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Homework Help: Move a pi bond into a cycloalkane

  1. Jun 21, 2008 #1
    Hello. This is driving me a little mad, and some assistance would be hugely appreciated.

    1. The problem statement, all variables and given/known data
    Show how the following transformation can be achieved (more than one step may be involved)

    2. Relevant equations


    3. The attempt at a solution
    The only approach i could come up with was the following:

    1. hydrohalogenation of the alkene substituent with H-Cl, putting a Cl on the more substituted C atom and an H on the less substituted C.

    2. dehydrohalogenation should then produce constitutional isomers, as there are 3 [tex]\beta[/tex] carbons, 2 of which produce the same molecule (i think) and both constitutional isomers have tri substituted double bonds. the required pruduct is then one of the constitutional isomers, methylcyclohexene.

    I just saw in another textbook something about endocyclic [tex]\pi[/tex] bonds being more stable than exocyclic [tex]\pi[/tex] bonds. Is this the case in this problem? and would that mean that the required product would be the only one formed?

    Thanks in advance for any help. Its late and i cant think anymore...
  2. jcsd
  3. Jun 22, 2008 #2

    You're correct there are 3 beta hydrogens, with the possibility of two different products from an E2 reaction. I'm not sure about different stabilities of endo/exo cyclic rings, but even if the two products were equally stable and the two reaction pathways had the same activation energy, you would still get 66% the product you were looking for, because there are twice as many ways to form the product you want.
  4. Jun 22, 2008 #3
    Thanks thanks thanks!
    really appreciate you taking the time for this :)
    I love these forums!
  5. Jun 23, 2008 #4


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    You wouldn't even need to hydrohalogenate/dehydrohalogenate. Look at the mechanism of the acid-catalyzed E2 elimination, first step. The product is a carbocation at the 3o carbon. Loss of H+ will give you the endocyclic isomer. Any exocyclic product would be recycled again and eventually the thermodynamic product is obtained.
  6. Jul 6, 2008 #5
    Using what little I know from basic organic chemistry, I would simply 1) brominate and 2) run an E2 reaction using a sterically-encumbered base such as tert-butoxide.
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