Question on Lewis structures

  1. 1. The problem statement, all variables and given/known data
    We need to draw the Lewis structure for certain compounds, such as C2H3Cl.

    2. Relevant equations
    None. I know how to draw Lewis structures, but I have a question: How do we know how to distribute the available valence electrons?

    For example, in drawing C2H3Cl, the right way to do it would be to have 2 H bonded with a C, this C would have a double bond with the other C, who would be bonded with the remaining H as well as the Cl, who would have 3 lone pairs of electrons.

    But how do we know this is the right arrangement? Why no draw it with all H bonded with one C, both C linked through a single bond, and C and Cl linked through a double or triple bond (double: C would have a lone pair of electrons, triple: C would no have a lone pair of electrons).

    3. The attempt at a solution
    See 2.

    Does it have something to do with the fact that it looks more symmetrical when you have both C in the middle with a double bond, and 2 atoms on each side with a single bond?

    Thank you for your help, I hope I was clear enough.
     
  2. jcsd
  3. GCT

    GCT 1,769
    Science Advisor
    Homework Helper

    Any person who had taken organic chemistry should know that C2H3Cl is an organic halogen, Chloro-ethene.

    An alternative method to hypothesizing the structure.......

    8(C)+3(H)+7(Cl)=18 electrons

    H-----Cl
    \ ---/
    C=C
    /--- \
    H---- H

    If you had a single Carbon-Carbon bond, you would need a lone pair on the Carbon, the electron count then would not equal 18 for the compound (correct me if I'm wrong).

    There is no rule for determining the Lewis structures from the chemical formula, this particular method of describing the structure of a compound requires strategizing, as with most of the concepts in inorganic chemistry.

    Your textbook should list several factors which can guide you to choose an "adequate" structure over another; most of them relate to the relative thermodynamic free energy of the proposed structures.

    Remember, always consider the octet principle, and be sure to distinguish which elements can more then 8 electrons around it; Carbon should have 8 electrons around it in most cases, it doesn't have any d orbitals to contain additional valence electrons.
     
    Last edited: Jan 15, 2007
  4. Well, first of all, Chlorine and all halogens make one bond just like hydrogen. End of story (at least as far as you're concerned.) So you're not going to see a Cl double or triple bond.

    Carbon makes 4 bonds. It doesn't like lone pairs (carbocations are the exceptions but are generally very unstable) so don't think of that as an option unless the problem states you're drawing a carbocation.
     
  5. It's all clear now!

    Thank you! (I have taken 3 days of Organic Chemistry so far, so I am sorry if I ask stupid question. As for Gen Chem I, that was some time ago).

    I found that going over things before the teacher, asking questions on this forum, and then hearing the teacher talk about them makes it a lot easier to learn & understand.
     
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