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Difference between the acid strength of inorganic and organic acids

  1. Sep 24, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi. I'm pretty much a beginner in organic chemistry and I'm just wondering something.

    For example, I know that HCl is a stronger acid than HF.

    But... then why the following happens if a have an organic acid (e.g a carboxylic acid)?

    According to what I see, a carboxylic acid with a Cl substitutent is weaker than one with a F substituent.

    2. Relevant equations



    3. The attempt at a solution

    I also know that because of electronegativity the difference between H and F is higher than that of H and Cl. So, that means that the bond between H and F is stronger, and therefore, the compoud is weaker ... am I right?
     
  2. jcsd
  3. Sep 24, 2012 #2

    AGNuke

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    All Organic acids are weak acid (Generally). To determine the acidic strength, try forming their conjugate base on paper. Check their stability.

    For F and Cl substituted Carboxylic acid, F exerts its inductive effect more than Cl, so it stabilizes the conjugate base by shifting the negative charge from -COO group to a wider area.
     
  4. Sep 25, 2012 #3
    There is a huge difference in the trend in acid strength if we are looking at the atom directly bonded to the acidic hydrogen, or a more remote atom.
    For directly bonded atoms:
    HI > HBr > HCl > HF
    H2Se > H2S > H2O

    For more remote atoms:
    HONO2 > HOPO2
    HOSO2OH > HOSeO2OH

    Organic acids also follow this latter trend:

    CH2F.COOH > CH2Cl.COOH > CH3.COOH
     
  5. Sep 25, 2012 #4

    AGNuke

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    The trend depends majorly on the stability of conjugate base. The better the CB is able to handle the negative charge on itself, the more acidic the acid. Simple Enough.
     
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