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Homework Help: Elimination reactions of cyclohexane derivatives

  1. Jun 14, 2018 #1
    1. The problem statement, all variables and given/known data
    When trans-2-methylcyclohexanol is subjected to acid-catalyzed dehydration, the major product is 1-methylcyclohexene.
    IMG_20180614_192151.JPG
    However, when trans-1-bromo-2-methylcyclohexane is subjected to dehydrohalogenation, the major product is 3-methylcyclohexene. IMG_20180614_192708.JPG
    The attempt at a solution
    Obviously, this is an E1 mechanism and the hydride shifts occur from either of the adjacent (to the carbocation) carbons in each case.
    Also, -Br is a much better leaving group than -OH (but I don't know if it's relevant here).
    The 3° hydrogen in case 2 is more acidic due to the greater -I effect of -Br. Right?
    How will this affect the outcome of the reaction?
    How can we account for the different outcomes anyway?
     
  2. jcsd
  3. Jun 14, 2018 #2

    mjc123

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    Is the mechanism the same in both cases?
    Is there a hydride shift?
    Is the leaving group -OH?
     
  4. Jun 14, 2018 #3

    TeethWhitener

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    Are you sure about that?
    Edit: ninja'd by mjc123.
     
  5. Jun 14, 2018 #4
    Edit: I was actually wondering now that case 2 outcome should've been 2-methylcyclohexane instead (due to the more acidic hydrogen)?
     
  6. Jun 14, 2018 #5
    Well, I think case 1 is definitely E1.
     
  7. Jun 14, 2018 #6

    TeethWhitener

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    Why? and do the same considerations apply for case 2?
     
  8. Jun 14, 2018 #7

    TeethWhitener

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    Side note: this molecule doesn't exist (why?)
     
  9. Jun 14, 2018 #8
    The protonated hydroxyl - carbon bond breaks, that's for sure.
    Since you objected, I'm reconsidering whether the 2nd is E1.
     
  10. Jun 14, 2018 #9
    I'm sorry I meant 2-methylcyclohexene
     
  11. Jun 14, 2018 #10
    Oh, no. The 2nd is an E2 mechanism
     
  12. Jun 14, 2018 #11
    Ok, so I see that in case 2, the base will attack the 2° hydrogen in preference to the 3° one.
     
  13. Jun 14, 2018 #12

    TeethWhitener

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    Ok. (You still haven't said why, but) we can move on. What will the intermediate look like in both cases?
     
  14. Jun 14, 2018 #13

    TeethWhitener

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    Of course this is true, given the product, but why?
     
  15. Jun 14, 2018 #14
    For E2, it's 1°>2°>3°
     
  16. Jun 14, 2018 #15
    I get it. I mainly got confused because of thinking that both were E1. Also, I went into thinking inductive effects and all.
     
  17. Jun 14, 2018 #16

    TeethWhitener

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    Why do you think this? It's not right.
     
  18. Jun 14, 2018 #17
    Why? That's what we're taught.
     
  19. Jun 14, 2018 #18

    TeethWhitener

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    What is the E2 product of 2-bromobutane?
     
  20. Jun 14, 2018 #19

    TeethWhitener

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    Remember Zaitsev's rule.
     
  21. Jun 14, 2018 #20
    2-Butene + 1-Butene
    (major) (minor)
     
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