October 20, 2021, 11:32:11 AM
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Topic: I'm weak with IR and need to identify important peaks in this spectrum.  (Read 3673 times)

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Offline PandHero

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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

Offline Borek

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Please read the forum rules.

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Offline PandHero

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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

Offline Babcock_Hall

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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?

Offline PandHero

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I know about it in theory but I don't know how to identify it.

Offline Babcock_Hall

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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


Offline PandHero

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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

Offline Babcock_Hall

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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.

Offline PandHero

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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.

Offline PandHero

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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.

Offline Babcock_Hall

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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
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.

Offline marquis

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Is that a perkin elmer instrument you are using?  And if I may, which one is it?  Thanks in advance.

Offline PandHero

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I don't believe it is. The one I used was at my college campus, so it's pretty dated.

Offline marquis

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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.

Offline Babcock_Hall

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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.
« Last Edit: July 05, 2021, 11:58:43 AM by Babcock_Hall »

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