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Topic: identity of unknown compound  (Read 6989 times)

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Offline sorianoelias3

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identity of unknown compound
« on: April 26, 2014, 10:42:45 PM »
hey everyone! thank you for reading my post, i recently started my last lab assignment in organic chem. bu I ran into some trouble while the first day was finished.  I was given an unknown mixture of a white solid at the bottom of a slippery, strong smelly liquid.  Started off by placing the bottle in ice for twenty minutes, that caused a lot of the solid to precipitate.
after precipitation, Filtration was completed and obtained about 3 grams of solid with perhaps 20mL of liquid, which i later fractionally distilled. i attached all my notes so far for the solid, and the tests which i completed. I obtained an HNMR and IR but they don't seem to make much sense when seen combined, i also have the theoretical compound's structure, what you guys think?
I am going to do further testing on the sample, but I wanted to get a second opinion on it, and if anyone could advise me of additional tests that I can perform.  thanks a lot =]

Also, i like the structure i came up with on the left better only because it seems that the aromatic ring might be para substituted, it seems there is a double peak at around the 7ppm
« Last Edit: April 27, 2014, 03:04:02 AM by sjb »

Offline discodermolide

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Re: identity of unknown compound
« Reply #1 on: April 27, 2014, 12:28:51 AM »
The left hand structure has a CH2COOH group.
I don't see the CH2 signal in the NMR.
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Offline critzz

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Re: identity of unknown compound
« Reply #2 on: April 27, 2014, 10:57:09 AM »
Look at the amount of signals in the aromatic area (7 - 8 ppm) and their multiplicity.
This should give you the information of the position/amount of substitutions.

Offline discodermolide

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Re: identity of unknown compound
« Reply #3 on: April 27, 2014, 01:36:50 PM »
The NMR you show only goes from 7ppm to 11 ppm.
If it is the phenyl acetic acid compound you suggest as the structure where is the NMR signal for the CH2CO2H group? I would expect it to be around 3.5.
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Offline sorianoelias3

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Re: identity of unknown compound
« Reply #4 on: April 27, 2014, 05:47:45 PM »
yes! i was looking at the NMR closely and came up with this attached image.  i think it makes more sense than the other two, i counted most of the Hs and they seem to fit properly, the IR however, does not seem to be too helpful.  i think ill have to do it once again to see if I get a better signal.  when i use DCL3 to dissolve it, should i use only a few drops and a lot of solid or is there something I'm messing up?
this would also make a bit more sense, since the Tollen's test came up negative twice i suppose =/

But what throws me off a bit is the 3288 signal on IR since if accurate, it would be an alkyne

Offline discodermolide

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Re: identity of unknown compound
« Reply #5 on: April 27, 2014, 10:09:28 PM »
Sorry but it's not that compound, there is no signal for the CH2OH group.
How about a C=C attached to the aromatic ring? Like a cinammic acid. See if something like that would fit.

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Offline sorianoelias3

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Re: identity of unknown compound
« Reply #6 on: April 28, 2014, 12:24:09 PM »
hmmm i see what youre saying here, i took another look and searching past assignments i figured this could be a possibility, but im not sure if the singlet is one or mixed with the other one. does this make more sense?
i got the two triplets and the two doublets, the singlet is there, but again... is it just one or should i have more than one in this case?

Offline discodermolide

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Re: identity of unknown compound
« Reply #7 on: April 28, 2014, 02:48:04 PM »
There is a singlet at 10.5 ppm, that is the COOH signal.
The other singlet at 7.2 is chloroform.
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Offline critzz

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Re: identity of unknown compound
« Reply #8 on: April 28, 2014, 02:54:52 PM »
That structure looks more like it, although the signal at 10,5 ppm is most likely coming from the proton of the carboxylic acid.

Is there any other signal at 5-6 ppm for the phenol?

If not, you could consider a halogen.


Offline sorianoelias3

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Re: identity of unknown compound
« Reply #9 on: May 02, 2014, 11:02:04 AM »
according to the person who did the NMR there are no more signals outside the range given, the only weird thing is that i did the test for a nitro group and this is what i got, it looks positive to me =/
and the melting point is 148-150 so it could not be the ortho substituted nitro group with the oh group. i will go ahead and try to make another test for halogen and COOH.
Also, a halogen? i did multiple tests for the phenol and they all came back positive, so its definitely there.

Offline critzz

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Re: identity of unknown compound
« Reply #10 on: May 04, 2014, 09:10:41 AM »
I'm pretty sure it's salicylic acid                C1=CC=C(C(=C1)C(=O)O)O

The peak at ~10.5 ppm actually IS the phenolic proton. The benzoic acid peak might be shifted even more downfield (>12 ppm) or so weak that it appears absent.

To be sure I checked a reference.

HNMR (400 MHz, CDCl3) δ10.37 (bs, 1H), 7.94 (d, J = 8,0 Hz, 1H), 7.54 (t, J = 8.0 Hz, 1H), 7.02 (d, J = 8,4 Hz, 1H), 6.95 (t, J = 7.4 Hz, 1H)

http://onlinelibrary.wiley.com/doi/10.1002/chem.201200092/full

Offline sorianoelias3

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Re: identity of unknown compound
« Reply #11 on: May 05, 2014, 02:10:55 PM »
thank you everyone!!
I will read a little bit into the NO2 test more and see why i got the result. i stole some pure acid and dissolved it into my liquid and the exact same results were obtained ^_^

now if I only knew the identity of my solvent ...hmmm

=]

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