[rogerwilco]

Ok so here's the proton NMR data I have:

C7H16O

0.98ppm: 9H - quartet
1.10ppm: 3H - singlet
1.25ppm: 1H - singlet
1.50ppm: 2H - quartet
1.75ppm: 1H - septet


Now, I have 2,2-dimethyl-3-pentanol as the answer, but if that were the case, the 9H at 0.98ppm would not be split, correct?  Or is there possibily some long range splitting?  I feel like I'm onto the correct answer here, but there are a few things that are making me second guess myself.  Any help here is greatly appreciated!

[Dan]

With shifts like that, doesn't it imply that there are no protons alpha to oxygen (or bonded directly to oxygen)?

I too am stumped...

[Dude]

Too difficult for me.  I have suspicions as to whether the listed criteria are possible.  As Dan indicated, the chemical shift values are too low for either an ether or an alcohol, the only two possibilities with a saturated structure.

[rogerwilco]

Well, according to my text (Solomons and Fryle), alcoholic hydrogens can exist at chemical shifts between roughly 1.0 and 6.0, give or take.  It shows a wide range possible.  I was more hung up on the splitting patterns.

[mike]

What about 2,3-dimethyl-3-pentanol?

[movies]

Well, according to my text (Solomons and Fryle), alcoholic hydrogens can exist at chemical shifts between roughly 1.0 and 6.0, give or take.  It shows a wide range possible.  I was more hung up on the splitting patterns.

The O-H protons can be anywhere in that range, but the Hs on the carbon bearing the OH would be shifted to about 3 or 4 ppm.

I don't see how you can get a 9H quartet.  Is that even possible?  Are you sure it's not a couple of singlets?  Or a singlet plus a triplet?

Y'know, I bet it's a doublet plus an overlapping triplet.  A doublet that integrates to 6H plus the septet would give you an isopropyl group, and the triplet plus the quartet at 1.5 would give you an ethyl group, so then Mike's suggestion would match nicely.

[Albert]

What about 2,3-dimethyl-3-pentanol?

Yes, I agree with you. This is the only reasonable answer.

[rogerwilco]

Yeah, 2,3-dimethyl-3-pentanol was actually my first idea for the answer, but a friend of mine working on it convinced me it couldn't be since there weren't three equivalent methyl groups in that structure (i.e. 9 equivalent hydrogens).

As far as leniency to the rules of NMR, I'm not too well versed, so when it comes to something like the possibility of having the 3H of carbon 1 and the 3H of the first methyl group exist virtually as the same peak (at virtually the same chemical shift) as the 3H on carbon 5, I wasn't sure how likely that was.  It does make sense that, assuming there is room for this type of thing, they would all show up at about the same spot, since the are virtually identical except that there is no methyl group off of carbon 4 to provide total symmetry.

So I assume the majority of you are thinking it's 2,3-dimethyl-3-pentanol?  Because now I'm very tempted to revert back to that answer too.

[Dude]

Sounds reasonable.  Were you given just the descriptions you presented or are you looking at the H-NMR spectrum and made the inference that it is a quartet?  If it is coincidental overlap of a doublet with a triplet for the 9 H integration, I would expect that the relative intensities of the peaks should deviate slightly from the ideal value (or there will be a shoulder) and the spacing between peaks should not be symmetrical (i.e. the quartet at 1.5 should look symmetrical and the quartet at 0.98 should look "funny").

[wereworm73]

I have a NMR spectrum of 2-methyl-3-pentanol, and the signal for the hydrogens highlighted in red is only 0.01 ppm away from the signal of those highlighted in yellow-orange.


  CH3 H
   | |
H3C-C-C-CH2-CH3
   | |
   H OH
 

That would back up movies' suggestion of a doublet & triplet overlapping.

Also, since there is no signal at around 3-4 ppm and the mystery molecule is known to be saturated and to contain 1 oxygen, that oxygen would need to be attached to a tertiary carbon. 

I think mike is right on the money about this molecule being 2,3-dimethyl-3-pentanol.

[Dude]

Good reasoning by Mike/ Movies/Albert, but I find it hard to believe that the lowest absorption would be called a quartet.

http://www.sigmaaldrich.com/spectra/fnmr/FNMR000512.PDF

[rogerwilco]

The "quartet" at 0.98ppm definitely is a little messy.  It's a photocopy of an NMR from a textbook I believe, so it's not the most clear thing in the world to begin with.

I think everyone's explanations for 2,3-dimethyl-3-pentanol help to reinforce my own initial thoughts (it's just that you guys actually have concrete reasons for it!).  It's obvious now that the carbon bonded to the O definitely has to be tertiary, otherwise there would be chemical shifts farther downfield.  The way the intergrals are given on the spectrum definitely seems to indicate the ratios I originally posted, and I think 2,3-dimethyl-3-pentanol has to be the best choice.  Everything else seems to sync up pretty well.


Oh and I just finally got Dude's PDF to work and that's exactly the spectrum I have.  I assume that's for the product we've been discussing?  And yeah, on that one it definitely doesn't look like a quartet, and like I said, on mine, the "quartet" is not symmetrical and does look sloppy.