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Which compounds are more basic than water?

  1. Sep 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Which of these compounds is significantly more basic than water?

    I have 5 compounds here. I have (1) propane with a carbanion on the end, (2)the same molecule as that but with an adjacent double bond to oxygen (its on the middle carbon), (3) C-C-Ominus, (4) C-C-Sminus, and (5) Br-.

    3. The attempt at a solution

    I ranked their bascisity as (inc bascisity) 5<3<4<2<1. I am pretty sure this is correct, except that the thiolate has a lower pka than the alkoxide. Which means it should be a weaker base. But sulfur is less electronegative than oxygen so it can't stabilize the negative charge as well, so it should be more basic. So I guess the increasing size of sulfur which makes it more acidic is more important than electronegativity?

    So, the pka of water, ethanethiol and ethanol are all generally around the same (14, 10.6, 16, respectively). Wheras propane is around 50. So I guess the answer would be the propane and acetone carbanions only?
     
    Last edited: Sep 4, 2011
  2. jcsd
  3. Sep 4, 2011 #2

    chemisttree

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    Is this acetone or does it too have a carbanion?
    Nope. Bromide is in the right place, though.

    The pKa tells you everything you need to know. Don't overthink it. Remember that the pKa is the pH at which half of the acid is deprotonated so water really should be 7 rather than something like ~15.5 when making comparisons like this.
     
  4. Sep 5, 2011 #3
    Acetone with a carbanion.

    bromide<thiol anion<alkoxide<acetone carbanion<propane carbanion

    This has to be right.

    Well then I have no clue. Is a pka of 10.6 significantly different than 7? What about 26? It seems pretty subjective.
     
  5. Sep 5, 2011 #4

    chemisttree

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    It is.

    I would say that something greater than two orders of magnitude more basic than neutral water is significantly more basic than water. YMMV
     
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