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whats more acidic: pyruvic acid or lactic acid

Discussion in 'MCAT Study Question Q&A' started by VandyDerm, Jun 4, 2008.

  1. VandyDerm

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    Hey guys, whats more acidic, pyruvic acid or lactic acid. im thinking lactic acid because both OH and the carbonyl contribute to acidity. I dont get why pyruvate is more acidic according to gay EK
     
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  3. unsung

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    Carbonyl is a withdrawing group --> stabilizes anion = weaker conjugate base = stronger acid.

    Hydroxy group is donating --> destabilizes anion = stronger conjugate base = weaker acid.

    Same rules as in all the aromatic stuff. The way I remember it- anything with a lone pair can potentially act as an electron donating group, alkyl groups withdraw through inductive effect, and anything with a delta plus (i.e. carbonyl, nitrile, etc.) is withdrawing.
     
  4. Vihsadas

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    Moving to study Q&A forum. :)
     
  5. spo01

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    Well lets take a proton (hydrogen) off each and compare stability shall we? There are 4 factors to look at in this easy approach: ARIO, which stands for Atom, Resonance, Induction, Orbital. That is the order from left to right from most important to least important.
    First, lets see what Atom the charge is on. We see that the negative charge in both is on an Oxygen, so that didn't do us any good.
    Second, we look at Resonance. Pyruvic acid and Lactic acid both have resonance, BUT pyruvic acid has even more resonance because of the extra carbonyl group that contributes to it. That's it, we're done, pyruvic acid wins.
     
  6. Vihsadas

    Vihsadas No summer
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    Wow...That's a great trick that I never bothered to learn. I guess I could kinda just tell when looking at two molecules based on those factors...
     
  7. sleepy425

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    the only problem with this is that the presence of the carbonyl in pyruvate does NOT increase resonance. The carbonyl is not in a position where it can participate in resonance. Try drawing a valid resonance structure that involves the carbonyl and you'll see that it's not involved. If you go to the next thing, which is induction, that's where the actual effect is. You have a large delta + on the carbonyl which is adjacent to the carboxyl group. This decreases electron density in the carboxyl group which makes it better able to stabilize the negative charge that develops as the proton is lost. but it's not a resonance effect, it's an inductive effect.
     
  8. sleepy425

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    oh, and actually, in this case, the hydroxyl group of lactate is not electron donating because it cannot participate in the resonance. so the inductive effect is applied here and since oxygen is very electronegative, it also makes lactate more acidic, but not as acidic as pyruvate because it's not as strong of an electron withdrawing group as a carbonyl. you can see this if you look at the trend of pKas:

    propanoic acid (pKa=4.87ish), lactic acid (pKa 3.85ish), pyruvic acid (2.39ish)
     
  9. Doctor D

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    I understand the resonance and induction parts of your system to determine which one is more acidic but could somebody explain with an example how the type of atom or orbital could effect this. Thank You.
     
  10. spo01

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    Why does it not have a valid resonance structure? I drawed it out and it seems to work fine, the only problem I see is that it doesn't fill the octet on one of the oxygens which I admit is ugly but it technically is still an acceptable resonance contributor. Correct me if I'm wrong here.
     
  11. spo01

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    These are just guidelines because there are exceptions ofcourse and things like the type of solvent can affect it (protic vs aprotic), but generally speaking this works most the time:

    for the ATOM: As you go from left to right in the same row you are increasing electronegativity. Thus a negative charge on a more electronegative atom is more stable. Now, as you go from top to bottom in the same column you are increasing the size of the atom and the larger the atom the better it can stabalize the charge. Remember, acidity/basicity is a measure of thermodynamics (*not* kinetics (rate/speed)), thus the more stable the more acidic.

    ORBITALS: a negative charge on an sp hybridized carbon is more stable than a negative charge on an sp3 or sp2 hybridized carbon. This is because the sp orbitial is smaller and tighter than the other orbitals, thus it is closer to the nucleus of the atim. A lone pair of electrons in an sp orbital are held closer to the positively charged nucleus and will be stablized by being close to the nucelus (most common exception: negative charge on sp C-C vs a negative charge on NH2. The former is more stable than the latter).

    This useful trick was found in "Organic Chemistry I as a Second Language". There are a ton of neat little 'tricks' in there that have helped me tremendously.
     
  12. sleepy425

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    I think I know what you're talking about (I think you took the electrons from the carboxyl C=O and pushed them onto the C-C double bond and pushed the ketone C=O electrons up to the oxygen) but that's not a reasonable resonance contributor because the octet on the oxygen is unfilled. that is so unfavorable because there would be a positive charge on oxygen as well as an unfilled octet, so it does not actually contribute in a significant way to resonance. even if it did, which it really doesn't, it would actually destabilize the negative charge because it would create a cross-conjugated system that diminishes the normal carboxyl group resonance.


    this brings up an important point: just because you can draw a resonance structure for something doesn't mean that it is a significant resonance contributor.
     
  13. Kaustikos

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    What? That defies how acidity/basicity works for substituted alkanes. They are significant. It's just...relative.:smuggrin:
     
  14. sleepy425

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    um, not sure what you're talking about with substituted alkanes (do you mean like substituted alcohols or carboxylic acids or carbonyls?) but some resonance structures are just not significant contributors at all, such as the one mentioned in the previous post. my point was, sometimes you draw a resonance structure and it contributes significantly to the true structure, but sometimes, like that time, you draw a resonance structure and its contribution is so minimal (and by minimal in that case I mean zero). like I said, in that case, either way, if that resonance structure were significant, which it absolutely is not, it also would not have increased acidity.

    the point that I was trying to make is that it is relative and because it's relative, some resonance structures you can draw are not significant contributors.
     

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