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How do I rank these leaving groups?

  1. Feb 15, 2016 #1
    1. The problem statement, all variables and given/known.
    Rank the following leaving group in order of increasing ability to leave?
    A) H2O B) NH2 C) OH D) I E) NH3

    2. Relevant equations
    Also not entirely sure the order of H2O and NH3. What should I be looking for to answer this portion of the question?
    Also my study group believes the better leaving group is a halogen. Is that correct?

    3. The attempt at a solution
    I think the answer is
    B<C<D<A<E
    My study group thinks it may be B<C<A<E<D
     
    Last edited by a moderator: Feb 15, 2016
  2. jcsd
  3. Feb 15, 2016 #2

    fresh_42

    Staff: Mentor

  4. Feb 15, 2016 #3
    No it is not. What you had linked to is in regards to the bond dissociation of a molecule when creating, what I believe to be, a free radical. Or when the two substances dissociated from each other leave with an unpaired electron. A leaving group in my question deals with one that leaves with a charge or in the case with H2O and NH3, had a charge when attached but when it leaves the two groups have a zero charge.
    Here is a wiki article fresh_42 on the leaving group.
    https://en.wikipedia.org/wiki/Leaving_group
     
  5. Feb 15, 2016 #4
    I believe that the most effective leaving group is going to be the weakest base. So in this case the best way to figure it out would be to look at a table of pka values and rank them in order of which pka values are the lowest.
     
  6. Feb 15, 2016 #5
    leaving-groups-copy.jpg
     
  7. Feb 15, 2016 #6
    I got that image from http://www.masterorganicchemistry.com/2011/04/12/what-makes-a-good-leaving-group/
    So from that graph and using the pka values the order should be b<c<e<a<d.
    It is very odd to me that a charged halogen is a better leaving group than a zero charged NH3 or H2O. My reasoning for why I think this is odd is because I tend to think the stability or ΔG is greater (G is lower in number) in the H2O and NH3 and the ΔG for the I- is to a lesser extent (or higher number in free energy). Due to the lower energy of the H2O and NH3 they would be a better leaving group. I know that I am wrong in this thinking but I am I totally off base here? Maybe I have the ΔG values incorrect and the halogen is more stable? Other than the pka values what would be a good way to develop my intuition as to why these halogens (except F) are better at leaving? Thanks!
     
    Last edited: Feb 15, 2016
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