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IR Spectroscopy

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    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
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    (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
    If 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).
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    (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
    You can kinda tell the difference between them using IR - look at the fingerprint region. However NMR would be a much better technique to use.
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    It'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.

    Sorry I couldn't be more help.
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    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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    Thanks for the quick responses, much appreciated.
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    (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
    Well 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.
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    (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.
    No, it would just be CH3CH2+...with the COOH removed.
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    (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
    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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    (Original post by adamcusirinzon)
    No, it would just be CH3CH2+...with the COOH removed.
    Oh of course, I was thinking of alcohols for some reason but yeah thats what I was trying to get across.
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    (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!
    Those are strange spectra...
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    (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!
    Thanks. 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?
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    (Original post by fayled)
    Those are strange spectra...
    How so? They're just fairly low resolution.
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    (Original post by illusionz)
    How so? They're just fairly low resolution.
    On mine its just came up with a red circle with 'forbidden' written along them
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    In my chemistry course if we were asked a question like that the answer would always be the fingerprint region below 1500 cm-1 as this is unique to each substance. This thread was started ages ago but nobody other than illusionz suggested this
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    (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.
    No. The intensity is related to the dipole moment of the bond.
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    (Original post by fayled)
    On mine its just came up with a red circle with 'forbidden' written along them
    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.
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    (Original post by illusionz)
    No. The intensity is related to the dipole moment of the bond.
    Aha, that makes sense; cheers.
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    (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.
    Yeah they don't seem to work if you copy / paste the images in. Best solution I've found is to take a screenshot!

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