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Role of salts in extraction

  1. May 24, 2010 #1
    1. The problem statement, all variables and given/known data
    In O-Chem lab, after the reaction is complete, NaHC03 and NaCl solutions are added. Then BME is added and aqueous layer removed.
    Why do you extract the mixture with salts, and what role does each one play?


    2. Relevant equations
    Ester synthesis
    trans cinnamic acid + methanol >sulfuric acid>>>> methyl trans cinnamate

    3. The attempt at a solution
    I have been confused about this for a while, I need to stop confusing myself more. I think that the salt reacts with the excess sulfuric acid, but not sure why two different salts would be needed.
     
  2. jcsd
  3. May 25, 2010 #2
    I think your reaction should read [tex]cinnamic acid +methanol+sulfuric acid \to methyl cinnamate[/tex], don't use >>>>>> for arrows, it only confuses things. Think about what [tex]NaHCO_3[/tex] is in relation to [tex]H_2SO_4[/tex]. Why would you want to wash the reaction mixture with a basic solution after the reaction is over? Also, please be clear on which BME you are reffering to, 2-mercaptoethanol or methyl tert-butyl ether. I am assuming you mean the latter, which is immiscible in water but will solvate your product, methyl cinnamate. The proper way to ask the question that you presented is "Why is the reaction mixture washed with NaHCO3 and extracted with methyl tert-butyl ether, followed by addition of NaCl to the organic layer?". NaCl is added to absorb any water molecules in the organic solvent. Bottom line is, the reaction is completed, add NaHCO3 to neutralize any leftover acid, followed by addition of methyl tert-butyl ether and extraction in a separatory funnel. The aqueous layer is then discarded to remover the things that are soluble in water. NaCl is then added to dry the organic solvent. This is followed by evaporation of the solvent and hopefully, crystallization.
     
  4. May 25, 2010 #3
    Right, thanks for the pointer!

    Yes you're right about the methyl t-butyl ether. The text refers to it as BME, but it's labeled Mtbe in the the lab... why the inconsistency? don't ask a lowly lab rat like me, I'm sure that there was/is a good reason.

    Okay so salts to neutralize, good! However, MgSO4 is used to dry the organic layer. I think both salts are extracted in the aqueous layer.

    Then Rotavap and bada-boom a crude liquid product that had a nice strawberry aroma. No recrystallization, because the crude and pure IR spectroscopy are very similar which is all that was required for results and analysis.
     
  5. May 26, 2010 #4
    I checked again on the sat. NaCl wash. It is used to further dry the solution because it attracts water molecules to the concentrated solution. However, in practice this is probably overkill, especially when working with a solvent that is essentially insoluble in water. It would probably be suitable for extracting something in ethyl acetate or THF. I seemed to find that extraction with an insoluble organic solvent followed by evaporation and drying with NaCl was more than enough to dry the organic layer.
     
  6. May 26, 2010 #5

    chemisttree

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    The NaCl is added to help you get a more rapid, clean break in the extraction. Unreacted cinnamic acid in the sodium form can act as a detergent in this extraction and a loose emulsion can form. Adding NaCl helps to decompose any emulsion and assists with a clean break. It might help to dry the MTBE layer and lessen the amount of MgSO4 you need to use as well. Water is sparingly soluble in the solvent and during the rotovap step it forms an azeotrope (BP = 52.9oC at 760 Torr, 3.5% water) which helps to dry it. Without the salt, only 1.5% water is present in saturated MTBE so the azeotrope can be used to significant advantage even if no MgSO4 is used.
     
  7. Aug 4, 2010 #6
    Yeah thanks for your help, writing down all the steps for the extraction helped a lot.
     
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