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Probably a dumb question...

Discussion in 'DAT Discussions' started by Cofo, Jul 29, 2011.

  1. Cofo

    Cofo Super Cool Member
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    but I am just making sure my thinking is right!

    Is a carboxylic acid more acidic than a tertiary amine (if a hydrogen bonded to the nitrogen leaves)???

    Because if a tertiary amine was deprotonated...
    it was be very stable, as it would have no charge!

    If a carboxylic acid was deprotonated...
    it would have a negative charge on the oxygen.

    So...now it has me thinking, that perhaps a tertiary amine is more acidic than a carboxylic acid.
    Anyone help me!!!
     
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  3. radmazindds

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    My understanding is that a strong acid has a weak conjugate base. So let's think through it that way. Strong anything means highly unstable and very reactive.....right? (I hope so, or all my studying's been for nothing.)

    Carboxylic acid's (COOH) conjugate base would be COO- --a negative charge. You would think this is unstable BUT this is resonance stabilized therefore VERY weak (stable/unreactive) as a base and a STRONG acid.

    Tertiary amine's (R3N) conjugate base would be ... how can you have a tertiary amine act as an acid? There are no protons to donate or available spots for electrons to attack. This is a base. Unless you have +NHR3. Then that's an acid (Bronsted-Lowry acid [proton donor]). But it's not a tertiary amine... is it?

    That's my two cents. I'd love to be corrected if I'm wrong, cause I'm curious, too. Good question--got me thinking. Hope it helped!

     
  4. AlbinoPolarBear

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    This is an interesting question. Not dumb at all.

    The COO- ion is stabilized by resonance is a very good conjugate base.

    COOH <-> COO-

    If you know anything about nitrogen-containing compounds, you should know that they make VERY GOOD nucleophile/bases. The lone pairs of the nitrogens hate to be alone, and they usually try to pick up something, anything, to make them feel not lonely (kinda like a girl who just got dumped). I've done research with nitrogen containing compounds, and the rule of thumb is... if your estimated synth time is x, be prepared to spend 7x the time. Saying that nitrogens are annoying is an understatement.

    So the equilibrium a tertiary amine will go something like this:

    R3NH+ <-> R3N

    So right now, there is a fight between competing factors for the abstraction of the hydrogen. Keep in mind, these are in equilibrium terms.

    Carboxylic acids proton abstraction:
    Pro: Conjugate base very weak and can be stabilized via resonance
    Con: Formal negative charge, no longer neutral compound

    Tertiary amine proton abstraction:
    Pro: Molecule will now be neutral
    Con: Nitrogen won't mind grabbing another hydrogen from somewhere else (think lonely girl analogy)

    So.. in the end. Who wins out? We gotta go to the judges with objective stats. (remember, the lower the pka, the more acidic the proton).

    The carboxylic acid (pka = 4).

    The tertiary amine is still a very good opponent (pka = 10).

    Hope this helps!
     
  5. toothhornet88

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    Yes, Carboxylic acids are about twice as tronger as tertiary amines!
    Just know the CARDIO mnemonic that Chad talks about in section 1.7 of Organic Chemistry Videos when comparing strength of acids or bases

    C-charge (the more electronegative = more stable thus more acidic)
    A-atom (the bigger the atom =more stable thus more acidic)--> only use this comparison when two atoms are in same period
    R-reasonance (more resonance= more acidic)
    D-dipole (not sure about this one)
    I-induction (EDG make bases stronger, EWG make acids stronger)
    O-orbital (sp3>sp2>sp when comparing acidity strengths)
     

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