Infrared Spectroscopy

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Pyruvic Acid
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#1
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#1
Would a broad absorption at 3350 cm–1 indicate that the sample being analysed has a -COOH group?
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cefox
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(Original post by Pyruvic Acid)
Would a broad absorption at 3350 cm–1 indicate that the sample being analysed has a -COOH group?
The 2500 - 3300 cm-1 is stated as the usual range for the O-H absorption in a carboxylic acid (using OCR data sheet here) so 3350 is very close.....it’s a bit of a tricky one but, having said that, exam questions aren’t usually ambiguous! AQA or Edexcel data may be slightly different. Have you estimated that value from a diagram of the spectrum and do you have any other evidence that it may be a carboxylic acid? It’s poss you might have an alcohol or phenol group with a C=O separately elsewhere in the molecule in a different C atom maybe....so not a c acid but alcohol plus aldehyde or ketone?
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Pyruvic Acid
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#3
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(Original post by cefox)
The 2500 - 3300 cm-1 is stated as the usual range for the O-H absorption in a carboxylic acid (using OCR data sheet here) so 3350 is very close.....it’s a bit of a tricky one but, having said that, exam questions aren’t usually ambiguous! AQA or Edexcel data may be slightly different. Have you estimated that value from a diagram of the spectrum and do you have any other evidence that it may be a carboxylic acid? It’s poss you might have an alcohol or phenol group with a C=O separately elsewhere in the molecule in a different C atom maybe....so not a c acid but alcohol plus aldehyde or ketone?
It was part of question 6b of https://pmt.physicsandmathstutor.com...m%206%20QP.pdf

I thought D would be the carboxylic acid due to te broad absorption but apparently C is actually the carboxylic acid : https://pmt.physicsandmathstutor.com...m%206%20MS.pdf
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cefox
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(Original post by Pyruvic Acid)
It was part of question 6b of https://pmt.physicsandmathstutor.com...m%206%20QP.pdf

I thought D would be the carboxylic acid due to te broad absorption but apparently C is actually the carboxylic acid : https://pmt.physicsandmathstutor.com...m%206%20MS.pdf
It’s helpful to see the actual question.
I think their reasoning is that CB and D both have the C=O peak.
Then we are meant to deduce that D
‘s broad absorption at 3350 is and alcohol O-H peak as it is just outside the range for the COOH O-H peak....but only just!

Therefore C is either c acid or ester. I can see they accepted either despite not telling us about any other peak for compound C.

It’s a misleading question because the 3350 is so close to the c acid range and also it says it is broad...which you are right to think would be COOH O-H. Alcohols can also be fairly broad.

Seems to me you are doing a good job of applying your theory and I would put this Q down really as a bit of a bad job!! On their part, not yours! New spec Q tend to be more clear cut so don’t worry.
Wishing you the best.
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cefox
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(Original post by cefox)
It’s helpful to see the actual question.
I think their reasoning is that CB and D both have the C=O peak.
Then we are meant to deduce that D
‘s broad absorption at 3350 is and alcohol O-H peak as it is just outside the range for the COOH O-H peak....but only just!

Therefore C is either c acid or ester. I can see they accepted either despite not telling us about any other peak for compound C.

It’s a misleading question because the 3350 is so close to the c acid range and also it says it is broad...which you are right to think would be COOH O-H. Alcohols can also be fairly broad.

Seems to me you are doing a good job of applying your theory and I would put this Q down really as a bit of a bad job!! On their part, not yours! New spec Q tend to be more clear cut so don’t worry.
Wishing you the best.
...sorry I meant just C and D in my second line above,,,,not CB and D!
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Pyruvic Acid
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#6
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#6
(Original post by cefox)
It’s helpful to see the actual question.
I think their reasoning is that CB and D both have the C=O peak.
Then we are meant to deduce that D
‘s broad absorption at 3350 is and alcohol O-H peak as it is just outside the range for the COOH O-H peak....but only just!

Therefore C is either c acid or ester. I can see they accepted either despite not telling us about any other peak for compound C.

It’s a misleading question because the 3350 is so close to the c acid range and also it says it is broad...which you are right to think would be COOH O-H. Alcohols can also be fairly broad.

Seems to me you are doing a good job of applying your theory and I would put this Q down really as a bit of a bad job!! On their part, not yours! New spec Q tend to be more clear cut so don’t worry.
Wishing you the best.
Thank you!
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