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Organic chemistry question. Mcat review question

  1. Mar 5, 2010 #1
    If you dehyrate 1-butanol, wouldn't you get 2-butene due to rearrangement? This is not a hw question. This is an mcat review question. The question asks: when 1-butanol is dehydrated, the new compound's heat of hydrogenation is most likely? The answer is between 30 and 33 kcal/mol

    It gives you a table of heats of hydrogenation
    1-butene 30.3
    1-pentene 30.1
    cis-2-butene 28.6
    trans-2-butene 27.6

    I'd assume that the new compound would have a heat of hydrogenation similar to trans-2-butene since the carbocation would undergo rearrangement. It is more thermodynamically favored. However, the answer that the review book gives favors a product that is more kinetically favored. I guess 1-butene would produce faster but if time allows, wouldn't it convert to a more thermodynamically favored product? Could anyone explain? Thank you
  2. jcsd
  3. Mar 6, 2010 #2

    Char. Limit

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    In order for a reaction to be thermodynamically favored, it must be reversible. This reaction, I believe, isn't, so you would use the more favored product.

    Also, neither of the 2-butenes appears in the multiple choice question, methinks...
  4. Mar 11, 2010 #3


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    If you use a very strong acid and choose the conditions properly, you can see rearrangement but that would likely lead to addition side reactions as well. Usually dehydration reactions are performed to maximize the actual dehydration without leading to further reactions, though. Beta elimination of the proton is very fast and leads to the alpha olefin under most condtions used for dehydration (ie. acid not too strong and acid's counterion not too good a nucleophile, etc...).

    When you hear the sound of galloping hooves, think "Horses!" not zebras....
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