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Explaining the mechanism of a reaction and drawing its Energy diagram

  1. Oct 20, 2014 #1
    1. The problem statement, all variables and given/known data
    This was a lab. I started with Maleic Acid. We performed a temporary corbonication to see whether or not the single bond would rotate to create fumeric acid, maleic acid or both.
    The question that I have to answer that I cannot answer is: Explain the mechanism of the reaction. & Draw an energy diagram for the two-step reaction mechanism while including a torsional strain

    2. Relevant equations

    Would ΔG = ΔH - TΔS useful?


    3. The attempt at a solution

    I tried to find the bond energy of the starting and end product but it didn't work. Not just that, my produc ( according to my experiment ) has a mix of fumeric AND maleic. So how can I draw an energy diagram if i only have a % of fumeric and maleic acid ( i don't have the weight at the end -- didn't have to record it)
     
  2. jcsd
  3. Oct 20, 2014 #2

    Borek

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    You can't draw an exact diagram, but the general scheme should be rather obvious.

    I know it is a typo, but you managed to write something that fits several similar words and I have no idea which one it is. Quite funny if you ask me :)
     
  4. Oct 20, 2014 #3
    This is odd I was specifically asked to draw a two step energy diagram ( unless that is what you meant). And my apaologies! I mean carbocation. Silly me.

    Edit: I realized what you meant. My question was more like this: should my Delta H be - or positive in the graph ( I think its my delta H right?)
     
    Last edited: Oct 20, 2014
  5. Oct 20, 2014 #4

    epenguin

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    How do you perform a carbocation then?
     
  6. Oct 20, 2014 #5
    I don't understand if that is sarcasm or an honest question? I wasn't really introduced to what a carbocation was. I assumed it was like hydration but with carbon ( sorry if my terminology "sucks"). And you perform a carbocation by giving a charge to the carbon, no? Also, I don't understand how your answer pertain to my question.
     
  7. Oct 21, 2014 #6

    Borek

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    epenguin points to the fact you can't draw a diagram not understanding what is happening in the mixture, and you clearly have no idea.

    Carbocation is an object, not a procedure. If you were not yet told what carbocation is, you have to check it by yourself - try books, google, wikipedia.
     
  8. Oct 21, 2014 #7

    epenguin

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    I might be getting pedantic in my old age. Helpers here very often guess and psyche out what posters really mean and don't say and indeed don't know, managing to do it somehow by logic and familiarity with standard syllabuses, but there must be some limits to what we even can do. I cannot imagine any Uni teacher would accept such sloppy wording, so maybe it is better for you for it to be pulled up here than later.

    It was disconcerting to read that you "performed" what must be an experimental procedure to which you give the name of a somewhat theoretical entity. So follow Borek's recommendations plus it must be possible to state what you did, observed and are trying to explain.
     
  9. Oct 21, 2014 #8
    We buy our lab books. Were given lab time and we do it. There are instructions to follow to but that is all. I can photocopy the pages too ... Even my theory book doesn't cover carbocation. Also another thing is I'm French attending an english school so I apologize for the language. Secondly, I can only make assumption and I agree I didn't say everything.

    I know I didn't state everyrhing. Like we dissolved maleoc acid in water. My end solution was a mix of maleoc acid and feric acid that we got through reflux and we then used a filter to get the solid back. Dissolved it again to use it on our TLC palate. My Rf values not 3.4. I must have typed to fast and didn't re-read. It is .83 for my solvent compared to fumeric. Might still be wrong but it followed the class trend.

    Now for an energy diagran., I do know how to write them. I know I put in energy by boiling my initial solution ( as I said maleoc acid in water + HCl) and I cooled it down in an ice bath. My main question was to know if I should write my end result to have the same energy as my initial point.

    P.s: sorry for not being helpful or missing facts. I havent taken chemistry in a year
     
  10. Oct 21, 2014 #9

    Borek

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    You ended with the mixture, so at least part of the maleic acid was converted into something else, and two different compounds rarely have exactly the same energy. Not all maleic acid was converted, which suggests the difference in energies is small.

    In strong acid solutions maleic acid is protonated to a carbocation, which later losses H+ and becomes - again - a butenedioic acid. However, there are two such acids (cis and trans), and which one is produced is mostly random.

    Honestly, I don't understand what sense this experiment makes if you are not explained the mechanism. Not your fault.
     
  11. Oct 21, 2014 #10
    We just touched upon that. ( this morning). Well I thought the trans isomer was more stable due to less electronic interference. I mean if I draw the torsional strain diagram, wouldn't it be more stable at 180 degrees? ( which would be the trans isomer). As for the carbocation I understand how it is possible in some case and not in others ( then again might be unrelated to this particular question -- I mean nucleophilic substitution)
     
  12. Oct 21, 2014 #11

    Borek

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    Trans is slightly more stable, yes, but the difference is so low it doesn't matter much in a hot solution, hence the mixture produced is equimolar.
     
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