[pfnm]

Hi,

I am trying to assign the peaks on the 1H-NMR spectrum for two unknowns. I have identified them as 2-propanol and benzyl alcohol.

For 2-propanol, I have an unmagnified spectrum (taken at 500Mz), and one where the spectrum is zoomed in on one peak.
I believe it's the alcohol peak.

But how do I describe it? Is it a doublet? Why is it split? Is it coupled to anything else? (It has a coupling constant that suggests it coupled to CH...but I thought the OH group did not couple).

(I apologise if the size is small large - the two numbers on the peak I'm looking at are 3.144ppm and 3.186ppm...the triplet upfield is for the 2 methyl groups and the septet downfield is for CH)



And magnified on the alcohol peak:



Then for benzyl alcohol:

Why are there two peaks, one at 1.868 and another at 2.178ppm? All the other spectra I was given (C-NMR, MS, IR) show the compound to be benzyl alcohol.

The peak at 4.637 (singlet) is for the CH2, the peaks around 7 are for the aromatic hydrogens...then one of those peaks at 2.178 and 1.868 is for the alcohol. Perhaps both?

The one at 2.178 integrates for 1H. The peak at 1.868ppm has no integration given...what is it?

The solvent used in both instances was CDCl3 and I believe the peak at 0ppm is TMS.



Lastly, the mass spectrum for benzyl alcohol has a base peak at 79 m/z. I am trying to assign the peaks eg 77 for Ph+...what does the base peak correspond to? What fragment?



Thanks alot

[rucik5]

Hi,
I must say i was quite surprised by the behaviour (77/79) of benzyl alcohol - but that indeed is what you have. Here's the spectrum from a different website, for comparison: http://imageshack.us/f/813/msbnoh.gif/
I found in a book that... I'll keep it short. It's easier to see than to explain:



Now, the NMR issue. Finding an OH peak is very tricky; usually they stay around 3-4 ppm, but they may appear literally anywhere (in complex molecules) or not be there at all. Sometimes they couple, sometimes don't. That's due to to the physical properties of a molecule, how prone it is to interact via hydrogen bonding, how wet the sample is, what solvent do you use, how nicely shaped your nmr tube was, how good the spinning and shimming in the magnet was. Erm, but I digress. The peak at 2.1 is the alcohol indeed, your isolated CH2 doesn't couple to it and 5 remaining rings are on the ring. The peak at ~1.8 must be an impurity. On my NMRs i quite often see water as a sharp peak ~1.6. If not that, then just an impurity. Here's another spectrum of BnOH:


You're right about propanol but I don't think I can come up with any decent explanation why the OH peak is of 'that' shape. I'd say it is a broad doublet on the expansion, split by the CH in this case. But when you look at the full spectrum it actually looks like a singlet and that's how I'd quote it (could be wrong though).
Quick search through NMR database shows that OH of isopropanol is a sharp singlet.

Hope I helped a bit.

[orgopete]

This is just a rhetorical comment about the electron impact spectrum and is directed to those whose career path includes teaching organic chemistry. Examine the fragmentation path posted by rucik5. In order to assign the peaks of m/e 77 and 79, a student needs to use the posted or a similar path. My question is, "At the point this is introduced in your organic chemistry class, how many students are familiar with the reactions needed to anticipate this path?" To further complicate the issue, if students obtain real EI MS, the parent peak is either very small or absent. That is, a person must already know what the MW of a compound is in order to find the parent of these compounds.

My personal opinion is that EI MS should be replaced with a discussion of chemical ionization MS. CI MS will very frequently give a P+H peak. For identification of an unknown, I feel this is more important that the fragmentation pattern if combined with other spectroscopic data, e.g, IR and NMR.

I think this is a rational approach and virtually mirrors the popularity of mass spectroscopy. MS is the oldest instrumental method. It was not as popular until CI MS became popular. I feel this is for two reasons. First, it gives the P+H (or P+?) peak that enables one to deduce the MW. The MW of an unknown is more valuable than the fragmentation pattern of an unknown. Second, CI was compatible with a broader range of samples. Sample introduction to a high vacuum and volatility had limited the utility of MS. The development of the liquid chromatography as to introduce samples to a mass spectrometer enabled a broader range of samples and an easier introduction system. This has become the very common atmospheric pressure chemical ionization mass spectroscopy.