Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: PandHero on March 24, 2021, 03:46:59 PM
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This is from a lab to separate an unknown binary mixture. This compound after performing a solubility test, I determined to be a carboxylic acid. I need help in identifying the peaks and determining what they are.
https://imgur.com/a/RXiROtN (https://imgur.com/a/RXiROtN)
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Please read the forum rules (https://www.chemicalforums.com/index.php?topic=65859.0).
You have to show your attempts at answering the question, this is a forum policy.
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Sorry I had a lecture here is my attempt, I'm not sure if all these peaks are relevant to the identity of the compound or if I am labelling them correctly.
https://imgur.com/a/M97PdhT (https://imgur.com/a/M97PdhT)
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Do you know anything about the relationship between the frequencies of C-H stretches and the hybridization of the carbon atom?
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I know about it in theory but I don't know how to identify it.
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What would you predict on the basis of the length and strength of various C-H bonds? You can check your predictions here:
https://www.cpp.edu/~psbeauchamp/pdf/spec_ir_nmr_spectra_tables.pdf
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I'm not sure if the sp2 C-H is correct or is that should be NH2. Also are the smaller peaks irrelevant, and at what point should I disregard the fingerprint region, my professor has been mixed on this.
One thing that is throwing me off is the fact that I know this is a carboxylic acid but I don't clearly see the O-H stretch of it.
https://imgur.com/ddwcKlq (https://imgur.com/ddwcKlq)
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2960 cm-1 is too low to be sp2CH. The carboxylic acid signals that I have seen do not look like your spectrum; therefore, I am also puzzled by this.
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https://imgur.com/a/0gw2DQd (https://imgur.com/a/0gw2DQd)
I do have this IR for the liquid portion of my unknown, however I determined this to be some type of neutral compound after the solubility test since it was insoluble in all the solvents up to H2SO4. I TA confirmed that these were the correct traits of my compounds.
I've also attached an image of my solid unknown. For the IR I dissolved it in a minimum amount of DCM. It had a bit of a yellowish tint. My liquid was clear, and the solid was dissolved in it and turned it yellow pre-separation.
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Hi I know it's been a while but I did more test and came to the conclusion (which was confirmed as correct by my TA) that the liquid product is mesityl oxide, and the solid is 3-nitrobenzoic acid. Just wanted to ass this as a follow up/conclusion to this.
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Sorry I had a lecture here is my attempt, I'm not sure if all these peaks are relevant to the identity of the compound or if I am labelling them correctly.
https://imgur.com/a/M97PdhT (https://imgur.com/a/M97PdhT)
You indicated a much broader region for C-H stretching (down to roughly 2000 wavenumbers) than is correct. One way for you to approach the problem now would be to find the signals that correspond with functional groups in the known structures. I am not certain that this is the best way.
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Is that a perkin elmer instrument you are using? And if I may, which one is it? Thanks in advance.
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I don't believe it is. The one I used was at my college campus, so it's pretty dated.
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Yes. If it was a Perkins elmer, many of them had functions of functional group search and peak search built in. The functional group search would have been the most help. The peak search usually requires a library. That may or may not be present and often cost extra. Anyway, the functional group search can also answer a lot of those questions. Most competitors also had a similar function on their instruments. Unless the school disabled it. Regards.
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@OP, If you have sp2C-H bonds in your molecule, where will they show a signal? If you have sp3C-H bonds in your molecule, where will they show a signal? A nitro group should give you a symmetric stretching signal and an asymmetric stretching signal, both of which are at least moderate in intensity.
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Hi I know it's been a while but I did more test and came to the conclusion (which was confirmed as correct by my TA) that the liquid product is mesityl oxide, and the solid is 3-nitrobenzoic acid. Just wanted to ass this as a follow up/conclusion to this.
@OP, To what do you assign the broad signal in the vicinity of 2950 wavenumber?
EDT
This sample looks too concentrated to me.
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This is from a lab to separate an unknown binary mixture. This compound after performing a solubility test, I determined to be a carboxylic acid. I need help in identifying the peaks and determining what they are.
https://imgur.com/a/RXiROtN (https://imgur.com/a/RXiROtN)
Is hard to identify a compound only with IR
Don't see any carboxylic peak. The carboxylic peak are very large, usually over 3200. Not this case.
The 3054 signal is C-H sp2 stretching
The 2998 signal is C-H sp3 stretching
I ' m not sure if the low signals at 2850 and 2685 can be attributed to an aldehydic group (usually they have two weak signal at around 2800-2700) .
It could be....because we know we have a C=O group (1700 signal)
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https://orgchemboulder.com/Spectroscopy/irtutor/nitrosir.shtml
PandHero,
Can you double-check the identities of the two compounds, and also which IR spectrum arises from the solid unknown versus the liquid unknown. In your spectrum labeled "IR Cont" I see things that are consistent with a carboxylic acid. In both "IR cont" and "Unknown IR 1" and I see some signals that might arise from a nitro group (see link), but from what you wrote above I thought that the carboxylic acid and nitro groups were part of the same compound. Some of the signals in the spectrum shown in "IR Cont" are too intense to allow for easy interpretation.