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Homework Help: Carboxyl acid strength and electronegativity How do they relate?

  1. Aug 8, 2006 #1
    Carboxyl acid strength and electronegativity... How do they relate??

    Here is a question that I had on a recent chem II exam.

    Which is the strongest acid?
    A. CH3COOH
    B. CF3COOH
    C. CCl3COOH (Cl for chlorine, not Carbon 13)
    D. CBr3COOH
    E. CI3COOH

    I selected choice A because the electronegativity is the smallest. A stronger bond would mean it would be harder to dissociate and for that reason I thought A was right. It turns out the answer is B. I believe the answer to be wrong, but maybe I just don't understand it. Could someone help me out and give a little explanation. I've been looking for the Ka values of these but was only able to find the one for CH3COOH, which is acetic acid. Thanks.
  2. jcsd
  3. Aug 8, 2006 #2
    fluorine has the highest electronegativity and thus pulls the electrons of OH-groups in the COOH-group towards the itself, making it relatively easy to lose the proton of the carboxylgroup. Structure:

    CF3 - COOH

    (electronegativities: F = 4.1, Br = 2.7, I = 2.2, Cl = 2.8 and H = 2.1)
    Last edited: Aug 8, 2006
  4. Aug 8, 2006 #3
    oh, so for that reason CF3COOH is the strongest acid out of the five? And I assume that CH3COOH is the weakest do to the same reasoning... Is there anywhere I can find the Ka values of these compounds? Thanks
  5. Aug 8, 2006 #4
    The -COOH carboxcylic group is actually constantly resonating, electrons are actually shared between the two oxygens and stabilise the proton. However, if you have groups attached that draw away the electronegative charge this resonating effect is no longer favourable and it will take less energy to dissacociate that proton.

    http://img197.imageshack.us/img197/8011/carboxcylicaciddx5.jpg [Broken]

    If you had groups that, via inductive effects, direct charge density towards that carboxylic group (say methyl [-CH3] groups) i would expect to find that the dissociation constant is higher*.

    Consider an aqueous solution, imagine the electrostatic charge of the two principle components, Water and the Carboxylic acid, as two magnets and a third magnet ( the proton ) which is oppositeley charged. Pulling the proton from the strong caroxylic magnet (analogy) with a weaker magnet is not going to happen easily, but if the carboxylic acid is weaker, H3O+ fomation is more favourable.

    Try The CRC Handbook of Chemistry & Physics for values of K.

    EDIT: * Corrected low Ka for High Ka :blushing:
    Code (Text):

     Strong Acid        Weak Acid
     Weak Base        Strong Base
       High Ka           Low Ka
    Last edited by a moderator: May 2, 2017
  6. Aug 8, 2006 #5
    Oh that kind of makes sense. It's not that the molecule is strongly held together, but the three Flourine on the left pull the rest of the molcule and the H is kind of dangling off on the end. I couldn't get the K vaules from there but if anyone could post them just to ensure that CF3COOH has indeed the strongest acid of the three then that would be great. Thanks again...
  7. Aug 8, 2006 #6
    CF3-COOH is a strong acid and dissociates completely in water and that can be the reason you can't find any K-value. I assume this because Ka of CCl3 - COOH is 2,0 x 10^-1 (it's directly under H3O+ in my table)
  8. Aug 8, 2006 #7
    The reason I chose CH3COOH as the strongest acid is because I remembered reading in my book that for binary acids HF was less than HCl which was less than HBr which was less than HI. But I guess the electronegativity matters here because the bond is stronger for a higher electronegativity but there are only two molecules, so it doesn't polarize it like the carboxyls... Is that how it works (why for binary electronegativity means weaker acid)? I know we went over an example in class, but I remember reading that in the book... unfortunately the book doesn't grade the exams so I should have never went with the book... Oh well thanks for clearing things up, I appreciate it a lot.
  9. Aug 9, 2006 #8
    for the acids HF,HBr,HCl,HI the atom radius is of importance.
  10. Aug 9, 2006 #9
    oh, ok. My book says that electronegativity is the reason why HF is the weakest, which to me makes sense. A stronger bond is harder to pull apart, and something that is harder to dissociate is a weaker acid... I tried to apply this thinking to the -COOH's and it backfired... There's way too many exceptions in chemistry for it to be one of my favorite courses...:rolleyes:
  11. Aug 9, 2006 #10
    Variety is the spice of life
  12. Aug 9, 2006 #11
    ^it's also the reason I probably will be losing $15,000 in scholarship money...
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