Carbonyl stretching in IR

[GBALA]

I got a IR spectrum of my unknown compound and it shows a peak which is sharp(small in size) but not as strong as it should be for a carbonyl stretching at 1732. Also a small and bit broad peak at 3356 which confuses me the possibility of O-H stretching which is normally very broad. Please help me regarding this

[khatereh]

How did you run the IR spectrum?have you made a disk with KBr ?or is it in Nojul mull? or have you made a solution?

 

 

 

I got a IR spectrum of my unknown compound and it shows a peak which is sharp(small in size) but not as strong as it should be for a carbonyl stretching at 1732. Also a small and bit broad peak at 3356 which confuses me the possibility of O-H stretching which is normally very broad. Please help me regarding this

[GBALA]

None of the method you mentioned. I have given my sample in a near by institute and they analyzed by simply taking nearly 1 mg onto a panel and started taking the spectrum. This is my first time IR experience and help me what went wrong with my spectrum. Thanks for your time and consideration

[hypervalent_iodine]

Is it possible to attach a copy of the spectra?

[mississippichem]

None of the method you mentioned. I have given my sample in a near by institute and they analyzed by simply taking nearly 1 mg onto a panel and started taking the spectrum. This is my first time IR experience and help me what went wrong with my spectrum. Thanks for your time and consideration

 

Sounds like an attenuated reflectance IR experiment?

[khatereh]

Sounds like an attenuated reflectance IR experiment?

 

They probably used "diamond anvil cell" for their FTIR which can be used when small amount of samples is provided (your sample will be usable easily after getting spectrum). I doubt that it is reflectance IR experiment.

[mississippichem]

They probably used "diamond anvil cell" for their FTIR which can be used when small amount of samples is provided (your sample will be usable easily after getting spectrum). I doubt that it is reflectance IR experiment.

 

Yeah, I often use the diamond anvil setup. The word "panel" made me think reflectance but you're probably right.

[khatereh]

The free hydroxyl group ( I mean without hydrogen bonding) normally is not very broad in IR. I agree with hypervalent_iodine, if you provide the spectrum it will be great.

Also I should note that for characterization of an unknown compound, one method is not enough. HNMR also can confirm the idea of having carboxylic acid group ( broad peak around 12ppm).

Edited July 5, 2012 by shidokht

[hypervalent_iodine]

Yeah, I just assumed it was a diamond disk. It's hard to say what it is without seeing what the OP is calling slightly broad vs. very broad. Another thing to consider is minor contamination with some hydroxy containing compound.

[GBALA]

Sorry for not able to provide my spectrum. I accepted your point that IR analysis alone wont enough for finding the structure of an unknown compound. I got HNMR spectrum also but there is no peak related to carboxylic acid. The final peak at HNMR spectrum is at 4 ppm and it is a triplet. No peak beyond that ppm.

 

If it is due to a hydroxy containing compound's contamination then is it likely to end up with a small broad peak at ~3300?

[hypervalent_iodine]

If you're talking about the COOH protons and not the alpha carbonyl ones, you won't see that proton in the normal range for a proton NMR. You need to extend it to scan up to about 13 or 14 ppm. Are you even sure it's a carboxylic acid? Also, if you need full characterisation, you need to do more than IR and ¹H NMR. You should at least get mass spec analysis and a carbon NMR as well.

 

The peak might not even be an OH peak is really what I'm saying. Of it were and it were from the COOH in your product, it would be very obvious. That being said, since your carbonyl peak is small, what you are calling a slightly broad peak might be what you would expect. Forget the IR, in any case. Go get some real spectra

[khatereh]

So If you are not able to see any peak in carboxylic acid range of the HNMR , I think you should doubt that your compounds contains carboxylic acid group. see following IR spectrum

 

 

 

If your spectrum contains this kind of broad peaks at around 3300, and if it is this much small in comparison with other peaks. it is not a reliable peak to assign it as OH of carboxylic acid group.

 

you have to do other characterization to get the structure. at least from the reaction that you have done you should have an idea what you are expecting to see.

 

following websites can be helpful for characterization:

 

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng

http://www.chem.wisc.edu/areas/reich/handouts/nmr-h/hdata.htm

http://www.chem.wisc.edu/areas/reich/handouts/nmr-c13/cdata.htm