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Relative stability of ions

  1. Aug 16, 2012 #1

    AGNuke

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    We have to determine in which of the following options, the first ion is more stable than the second.

    1.
    Q1.png

    2.
    Q2.png

    3.
    Q3.png

    4.
    Q4.png

    5.
    Q5.png

    6.
    Q6.png

    Attempt at the question

    1. Since in the first one, +ve charge is due to bonding, not deficiency of electrons.

    2. First one can get lone pair from N.

    3. N can easily donate its lone pair as opposed to O.

    4. O can easily affect the -ve charge density by inductive effect.

    5. P can handle +ve on itself more appreciably than N.

    6. Backbonding is possible in 2nd case.

    The problem is that I have marked 4 options but only 3 options are right as per the answer key. Is it that the answer key is wrong?
     
    Last edited: Aug 16, 2012
  2. jcsd
  3. Aug 16, 2012 #2

    AGNuke

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    Changed Image Host. Now should be able to see the pics.
     
  4. Aug 16, 2012 #3

    chemisttree

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    Draw all of the resonance structures for the second choice and see if your reasoning holds up.

    So can the second one. Which one is most stable?

    Is that a methyl group instead of double bonded methylene on the nitrogen? If not, it looks like an ylide but you haven't drawn the carbanion. Right answer, BTW.
     
  5. Aug 16, 2012 #4

    AGNuke

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    1.
    A1.png

    I've blundered up big time. I now think the second one is more stable. :cry:

    2.
    A2.png

    Second one can also donate. (What's happened to me? :confused:).
     
  6. Aug 16, 2012 #5

    chemisttree

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    Keep working on that delocalization for 1 and 2. You haven't show them all yet. Good work so far! Remember, the more delocalized the charge the more stable.
     
  7. Aug 17, 2012 #6

    AGNuke

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    My Bad. Its carbanion. So, its possible that P can form pπ-dπ back bond.

    As for 1 and 2, the latter case seems to have 3 canonical structures, which should increase their stability. Therefore, 1 and 2 are out of answer.

    I noticed that the season of back bonding is showering over 4. S can also form pπ-dπ back bond to accomodate that -ve charge. And they say Electromeric effect is more effective in shifting electron density than Inductive effect.

    So, the answer boils down to 3,4,5. I really need to take sleeping pills... :bugeye:
     
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