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Acidity of compounds with oxygen-Nucleophility

  1. Jul 23, 2014 #1
    Rank the following in terms of decreasing nucleophility:
    CH3CO2- CH3OH CH3O- CH3CO2H NC-
    For compounds with oxygen, basicity parallels nucelophility.
    I attached answer from the book.
    Now, what I want to understand, is why CH3CO2H is more basic than CH3OH?
    I also attached the only pKa values given in the book.
    In my head, CH3CO2H is more acidic (is there a way to prove this using the info given in the acidity chart?)
     

    Attached Files:

  2. jcsd
  3. Jul 25, 2014 #2
    Even though methanol may be less nucleophilic than acetic acid, you must remember that acidity is based upon the stability of an acid's conjugate base. Acetate has resonance structures that allow it to stabilize the negative charge over two oxygen, making it a relatively stable base and thus making acetic acid a relatively strong acid (compared to other organic compounds).
     
  4. Jul 26, 2014 #3
  5. Jul 26, 2014 #4
    I don't understand why Chris K's answer is wrong. CH3COOH can hardly take another proton to become CH3CO2H2. As you said, it is acidic.
    Now, how am I supposed to know CH3COOH is a better nucleophile than CH3OH? Is it by rough memorization? Or is it a debated subject?
     
  6. Jul 26, 2014 #5
    Teemo, your answer isn't clear to me. What are you trying to say? That basicity is not entirely dependent on nucleophilicity in this case?
     
  7. Jul 26, 2014 #6
    What I'm saying is exactly what Chris K. is saying. The book answer is correct. I guess I didn't make myself clear last time about the relationship between resonance and nucleophilicity. First, keep in mind that a conjugate base will ALWAYS be more nucleophilic than its acid. Moving on. Like I said last time. Acetate can resonance stabilize its negative charge, whereas methoxide cannot. Nucleophilicity is dependent on electron density, which allows for the attack of an electrophilic entity. Because acetate is resonance stabilized, it is a weaker nucleophile than methoxide due to its widespread electron density, whereas the methoxide's electron density surrounds the oxygen atom (you can see electron density maps for this if not visualizable). So we know that acetate is a weaker nucleophile than methoxide. Now we look at acetic acid and methanol. Both differ from methoxide and acetate by one hydrogen atom attached to a nucleophilic oxygen atom. So we can tell that the effect of the hydrogen atom on each compound is roughly similar. As such, because methoxide is determined to be more nucleophilic than acetate, (due to two reasons Chris mentioned, size and resonance) and because the effect of the hydrogens in essence cancel out, we know that methanol is more nucleophilic than acetic acid.
     
  8. Jul 26, 2014 #7
    Teemo. There is something vital and crucial I may not be getting here.
    The book says:
    CH3COOH>CH3OH
    Chris K. and (you) seem to say:
    CH3COOH<CH3OH
    Did you misread the books answer? Maybe I'm too tired and I should sleep...
     
  9. Jul 26, 2014 #8
    I believe your book's answer is incorrect. If you look at the mechanism for esterification, the methanol attacks the acetic acid. If acetic acid was more nucleophilic, it would be more likely for the methanol to be protonated and attacked rather than the acetic acid.
     
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