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What does adding a base dropwise due to an acidic solution?

  1. Jun 24, 2013 #1
    1. The problem statement, all variables and given/known data
    In my organic chemistry lab we are synthesizing Benzocaine by utilizing .25g of p-aminobenzoic acid, 2.5 ml ethanol, and add .3 ml H2SO4 together. Once this solution refluxes we add 10% K2CO3 dropwise and a precipitate will form.

    Lots of details, but all the matters is that we have an acidic solution and are adding a base dropwise. What does adding it dropwise compared to dumping it in do compared to just dumping it?

    2. Relevant equations

    none

    3. The attempt at a solution
    That was a lot of details, but basically we have an acidic solution and we add a base Dropwise. What does adding the base dropwise do compared to just dumping the entire basic solution into the acidic mixture? I'm thinking it has to do with Le Chatlier's principle. Adding a small amount of base allows the solution to equilibrate slowly, while just dumping a huge amount of base in causes the mixture to crystallize (form precipitates) into less stable bigger chunks with unwanted side products and lead to an inaccurate yield.
     
  2. jcsd
  3. Jun 24, 2013 #2

    Borek

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    Organic chemistry is definitely not my forte, but the most obvious thing I can think of: what could happen in the high pH of excess potassium carbonate?
     
  4. Jun 24, 2013 #3
    hmm. I'm probably complicating a ton but excess base would make the solution predominately basic preventing certain products from forming (maybe the main product, which is benzocaine?). We are synthesizing benzocaine which involves turning a p-aminobenzoic acid into ethyl p-aminobenzoate. This involves ethanol attacking carbonyl and being deprotonated by HSO4-. Without the deprotonation (wont happen in basic conditions?) the ethanol can become a leaving group instead.

    This is a link to the mechanism: http://www.personal.psu.edu/users/j/m/jmg5214/writing_clip_image002_0002.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  5. Jun 24, 2013 #4

    Borek

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    Look for much simpler thing. What happens to esters in high pH?
     
  6. Jun 24, 2013 #5
    I want to say that they'll be hydrolyzed or fall apart?
     
    Last edited: Jun 24, 2013
  7. Jun 24, 2013 #6

    Borek

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    And that's exactly my line of thinking.

    Doesn't have to be right, but it is probably the simplest possible explanation.
     
  8. Jun 25, 2013 #7

    chemisttree

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    solubility of benzocaine is marginal in water... better in ethanol. You add slowly to react excess sulfuric acid and then to free-base the product, slowly. when you just dump things together you risk entrapping the ammonium form with the free base. when you recrystallize, you lose the ammonium compound. And you risk overshooting the neutral point and could produce a basic aqueous solution that could start to hydrolyze the product during workup.
     
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