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Mechanism of saponification

  1. Dec 6, 2014 #1

    Qube

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

    Problem: I am told that the first mechanism is operative in saponification.

    XlFfR.jpg

    3. The attempt at a solution

    However, this strikes me as wrong because the first mechanism involves the formation of dianion. Using hydroxide ion to abstract a proton and make that particular dianion seems like a very uphill process energetically.

    The second mechanism, according to her, is wrong because it involves the formation of a carboxylic acid in basic conditions. And acids should never exist in basic solutions.

    Er, this seems wrong. First, the acid we're forming isn't the result of some proton abstraction in basic solution. That would be hard - can you imagine abstracting a proton from HO-? Second, the second route seems highly favorable because once you form the -COOH you can deprotonate the -COOH - an energetically favorable move!
     
    Last edited: Dec 6, 2014
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  3. Dec 6, 2014 #2

    Bystander

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    Your question? Learn it "his" way for purposes of passing the course and the other way for real life later? Which is correct? Which is correct and how to win a debate?

    Hit the library, or Morrison & Boyd, or other available sources to satisfy yourself. "Fighting city hall" isn't terribly practical unless you are a skilled diplomat. Take what you find on the internet with many grains of salt. That said, what you've stated looks pretty much like what I remember (hated organic, still do), but confirm it elsewhere. Language barrier between prof and students?
     
  4. Dec 6, 2014 #3

    Borek

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    Bystander is perfectly right with his advice - my edition of Morrison & Boyd (Polish translation) shows the mechanism for both hydrolysis in acidic and alkaline conditions.

    What I don't understand is why your reaction equation doesn't start with an ester as it should - it starts with something that already looks like some kind of intermediate (which can be a hint).
     
  5. Dec 7, 2014 #4

    Qube

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    Questions:

    1) How's my reasoning?
    2) Who's right in reality?
     
  6. Dec 7, 2014 #5

    Qube

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    Well, I didn't want to go through the entire mechanism from start to finish since the beginning isn't relevant. It all begins the same way - nucleophilic attack at the carbonyl carbon.
     
  7. Dec 7, 2014 #6

    Bystander

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    Reasoning? Looks good, agrees with what I remember.
    Correct in fact? Again, I'll buy your argument rather than a dianion/double anion; 8.99 x 109(q1q2)/((.4 to .5 nm)εH2O) ~ 3 x 1026 J/mol to form the double anion if I haven't misplaced too many orders of magnitude.

    Seriously, hit the library; check Noller, Morrison & Boyd, and whatever you can find for introductory organic texts until you are satisfied with your reasoning. We can't make you believe in yourself --- that's your department.
     
  8. Dec 7, 2014 #7

    epenguin

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    Comes back to me I've seen something like that with a δ- on each O atom and two partial double bonds, O---C---O which represents the "tetrahedral intermediate" so maybe it was that with a bit of absentmindedness.
     
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