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Orbital Symmetry Controlled Reactions

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data
    http://avogadro.chem.iastate.edu/CHEM332/332Fall06Exam5Key.pdf
    Please go to page 5.

    2. Relevant equations
    No equations, just concepts.

    3. The attempt at a solution
    I tried the first problem (A), and got the solution wrong. I thought I'd check the concepts before attempting any other problem and checking the solution.
    I thought that it was photochemically allowed, not thermally because from what I understood, an odd number of double bonds in the initial reactant and a trans product indicated a conrotatory ring closure using light. Obviously, I was wrong.

    Could you please explain how I should solve this and similar problems?

    Thank you.
     
    Last edited: Nov 26, 2007
  2. jcsd
  3. Nov 27, 2007 #2

    chemisttree

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  4. Nov 27, 2007 #3
    I am sorry, I still don't have a clear understanding of what I should do. Could you please tell me how you would reason for just the 1st problem? Thank you!
     
  5. Nov 27, 2007 #4

    chemisttree

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    Do you understand that it is a 4+2 cycloaddition?
     
  6. Nov 27, 2007 #5
    Yes, the way I do it (correct me if I am wrong) is to count around the new sigma bond. The "1" would be the carbon attached to CO2Et, and "2" the other side of the double bond where the new sigma bond will be (hence the "2"), while the "4" comes from counting from the other side ("1" being the carbon to the right of the molecule). It is a cycloaddition because we have 2 double bonds forming a single bond. We have 2 less double bonds, and 2 new single bonds.

    What I don't understand is why it is thermally allowed and suprafacial on alkene and diene.
     
  7. Nov 27, 2007 #6

    chemisttree

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    The numbers "2" and "4" come from the number of electrons in the interacting system. You should not count carbons.... It means that a 2-electron system (isolated alkene) is interacting with a 4-electron system (conjugated diene), thus a 4+2 cycloaddition.

    Do you know what the terms 'suprafacial' and 'antarafacial' mean?
     
  8. Nov 27, 2007 #7
    I understand now, I was thinking of sigmatropic rearrangements. It is like a Diels-Alder reaction then?

    Suprafacial is when we have bonding of lobes of the same sign, while antarafacial has bonding of lobes of the same side as well as bonding of lobes of opposite signs.
    Do we have to draw the orbitals in order to know which applies to the problem?
     
  9. Nov 27, 2007 #8

    chemisttree

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    This is a Diels-Alder reaction. Rethink your definition of suprafacial. It doesn't refer to the sign of lobes. Also rethink what 'antarafacial' means. It would be helpful if you would revisit the link I provided and carefully read the whole thing.
     
  10. Nov 27, 2007 #9
    I did, but combined with the lecture, I got really confused and the prof. is not helpful at all.

    Here's a pattern I derived from all the related notes/websites/exams I read:

    In an electrocyclic addition:
    Even double bonds with trans product = thermal + conrotatory
    cis product = photochemical + disrotatory
    Odd double bonds with trans product = photochemical + conrotatory
    cis product = thermal + disrotatory

    Cycloaddition reaction:
    Even double bonds with trans product = photochem. + suprafacial
    cis product = thermal + antarafacial
    Odd double bonds with cis product = photochemical + antarafacial
    trans product = thermal + supra.

    Sigmatropic:
    Even double bonds with trans product - heat/conrotatory
    cis product - thermal/disrotatory
    Odd double bonds with trans product - photochem. + conrotatory
    cis product - thermal - disrotatory

    Does this sound correct? I will not learn this by heart even if it is correct, I will most likely find diene and triene example, but I just want to know if I am on the right path.

    Thanks!
     
  11. Nov 27, 2007 #10

    chemisttree

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    I'm confused as to why you are grouping these reactions as you have them. A cycloaddition is a type of electrocyclic reaction. Sigmatropic reactions are isomerizations. The same principles apply to all of these reactions if you understand the principles. It is best to understand the principles first and the types of reactions (a distant) second.
     
  12. Nov 27, 2007 #11
    This is how they were broken down in the lecture... I will go back and try to get the principle down first.
     
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