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Molecular Structure

  1. Feb 7, 2005 #1
    What are the rules for drawing Lewis dot diagrams, besides the

    1) Carbon usually has a full octet
    2) Some atoms become electron deficient, others electron sufficient
    3) The Duet Rule
    4) Link OH if written in the formula

    What the "other" rules?

    For example, i try to draw glucose, C6H12O6:
    *I make a hexagon of six single-bonded carbon
    *I attached a double-bond with oxygen to each carbon (carbon has 2 single bonds, and one double bond, so it obeys octet rule)
    *I finally attach two single-bonded hydrogen to each oxygen.

    Why doesn't this work? I have six carbon with octets, six double-bonded oxygen=carbon bonds (six oxygen), and two hydrogen at each oxygen

    C-C-C-C-C-C with C=O, where each oxygen then bonds with two hydrogen

    Why is this incorrect----what are the other rules for drawing lewis diagrams??
     
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  3. Feb 7, 2005 #2

    Gokul43201

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    In this case, your oxygen atoms do not have octets. That is the main problem. Typically, for oxygen to have a neutral octet, it can support only 2 bonds (either one double-bond or two single bonds), but you are trying to give it 4 each.

    In any case, expecting you to guess the structure of glucose by trial and error is quite unreasonable. It could take forever, and there's no good reason to make you try the gazillion possibilities before finding the correct one.
     
    Last edited: Feb 7, 2005
  4. Feb 7, 2005 #3
    But the oxygens do have an octet--a double bond with the carbon, and two single bonds with hydrogen (one double bond+2 single bonds gives 8 electrons, doesn't it?)

    Oxygen can have only "two" bonds for a neutral octet?

    If so, these are the kind of rules I should know about; are there more such rules?
     
    Last edited: Feb 7, 2005
  5. Feb 7, 2005 #4

    dextercioby

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    Last edited: Feb 7, 2005
  6. Feb 7, 2005 #5
    So then, the oxygens aren't the problem? Then what is?
     
  7. Feb 7, 2005 #6

    dextercioby

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    They do indeed.They have either double bond with the C or 2 simple bonds,1 with the C and one with the H.In this case...

    Daniel.

    P.S.Glucosis does not have a cyclic structure... :wink:
     
  8. Feb 7, 2005 #7

    Gokul43201

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    I know this Dexter. I was explaining to Bomba why his structure was infeasible.

    No, the oxygens are the problem. The only way to ensure an octet as well as the correct charge for oxygen is through 2 bonds and two unbonded pairs. With 4 bonds, you make an octet, but you only have 4 electrons contributed by O, whereas you need 6.
     
  9. Feb 7, 2005 #8
    Well then :smile: ....are there any other such rules I should know about?

    Just curious, what are the other general rules?
     
  10. Feb 7, 2005 #9

    Gokul43201

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    1. Know the number of valence electrons in different elements (O=6, C=4, N=5, H=1, Cl=7, etc.)
    2. After sharing electrons all atoms must have a valence octet (except H, which gets a duet)

    Really, these two are the only rules you need to know for most organic compounds.
     
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