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Homework Help: Producing ethanol

  1. Mar 9, 2008 #1
    producing ethanol....

    1. The problem statement, all variables and given/known data
    One way to produce ethanol for use as a gasoline additive is the reaction of water vapor with ethylene: H2C=CH2 + H2O ----> CH2CH2OH Rewrite this equation on a piece of paper using Lewis structures, and use those structures to answer the following question: Was it necessary to break all of the chemical bonds in the reactants in order to form the product, ethanol? Explain your answer.

    2. Relevant equations

    3. The attempt at a solution

    I rewote the equation on a piece of paper with the Lewis structures H2C= H-C-H, CH2= H-C-H, H2O= H-O-H, CH2= H-C-H, CH2= H-C-H, OH=O-H. I also wrote for ethanol: H-C=O=C-H (with single bonds of H coming off of the C on both top and bottom, due to format limitations, I can't right it..:( Is this right? Was it necessary to break all of the chemical bonds to form reactants?
    Thanks!
     
  2. jcsd
  3. Mar 9, 2008 #2
    errmm no you don't have to break all the bonds.....

    do you know the mechanism for this reaction?
     
  4. Mar 10, 2008 #3
    I am not sure about the mechanism for this reaction...I am totally new at this chemistry stuff! Thanks for your help:)
     
  5. Mar 10, 2008 #4
    this is called electrophilic addition reaction. basically electrophiles are species which are attracted by negatively charged species. so in ethene there is a pi bond in the C=C.

    now, in water, there is oxygen atom which is very electronegative. it tends to pull the shared pair of electron across the O - H bond towards itself. hence, it gets a partial negative charge. the hydrogen consequently gets a partial positive charge. this is why water is polar.

    when water molecules approach the pi bond, the pi electrons further repel the shared pair of electron in the O - H bond causing heterolysis. H+ and OH- are formed.

    the H+ is attracted towards the pi bond(consisting of electrons). the pi electrons (in the pi bond) forms a covalent bond with the H+.

    now, the ethene molecule becomes CH3C(+)H2. this is known as a carbocation. the carbon has a positive charge.

    remember the OH- ion? it then forms a covalent bond with the carbocation(C+). ethanol is finally formed.

    this is the simplest example of electrophilic addition reaction.

    so, the only bonds broken are the pi bond of the C=C (not the sigma bond) and O - H from water.


    i hope this is your level of chemistry...
     
  6. Mar 10, 2008 #5
    So I don't have to break all the bonds to form ethanol, just the C=C and the O-H?
     
  7. Mar 10, 2008 #6
    another much more simple approach was to compare the lewis structures of both ethene and ethanol, to see which bonds have changed. it would be obvious that the C = C was broken as well as H - OH bond.

    and i guess this was the approach required.... sorry if i complicated things.....
     
  8. Mar 10, 2008 #7
    in short yeah.....
     
  9. Mar 10, 2008 #8
    No I think I am the one complicating things!! I see what you are saying. I am taking this chemistry course online, and i am not sure that was the greatest idea! Oh well, thank God for people like you...
     
  10. Mar 10, 2008 #9
    anytime..... good luck
     
  11. Mar 13, 2008 #10
    I think Glucose + Yeast also can become Ethanol
     
  12. Mar 13, 2008 #11
    yeah, but in the absence of oxygen (anaerobic)
    however only about 15% ethanol will be obtained...
     
  13. Aug 19, 2008 #12
    Re: producing ethanol....

    Besides that, I noticed that Ethene + Water also can form Ethanol but 3 requirements is needed.
    1. temperature 300°C
    2. use H₃PO₄ as catalyst
    3. pressure 65 atm
     
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