i hate NMR's!!!!!!!

[musiclink213]

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my lab notebook is due tomorrow, and of course i wait until last minute to write all the discussions and analyze all the compounds and figure out waht i have using NMR and IR spectra. grr.... i can't do it!!!!! i hate that damn NMR!!!!!!

anyone know what it means when in a proton NMR there's only one peak? does that mean it is being shielded?

 

[femdoc]

Give more info...

 

[twinklz]

It usually means that all the protons are "equal"...for instance if you looked at an NMR of cyclohexane, you would only see one peak. But if you add a functional group to one of the carbons, you'd start getting multiple peaks (and then you have to worry about splitting and stuff). Actually, I like analyzing NMRs...its like a puzzle!

 

[Grumpy Bear]

I may be oversimplifying your question, but doesn't it just mean that there's only one type of hydrogen in the compound (i.e. all the hydrogens are equivalent by virtue of having identical connections and surroundings)? Ex: CH3CH3

 

[jlee9531]

NMRs were fun!

one peak? that sounds like equiv Hs.
i dont have notes with me or anything but yeah they be equiv.

and then just use the carbon nmr to figure out the rest.

 

[YaFwiendToluene]

if its only one peak on the NMR, then you may be dealing with a symmetric molecule. I can't really tell from the information that you provided...but is it like a 6H singlet...or what? Did you integrate the peak? did you find a carbonyl functional group in the IR spectra....

Anyway....say I had a simple ketone(acetone).....acetone is a symmetric molecule....it would show up as a 6H singlet, representing two methyl groups in the same "field". You'd see it downfield a little more though because of the electron withdrawing effects from the carbonyl group.

As stated by femdoc.....more information is needed.....but its sounding like symmetry to me. Run a Carbon Spec. and see what ur dealing with.

 

[musiclink213]

the H NMR i have has a singlet at 7.5, a doublet at around 7.3and another doublet at raound 6.8. then, i have a doublet at 2.5 and a singet at 2.3

i've also got a C NMR. a singlet at 198.20, peaks at 135.4, 134.4, and 133.3 (i'm not sure if those are really close singles or a triplet). then a doublet from 128.85-128.3. a singlet at 28.1, and a singlet at 19.7

it also doesn't help that it's not even my product, seeing as i had none from the lab cause my prof. gave us the wrong instructions, and old alumina that wasn't reactive. this lab is driving me crazy!!!!!!!!!!!

 

[jlee9531]

(all of us trying to run back to our notes to help you figure this out haha....i can picture it now....)

 

[musiclink213]

(all of us trying to run back to our notes to help you figure this out haha....i can picture it now....)

lol, that would be funny. but you don't have to go back to notes. maybe that knowledge can be retained? no? oh, who am i kidding. we did NMR a month ago, and i can't remember it. but at least today was the last day of classes until finals start, so i can sleep for 4 whole days!!!!! yay!!!!!!!!

 

[Rendar5]

Originally posted by musiclink213
the H NMR i have has a singlet at 7.5, a doublet at around 7.3and another doublet at raound 6.8. then, i have a doublet at 2.5 and a singet at 2.3

i've also got a C NMR. a singlet at 198.20, peaks at 135.4, 134.4, and 133.3 (i'm not sure if those are really close singles or a triplet). then a doublet from 128.85-128.3. a singlet at 28.1, and a singlet at 19.7

it also doesn't help that it's not even my product, seeing as i had none from the lab cause my prof. gave us the wrong instructions, and old alumina that wasn't reactive. this lab is driving me crazy!!!!!!!!!!!

Stuff around 7 are aromatic peaks. So I'd say that you have a monosubstituted benzene ring, which probably accounts for the singlets and two doublets in the aromatic area. Is there a carbonyl peak on the IR? If so, where is it at?

 

[Rendar5]

just a random guess (I'd need to check the IR carbonyl frequency), but it might be benzyl aldehyde.

 

[Moose1]

you've got to give more info. what was the reaction? in other words, what kind of functional groups could be present? nitriles? Bromides? Chlorides? they wouldn't show up in the C or H NMRs but they will affect the spiltting, especially in the aromatic region if they are on phenyl groups.

also, what are the integrations? those are important.

i work as an organic chemist and i've analyzed 1000+ NMRs. this should be a piece of cake but i need more info. also, are you sure about the C NMR and its "doublet?" could it be actually two singlets?

 

[Moose1]

Originally posted by Rendar5
just a random guess (I'd need to check the IR carbonyl frequency), but it might be benzyl aldehyde.

nope

aldehyde protons are way up there...benzaldehydes is at 9.94 ppm

the rest of it is multiplets from 7.82-7.41

banzaldehyde contains no groups that would be in the low ppm range.

the stuff that throws me is the double and singlet around 2.5 ppm.

 

[pbn2au]

hey I'm trying to think of some ideas for this compound but I need some more info:
- do you have integration values for the proton-NMR? Those would help.
- do you know the formula? (because we're determining structure here, correct?)
- Also, is this an NMR taken in the laboratory (by your instructor?) or did it come from an article/web/etc.
(the question is whether the NMR is of a crude product)

Any IR data would help too...

Clearly there are 5 distinct proton environments, and possibly 7 distinct carbon environments - this smells like a styrene-esque something or other. My guess is something ala trans-4-phenyl-3-buten-2-one (but this does not account for all the signals observed). The more you could offer the better - and if not, good luck w/that

(I got my "NMR on" a lot during undergrad)

 

[Pinkertinkle]

Ahh don't worry about NMR's. On the last MCAT, the only NMR involved a H NMR and you had to decide which of the 4 compounds matched that NMR, easily solved since each compound had a different number of hydrogens.

 

[Rendar5]

Originally posted by Moose1
nope

aldehyde protons are way up there...benzaldehydes is at 9.94 ppm

the rest of it is multiplets from 7.82-7.41

banzaldehyde contains no groups that would be in the low ppm range.

the stuff that throws me is the double and singlet around 2.5 ppm.

I was thinking benzyl aldehyde, not benzaldehyade (which would be phenyl aldehyde). Ph-CH2-CHO as opposed to Ph-CHO. It is a wrong answer, though.

 

[Moose1]

Originally posted by Rendar5
I was thinking benzyl aldehyde, not benzaldehyade (which would be phenyl aldehyde). Ph-CH2-CHO as opposed to Ph-CHO. It is a wrong answer, though.

my intention is not to be a brat, but actually benzyl aldehyde is analogous with benzaldehyde.

i think you meant tolualdehyde

 

[Tankitytank]

Go here (SDBS)

http://www.aist.go.jp/RIODB/SDBS/menu-e.html

Enter the NMR shifts, search for your compound, and check their spectrums with yours.

 

[Rendar5]

Originally posted by Moose1
my intention is not to be a brat, but actually benzyl aldehyde is analogous with benzaldehyde.

i think you meant tolualdehyde

You're probably right. I was never good at naming aldehydes and just came up with the name benzyl aldehyde off the top of my head. I meant PH-CH2-CHO, so that's tolualdehyde I guess.

I've analyzed a ton of NMR's too, but they were for organometallic sandwich compounds.

 

[musiclink213]

i don't have the IR cause i had no product, so it was something that the prof. gave us cause they screwed up with the instructions and chemicals, so we just have to talk about what we should have gotten based on the NMR data that he obtained when he did it. I don't know what the formula is, although I just realized that if i have the starting compounds, i can Work out the end product! then i can just make it up with relation to the NMR. i started with paraxylene (sp?), so there is definitly and aromatic ring, and i did friedel-crafts Rx with acetyl chloride.

it's really sad that it took me around 3 hours to figure out that i could do a reaction mechanism, but that's what having a quant exam early in the morning and no sleep will do. thanks a lot for the help.

 

[drlexygoat]

I give MAD props to those who can still pull this NMR stuff out of their heads. I think I lost all of my spec knowledege 2 minutes after my organic 2 final last year. NMR was always just a bunch of peaks and valleys to me! Wish I could help.

 

[asama527]

LOL!!!
It could be grease. Silicon grease makes one giant, sharp peak. 🙂

 

[Rendar5]

...I feel embarassed, but I completely forgot where silicon grease shows up.

 

[asama527]

It shows up near the DMSO point but not quite overlapping with it.

 

[asama527]

Originally posted by musiclink213
the H NMR i have has a singlet at 7.5, a doublet at around 7.3and another doublet at raound 6.8. then, i have a doublet at 2.5 and a singet at 2.3

i've also got a C NMR. a singlet at 198.20, peaks at 135.4, 134.4, and 133.3 (i'm not sure if those are really close singles or a triplet). then a doublet from 128.85-128.3. a singlet at 28.1, and a singlet at 19.7

it also doesn't help that it's not even my product, seeing as i had none from the lab cause my prof. gave us the wrong instructions, and old alumina that wasn't reactive. this lab is driving me crazy!!!!!!!!!!!

What are you integration ratios (how do the areas under each peak compare?)

 

[16846]

2 degrees in chemistry and an entire semester of NMR later I still have absolutely no idea on how to read those little peaks and valleys... 😕

 

[NRAI2001]

NMR sucks, its the thing I most disliked about O chem.😡

 

[YaFwiendToluene]

If it was a simple friedel crafts.."acylation"..just draw out the thermodynamically favored product......p-xylene with an acyl group on carbon 2 of the ring. Then match the HNMR data with the hydrogen atoms.

 

[musiclink213]

Originally posted by YaFwiendToluene
If it was a simple friedel crafts.."acylation"..just draw out the thermodynamically favored product......p-xylene with an acyl group on carbon 2 of the ring. Then match the HNMR data with the hydrogen atoms.

thats what i ended up doing. i don't understand why i needed the NMR's though, especially since i had 0 product, so i couldn;'t even say if what i had was true product. and when i say 0 product, i mean 0 product. my empty container and my container at the end weighed exactly the same. i had no product, and neither did anyone else. the NMR data was of the professors, so it made no sense how we could compare our results to the actual results if we had no results! grrr......., chem lab makes me mad. 😡 but at least it's done, i just handed it in, with a bunch of BS for that discussion, but i think it'll be ok, cause i got all 12/12 on the other ones. and chem lab is only worth one credit, although i wish it were included in the lecture section, but it's not cause not everyone has to take the lab. oy, chem is nothing but stress.