NMR for isopropanol(2-propanol)

[Luthertaketwo]

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I can't figure out why this NMR is showing me a pentet for the lone hydrogen in the NMR instead of septet. Is it suppose to be doing this?

It shows:
A doublet at 1 PPM.
A singlet at 3.5 PPM
A pentet at 3.82 PPM.

The NMR should be, I think:
A doublet at ~1PPM
A septet at ~3.5 PPM.
A singlet at ~4 PPM.

Also, it seems strange that the pentet is showing as being the most downfield, when i would think that the lone proton on the alcohol should be furthest downfield.

 

[jlee9531]

about the hydrogen on the alcohol....

you shouldnt always think that it is the most downfield since that H is very unpredictable in HNMR. The chemical shift can change on a dauly bases depending in temperature, concentration etc...

these changes can occur prolly cause of the hydrogen bonding the OH group can form...thus masking the deshielding effect of the O.

the pentet you see would prolly feel the most deshielding effects from the oxygen...hence it is the most downfield.

 

[Luthertaketwo]

I'm just going to burn this worthless ochem book. All they say is that a proton such as that will not couple other protons. This NMR stuff is really starting to piss me off.

 

[Hamface16]

Actually the data you have sound about right. The lowest downfield at 1ppm are the 6 hydrogens of the methyl groups which are a doublet b/c they are next to the one hydrogen on the methylene. The next downfield should by the hydroxyl hydrogen, which rapidly exchanges in an NMR solution and shows up not as a singlet (s) but as a broad small (bs) peak. Finally the most downfield will be the single hydrogen on the middle carbon, which is shifted so far downfield due to its proximity to the oxygen. Most probably it should show up as a doublet of a triplet, but if you are looking at a real spectrum not a perfect one use in an organic text one of the peaks might be hard to see.

Unlike CNMR where any carbon connected to an oxygen will appear a lot downfield, in PNMR the hydrogen connected to an oxygen will usually appear lower in the spectrum.

 

[Luthertaketwo]

Er... actually. I just looked really close and it does appear that it is actually a septet.

http://www.chem.ucla.edu/cgi-bin/webspectra.cgi/bp6/H?390,210

Need my glasses, I guess.

 

[musiclink213]

told ya you'd get better answers here.

 

[jlee9531]

nmr is fun man. think of it as a puzzle. unless you hate puzzles...then you are in for hell. .

and you can always turn to sdn for ochem geniuses. there are many. tho its been about 4 years since i did any. so ask others. im rusty haha.

 

[Luthertaketwo]

Originally posted by jlee9531
nmr is fun man. think of it as a puzzle. unless you hate puzzles...then you are in for hell. .

and you can always turn to sdn for ochem geniuses. there are many. tho its been about 4 years since i did any. so ask others. im rusty haha.

Oh, well thanks. I'm sure I will have to tap that. This NMR stuff kind of reminds me of when you first start doing organic and you are asked to make all the constitutional isomers of some generic chemical formula.

 

[Cerberus]

lab monkey work