

Yahoo Answers wrote:

Organic Chemistry, NMR spectrum please help?
please help me with this spectrum. I am having a hard time with it and have no idea where to start. I have the IR spectrum of this compound and it looks like it should be carboxylic acid or an alcohol. I need to identify the unknown in order to pass the class. thank you.
so the spectrum contains 3 major signals. the first signal from around 1-2 with multiple peaks and broad. the second signal is at 3, very sharp and strong. the third one is weak and broad around 3.7-4. the integration for the first one is 9, the interation for the second and third are around 1.5

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These are fairly nondescript spectra. That makes it a little harder, but the strategy is the same. Let's start with the NMR. The integral is given as 1.5:1.5:9. Since hydrogens come in 1's, 2's, and 3's, That could be 3:3:18, but that seems unlikely. That would virtually mean that all signals are CH3 groups (and a lot of singlets).

The IR has a strong OH peak, therefore it seems likely there will be an OH in the IR. Let's try the integral again with 1:1:6?. There doesn't appear to be an CH3 groups. Therefore a reasonable structure for 1:1:6 might be cyclobutanol.

If you were to recalculate the integral ratios, I think a much larger number is likely. As you can see from the spectrum, I have included a box for each hydrogen. By my calculation, it is 8? boxes. I added a question mark as there is some drift in the NMR on the left that isn't present on the right. The integral could be larger, 10?. There aren't any distinct CH3 groups, so let us assume for the minute it is a ring. There are two possible rings, cyclopentanol (C5H10O) or cyclohexanol (C6H12O).

http://www.curvedarrow.com/chem/ZZ7A8BE718.jpg

At this point, we can go to SDBS and look up the spectra of these two compounds and compare their IR spectra, peak for peak.

http://www.curvedarrow.com/chem/ZZ5A1C4CF4.jpg

From comparing the IR, it matches cyclohexanol.

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