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Hydrophobicity and colorimetry

  1. Sep 1, 2011 #1
    1. The problem statement, all variables and given/known data

    Given four compounds that are variations on amines (compound one has -OH funcitonal groups, compound 2 has a carboxy group, etc.). I am also given a bright yellow compound, which will ultimately be covalently linked to each of the four compounds via amide bond (it is a ringed aromatic compound with a carboxy group and a nitrate group). I am supposed to contrive an experimental approach using a colorimetric readout to order the compounds' hydrophobicity from most to least.

    Obviously, I am including only an overview without posting the actual compounds. I'm hoping I can just bounce around ideas.

    3. The attempt at a solution

    I was initially thinking that more hydrophilic compounds would be impeded from bonding with the colored compound by a shell of water molecules forming H-bonds. Then, a colorimeter could be used to measure relative strength of color. More color would mean that the compound bonded more fully to the colored compound, because it was less impeded by water and hence more hydrophobic. Then I could order them that way.

    My TA told me (somewhat cryptically) that parts of my idea were right but parts were missing. She then said to think about other products of amide linkage. The only other product I can think of is just NH3. Can someone tell me what's right and shove me in the right direction?

    Note: If I don't respond to your well-meaning comment, it's because it came after this problem set is due and I no longer really care. Unfortunately I've left physics forum as a sort of last resort. However, help would be appreciated!
  2. jcsd
  3. Sep 2, 2011 #2
    Is this really what you mean?
  4. Sep 3, 2011 #3


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    You are reacting four amine compounds with an aromatic carboxylate to produce an amide. The other product of that reaction is going to be water. The answer was more than cryptic, it was misleading. Maybe she meant to look at the other functional groups left over once the amine functionality was reacted away.

    You should think about the structure of the chromophore itself. Is the chromophore a charged or ionized species? What is the solvent? In more polar solvents like water, more hydrophobic molecules will tend to self associate rather than associate strongly with the solvent. What type of association will modulate either the color or the intensity of color?
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