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Lewis structure question

  1. Jan 14, 2017 #1
    Hi,

    For school, I have to give the Lewis structure of C3H5+ ion.

    There is a method explained, which is fairly easy, and has always worked. It has also worked for ions. However, this is the first time I have to give the Lewis structure of a positive ion.


    The valence of the C atom is 4 electrons, and 1 electron for the H atom. In order to meet the octet rule, the C atom needs 8 electrons, and the H atom needs 2 electrons. According to the method that is given, the amount of pairing electrons can be calculated by substracting the amount of valence electrons from the amount of electrons that are needed to meet the octet rule.

    I have learned to add electrons for a negatively charged ion, so I concluded I will have to substract electrons in a positive ion. So here I will have to substract 1 electron from the amount of valence electrons, due to the positive charge of 1+. Correct?

    So I thought:

    Valence electrons = 3C + 5H - 1 = 3 * 4 + 5 * 1 - 1 = 16 electrons
    Electrons to meet octet rule = 3 * 8 + 5 * 2 = 34 electrons

    So the ion is 18 electrons short. I have been told that these electrons will form bonds. So there are 18/2 = 9 electron bonds, so there should be a total of 9 bonds between all the atoms.

    Then I looked up the lewis structure for this ion, and there are a total of 8 bonds in the ion, and not 9. Where have I gone wrong here?

    This method has always worked so far...
     
  2. jcsd
  3. Jan 14, 2017 #2

    TeethWhitener

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    This compound does not obey the octet rule.
     
  4. Jan 15, 2017 #3
    Well, of course, it's not a noble gas. Maybe I have I understood it wrong, but as far as I understood it, the electrons required to obey the octet rule, will be used to form bonds. In this case, I calculated 18 electrons would be required to obey the octet rule, which are 9 pairs, so assumably 9 bonds in the total molecule. But there aren't 9 bonds between the atoms, there are only 8. This is what I do not understand, especially looking at the fact that this method of determining the Lewis structure has always worked for me. I double checked the valence for all electrons, checked my calculations several times, but cannot find where I went wrong.
     
  5. Jan 15, 2017 #4

    TeethWhitener

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    As I said, this compound (like most carbocations) does not obey the octet rule. One of your carbons will be a sextet. That means you can't use the octet rule to determine the number of bonds.
     
  6. Jan 15, 2017 #5
    Sextet? A hexavalent carbon? Unheard of!
    C3H5+ is the propene carbonium ion. It is formed (briefly) because it has two resonance forms, delocalizing the positive charge across the molecule.
    1. Simplify your Lewis method: 3C x 4 v.e. = 12
    count your v.e.'s 5H x 1 v.e. = 5
    positive charge so subtract 1 electron -`1
    16 valence electrons to work with = 8 electron pairs
    2. Using electron pairs, first, connect the atoms, then disperse the remaining electrons so that the octet rule is obeyed (if possible); use double and/or triple bonds as needed. You can have lone pairs of electrons on O, S, N, P etc.
    H2C=CH-CH2 (I cannot make this program format the molecules properly; there should be a p
    ..................⊕ positve charge on the right side carbon atom.)
    if there is a charge, place it on the least electronegative atom - in this case carbon, for the reason just mentioned, and because two resonance

    forms are possible, stabilizing the molecule:
    H2C-CH=CH2 ↔ H2C=CH-CH2
    .....⊕.................................⊕
    these two ⊕ should be spread between the two ends. Get an organic chem book and look up RESONANCE
     
    Last edited: Jan 15, 2017
  7. Jan 15, 2017 #6

    TeethWhitener

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    1. I should clarify: Sextet refers to the fact that the Lewis structure for the compound gives an electron count of six around the carbon with the formal positive charge. It does not refer to the number of bonds to carbon (just as a full octet of electrons around carbon doesn't imply octavalent carbon).

    2. Also, NB -- C3H5+ can be an allyl cation or a cyclopropyl cation.

    3. Just to be really pedantic, carbon can in fact be hexavalent: consider the carboranes, or the FeMoCo core of nitrogenase.

    Edit: to be really really pedantic, I should say that carbon is hexacoordinate in these situations, not necessarily hexavalent.
     
    Last edited: Jan 15, 2017
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