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Infrared spectroscopy of a cyclic alkene

  1. Oct 27, 2004 #1
    At school we extracted limonene from orange peels and we had to make an IR spectroscopy for it but I don't see anywhere how we can know the product has a ring constitution... I see a lot of information about aromatic rings but nothing for an alkene ring... Can anybody help me?
     
  2. jcsd
  3. Oct 27, 2004 #2

    chem_tr

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    Well, if it is what you mean, you can find some useful sites by googling "limonene" and "IR". It has an alcohol group, an isopropyl chain (if I am not wrong), and one or two double bonds. So all of them can be seen in their corresponding regions; also look for multiple regions for different modes. For example, a C-H vibration of an alkane/alkene at ca. 2900 cm-1 always has more powerful C-H deformation at ca. 1400 cm-1, etc. You can see a C=C signal at around 1600 cm-1, and of course, alcoholic O-H stretching is around 3500 cm-1, where its C-O deformations are around 1200 cm-1.

    Hope these help (don't forget to google).
     
  4. Oct 28, 2004 #3
    As known IR spectroscopy is a support method to determine the sturucture, without being known what the sample is, IR is not solely enough...

    In your example, limonene has two seperate double bonds. These double bonds are ordinary sp2 hybrids, so the overall cyclic structure doesn't exhibit a very different spectrum. The movements in the bond change slightly due to the not-too-much-strain. But if the structure were something like cyclopropane, then we would expect a change than the normal sigma bond. This change would be assgined to the abnormal change in the angle for sp3 hybrid. (Normally 109,5 degrees). Actuallt, in the FTIR spectrum of cyclopropane, the signal about 3000 is broadand smears to about 3200, where in the non-cyclic hydrocarbons this is sharper and generally not exceed 3000.

    Until now, what I wrote is a little off-topic, but this is to explain that only IR spectrum is not a tool to determine the structure if it's cyclic or not... Assuming that the sample is unknown, you should consult additional methods to determine "cyclicity". I hope I explained clearly.
     
  5. Oct 28, 2004 #4
    Thank you for the informations
     
  6. Oct 28, 2004 #5
    Well one method might be to find the known IR spectrum of limonene and compare it your results.
     
  7. Oct 28, 2004 #6

    chem_tr

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    So-crates, you are a genius :smile: You reminded me that there are IR atlases, and one of them must have an entry for limonene. asfd, if you are a university student, your library must have an IR atlas, in which you very likely find the IR of limonene and as so-crates said, compare them. Remember that only the range between 400 (or lower such as 100) and 1400 cm-1 is reliable in terms of comparison, since this is well known as "fingerprint" region, where only the same molecules can give the same pattern.
     
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