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A problem relating to density and layers

  1. Sep 24, 2004 #1
    Hi, I recently had a lab where I had to separate a mixture of benzoic acid and benzil. So, I dissolved the mixture in ch2cl2 and then added NaOH in order to convert the benzoic acid into its sodium salt. At anyrate, this then produced the aqueous and organic layers.

    However, I noticed that the organic layer (the yellow one) was on the bottom and the aqueous was on the top which makes sense since ch2cl2 is more dense than water, but there was a thin yellow layer on top of the aqueous layer. How did that yellow layer get up there if it is indeed more dense than water?

    My yield of benzil was a little low so I'm assuming that part of it was included in my benzoic acid.
     
  2. jcsd
  3. Sep 25, 2004 #2

    chem_tr

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    Hello, I remember that benzil converts into an acid derivative via the mechanism "benzil rearrangement" in alkali media, so you'll need to find another way to obtain benzil; have you tried Schiff condensation with small amines like pentylamine? Then the Schiff product precipitates, and after you filtrate, benzoic acid may be available in the filtrate, easily crystallized from water.

    When we come to the yellow line on top of aqueous layer, I think that it is an emulsion; you may try two things to avoid it:

    a) Hold the separatory funnel from the bottom, i.e., from the stopcock, and stir the funnel in round motions. This will help the "confused" upper layer to be in its right place.

    b) If it does not help, prepare a saturated NaCl solution and add to the medium until the layer disappears; after you add some NaCl, don't forget to shake the funnel. It is due to distribution; the compound easily soluble in organic layer has some solubility in aqueous layer. If you add a compound very soluble in water, then the organic compound cannot be dissolved in water anymore.
    Regards, chem_tr
     
    Last edited: Sep 25, 2004
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