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Exam Question!

  • 19-04-2011 1:36pm
    #1
    St3ph3n
    Registered Users, Registered Users 2 Posts: 22 ✭


    Hey would anyone know the answer to this organic chemistry question:

    (a)

    The pKa values of acetic acid, oxalic acid and malonic acid are given below. Explain the trends.
    CH3CO2H 4.7
    (CO2H)2 1.2, >6
    CH2(CO2H)2 2.8, 5.8

    (b)

    Which is the stronger acid 4,4-dimethyl-1,3-cyclohexanedione or 2,2-dimethyl-l,3-cyclohexanedione and why?

    Any help would be greatly appreciated! :D


Comments

  • #2
    SOL
    Registered Users, Registered Users 2 Posts: 1,155 ✭✭✭SOL


    The more stable the anion the better an acid it will be... think about this and the electron withdrawing power of the rest of the molecule in question for part a)

    for part b)

    For the second part, think about what happens when you deprotonate a ketone...
    Think about how the anion is stabilised... then think about which can do this better...

    if you are still stuck I can give you the answers...


  • #3
    St3ph3n
    Registered Users, Registered Users 2 Posts: 22 ✭St3ph3n


    Thanks for the help SOL but I think I might be a lost cause!If you could give me the answer I'd really appreciate it! :)


  • #4
    SOL
    Registered Users, Registered Users 2 Posts: 1,155 ✭✭✭SOL


    okay, well for the first part the acetic acid has a pKa of 4.7, if you look at the oxalic acid the pKa of the first proton is 1.2, so it leaves a lot more easily...
    This lets us conclude that the other carboxylate group is helping to stabilise the deprotonated form. If we look at malonic acid, we see that if we move the carboxylic acid one carbon more away the effect is weaker, but still there and the pKa is 2.8



    If we look at the second proton in the two diacids we can see that the deprotonated carboxylic acid is having the opposite effect so the more separated one is 5.8 but this rises to more than six in the adjacent one. This makes sense since a carboxylate anion is negatively charged and so will not be able to stabilise another adjacent negative charge.

    If you are wondering why the carboxylic acid withdraws electrons have a look at the periodic table and where oxygen is relative to carbon.

    For part b) when you deprotanate a ketone or aldehyde it will tautomerise to an "enolate" to put the negative charge on the oxygen. In one the site that would allow stabilisation by both oxygens is blocked...


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