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Which proton is most acidic?

  1. Jul 19, 2011 #1
    [PLAIN]http://img638.imageshack.us/img638/5747/asdfnt.jpg [Broken]

    My workbook says that protons A & B are both more acidic than C, with the answer being A (I do get that A should be more acidic than B).

    [PLAIN]http://img52.imageshack.us/img52/9800/asdfmi.jpg [Broken]

    I chose C though because de-protonation of C would access the more conjugated system. I would think there is more electron delocalization if you de-protonate C compared to A or B.


    I posted this question on another forum and got the response that the reason is because the alpha hydrogens are closer to the Oxygen (electron-withdrawing), but I am not entirely convinced. In my undergrad Orgo course, I was told to always go with the proton that yields that more stable conjugate base, and I would think the base that results from de-protonation at C is clearly much more stable than the one that results from either A or B.

    Is this a matter of kinetic control vs. thermo control or something? That's the only thing I can think of, where at low temperatures the proximity to the Oxygen makes A & B more acidic while at higher temperatures the yield would increasingly favor de-protonation of C. Just something I'm putting out there, but it's really just a shot in the dark.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 19, 2011 #2

    SpectraCat

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    I think you have come to that conclusion a little hastily. Have you tried to draw resonance structures for deprotonation at A or B? The key stabilizing feature of the resonance system you drew for deprotonation at C is that it includes a structure where the negative charge is localized on the oxygen in the anion.

    All three deprotonation sites result in enolate anions ... you might want to look up keto-enol tautomerism to help understand this a little better.

     
    Last edited by a moderator: May 5, 2017
  4. Jul 19, 2011 #3
    I do think I have a good handle on keto-enol tautomerism. If you draw those resonance structures out (de-protonation of A & B), you will see that the conjugated system upon de-protonation of C is still much more extensive because the aldehyde is a Beta-unsaturated aldehyde.

    I drew these side by side for you to compare (de-protonation of A #1 vs. de-protonation of C, #2):

    [PLAIN]http://img692.imageshack.us/img692/1476/asdfhug.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
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