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Bond lengths in Cycloocta - 1,3,6 - triene

  1. May 25, 2017 #1
    1. The problem statement, all variables and given/known data
    Which is the correct statement about the given compound?
    (a) Bond length between C2 and C3 = Bond length between C5 and C6
    (b) Bond length between C1 and C2 = Bond length between C6 and C7
    (c) Bond length between C6 and C7 < Bond length between C3 and C4
    (d) Bond length between C2 and C3 < Bond length between C3 and C4

    IMG_20170525_135334.JPG
    3. The attempt at a solution
    Well, since I can't predict anything about the inequalities, I think it's (a) as well as (b). But their answer is (c). But I don't understand how?
     
  2. jcsd
  3. May 25, 2017 #2
    Why can't you predict anything about the inequalities? Why do you assume untrue equalities?
    What is the difference between the double bond between 6 and 7 and those between 1 and 2, and 3 and 4? What effect would this have on bond lengths?
     
  4. May 25, 2017 #3

    SammyS

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    What academic discipline is the source of this question?

    In addition to knowing how bond length tends to differ for double bonds compared to single bonds in some qualitative way, it looks like you also need to take symmetry into account.
     
  5. May 26, 2017 #4
    That bond is vertical (w.r.t to other double bonds).
    So does that mean that there is min. repulsion between C6 & C7 (or min. strain on that bond) and hence that bond is shorter and stronger?
     
  6. May 26, 2017 #5
    12th.
    We've just done the Resonance theory, Hyperconjugation etc. & properties and reactions of benzene, phenol & haloarenes etc.
    So I know nothing about cycloalkenes
     
  7. May 26, 2017 #6

    DrClaude

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    I don't think that repulsion is the way to go.

    My guess is that you have covered π conjugation. There is something from that that you can apply here to get more information about the nature of the different double bonds in the molecule.
     
  8. May 26, 2017 #7

    SammyS

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    That the bond is depicted as being vertical is not relevant.

    Look at the two bonds in each direction from the bond of interest. Then answer:
    .
     
  9. May 26, 2017 #8

    The p orbitals of C6 & C7 are the most conjugated, seeing the geometry of the molecule?
     
  10. Jun 4, 2017 #9
    Sorry I have time to post recently. So, as I said, are the p orbitals of C6 & C7 most conjugated?
     
  11. Jun 5, 2017 #10
    What would they be conjugated with?
     
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