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Ochem: flame test; soluble in H2SO4

  1. Oct 28, 2009 #1
    I have two questions about organic compounds in particular. I would be very happy if someone could point me in the right direction for more information.

    First, I'm having trouble finding information about the use of flame tests for qualitatively classifying organic molecules. I've looked in the indexes of my organic textbooks and don't see "flame test," nor do I recall learning about it. Most of the information I find relates to metals and inorganic salts. Do brightly colored flames suggest an organometallic compound? For example, green can indicate the presence of copper (and other elements, I know) so does a green flame from an organic compound mean it might be an organocuprate?

    Second, what type(s) of organic molecule is soluble in concentrated H2SO4 and not in any other solvents, at least not those that an organic chemist would commonly use for a solubility test? Again, most of what I'm finding are inorganic substances, for example certain metals dissolve in sulfuric acid but not in other concentrated strong acids.

    Thanks :smile:
     
  2. jcsd
  3. Oct 30, 2009 #2

    chemisttree

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    First, no one uses a flame test to identify an organic compound unless you are trying to tell the difference between an aromatic compound and just about everything else on the planet!
    Flame tests in organic chemistry are almost useless.

    Secondly, solubility in concentrated sulfuric acid almost useless in the determination of an organic compound. It is the last thing I would try.... and I have tried many things! I once worked with an otherwise sparingly-soluble aromatic carboxylic acid that was soluble in sulfuric acid. I was making peroxides and the sulfuric acid helped solubilize the fairly stiff backbone carboxylic acid-terminated nylon peroxide compound. Not something you would normally encounter! You most likely have an inorganic compound.
     
  4. Oct 30, 2009 #3
    My question was inspired by an organic chemistry lab problem set. We were given spectra and the results of a few tests. A surprising number of the compounds were reported to be soluble only in concentrated sulfuric acid. My understanding is that organic compounds that are difficult to dissolve often have aromatic and other functional groups that give rise to extensive non-covalent interactions. Still, I would expect these to have some solubility in benzene or phenol. The green flame test is indicates an alkyl halide. It doesn't sound like anyone really does these tests anymore.

    Thanks for the info.
     
  5. Nov 1, 2009 #4

    chemisttree

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    Yes you can tell if an organic compound contains a halide by doing that test with a copper wire. It is the copper itself that gives rise to the green flame and the halide reacts with the copper in the flame to do so. The test is called the Beilstein test and is useful in rapid identification of PVC or other halide containing plastics.
     
  6. Nov 4, 2009 #5
    Hi,

    The test is what I am actually searching about and I have done this test in the chemistry lab and it really gives such positive results and really it is good for handling plastics.


    Thanks!
     
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