IR Spectroscopy
Chemistry discussion, revision, exam and homework help.
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IR SpectroscopyHow could you differentiate between two carboxcyllic acids (for example) using IR spectroscopy? They should have troughs at the same frequency because they both contain the same functional groups, so how could you tell between ethanoic acid and propanoic acid? Would the trough representing the C-H be more prominant in propanoic acid as there are more C-H's, or something along those lines?
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Re: IR SpectroscopyIf this is for A level, you probably won't be asked to work out the structures of two compounds with only one functional group (which is the same) from the IR trace; you'll only be asked to work out the structures of compounds with different functional groups, e.g. propan-1-ol and propanoic acid (the former having a R-OH group and the latter R-COOH).(Original post by Perpetuallity)
How could you differentiate between two carboxcyllic acids (for example) using IR spectroscopy? They should have troughs at the same frequency because they both contain the same functional groups, so how could you tell between ethanoic acid and propanoic acid? Would the trough representing the C-H be more prominant in propanoic acid as there are more C-H's, or something along those lines?
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Re: IR SpectroscopyYou can kinda tell the difference between them using IR - look at the fingerprint region. However NMR would be a much better technique to use.(Original post by Perpetuallity)
How could you differentiate between two carboxcyllic acids (for example) using IR spectroscopy? They should have troughs at the same frequency because they both contain the same functional groups, so how could you tell between ethanoic acid and propanoic acid? Would the trough representing the C-H be more prominant in propanoic acid as there are more C-H's, or something along those lines?
ThanksLast edited by illusionz; 18-05-2012 at 19:17. -
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Re: IR SpectroscopyIt's not 100% that peaks are more prominent because there are more C-H's, but sometimes it does happen in that way. I guess the 'tell tale' sign in propanoic acid would be possibly having a very broad absorbtion peak at the OH bond in a carboxylic acid.
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Re: IR SpectroscopyWell you can kinda tell the difference by looking at the fingerprint region for the two and comparing them, but you can't take an IR of an unknown compound and determine that it's ethanoic acid.(Original post by Perpetuallity)
How could you differentiate between two carboxcyllic acids (for example) using IR spectroscopy? They should have troughs at the same frequency because they both contain the same functional groups, so how could you tell between ethanoic acid and propanoic acid? Would the trough representing the C-H be more prominant in propanoic acid as there are more C-H's, or something along those lines?
ThanksLast edited by illusionz; 18-05-2012 at 19:00. -
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Re: IR SpectroscopyNo, it would just be CH3CH2+...with the COOH removed.(Original post by fayled)
I suppose they would be mostly the same.
The only thing I could suggest is that you would use a mass spectrometer - there would be CH3CH2CH2+ ions present as fragment ions when analysing propanoic acid, but not for ethanoic acid. -
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Re: IR SpectroscopyJust so you can see, here are spectra of the two.(Original post by Perpetuallity)
How could you differentiate between two carboxcyllic acids (for example) using IR spectroscopy? They should have troughs at the same frequency because they both contain the same functional groups, so how could you tell between ethanoic acid and propanoic acid? Would the trough representing the C-H be more prominant in propanoic acid as there are more C-H's, or something along those lines?
Thanks
Ethanoic acid:
Propanoic acid:
You can see that the fingerprint region is slightly different, so if you had reference spectra of high resolution you could tell two samples apart, but you couldn't just take the spectrum and think, ahah, that's propanoic acid!Last edited by illusionz; 18-05-2012 at 19:05. -
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Re: IR SpectroscopyOh of course, I was thinking of alcohols for some reason but yeah thats what I was trying to get across.(Original post by adamcusirinzon)
No, it would just be CH3CH2+...with the COOH removed. -
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Re: IR SpectroscopyThose are strange spectra...(Original post by illusionz)
Just so you can see, here are spectra of the two.
Ethanoic acid:
Propanoic acid:
You can see that the fingerprint region is slightly different, so if you had reference spectra of high resolution you could tell two samples apart, but you couldn't just take the spectrum and think, ahah, that's propanoic acid! -
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Re: IR SpectroscopyThanks. Also does the number of C-H's effect the relative absortion frequency of C=O or O-H? Or in other words, does the molecular composition of R (in RCOOH for example) affect the absorbtion frequencies of the functional groups?(Original post by illusionz)
Just so you can see, here are spectra of the two.
Ethanoic acid:
Propanoic acid:
You can see that the fingerprint region is slightly different, so if you had reference spectra of high resolution you could tell two samples apart, but you couldn't just take the spectrum and think, ahah, that's propanoic acid!Last edited by Perpetuallity; 18-05-2012 at 19:15. -
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Re: IR SpectroscopyHow so? They're just fairly low resolution.(Original post by fayled)
Those are strange spectra... -
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Re: IR SpectroscopyOn mine its just came up with a red circle with 'forbidden' written along them(Original post by illusionz)
How so? They're just fairly low resolution. -
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Re: IR SpectroscopyNo. The intensity is related to the dipole moment of the bond.(Original post by Perpetuallity)
So the number of C-H's doesn't effect the relative absortion frequency of C=O or O-H? So there's no way of telling them apart. -
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Re: IR SpectroscopyAh. They're from a large database run by some japanese chemists. I would have presumed my university pays a bit of money for all IP addresses associated with it to be allowed access. But the OP seems to be able to see them so no idea why you can't.(Original post by fayled)
On mine its just came up with a red circle with 'forbidden' written along themLast edited by illusionz; 18-05-2012 at 19:17. -
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Re: IR SpectroscopyAha, that makes sense; cheers.(Original post by illusionz)
No. The intensity is related to the dipole moment of the bond. -
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Re: IR SpectroscopyYeah they don't seem to work if you copy / paste the images in. Best solution I've found is to take a screenshot!(Original post by illusionz)
Ah. They're from a large database run by some japanese chemists. I would have presumed my university pays a bit of money for all IP addresses associated with it to be allowed access. But the OP seems to be able to see them so no idea why you can't.
