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H3CHCHC=NH Resonance in Sappling

  1. Aug 29, 2015 #1
    1. The problem statement, all variables and given/known data
    This is a problem with Sappling involved. I was given a Lewis Structure, told it was an anion (no charges were given just that it was one) and told to fix the structure and draw a resonance.

    2. Relevant equations
    No Equations.

    3. The attempt at a solution
    The problem starts out with the basic structure, there is a double bond between the carbon and the nitrogen, no charges and no lone pairs on the structure.

    I counted up the number of valance electrons for this structure and got 23e-, since it's an anion, I added 1 extra electron (note they did not say the charge on the anion just that it was one). Now I have a total of 24 valence electrons in the molecule. The bonds take up 20 e- and that leaves 4 e- left over. I distributed a pair to the Carbon with 3 bonds (the first CH, or Carbon-2) and to the Nitrogen. This makes the Nitrogen neutral and gives the 2 carbon a -1 charge and the entire molecule has a -1 charge, an anion. But this is incorrect according to Sappling and I can't figure out another way to arrange the electrons so that the molecule makes sense.

    As for the Resonance I pushed the electrons I had placed on the first model into different positions, putting a double bond between carbons 2 and 3 and a single bond between carbon 3 and the nitrogen. Also I gave the nitrogen 2 sets of lone pairs and a -1 charge. This resonance is correct according to sappling, but the initial model I amended is not.

    I know it's not recommended to post a picture on here, but I'm drawing models and I don't see any other way to accurately describe what I have done, than to post a picture of my drawing so here it is:
    http://imgur.com/6U0x4CI
     
  2. jcsd
  3. Sep 1, 2015 #2
    The 'original' drawing is missing a hydrogen atom, of course, which you added later.
    "Resonance" is the character of having TWO significant resonance forms - forms in which the atoms are in the same place, but the charges are not. The actual structure has the charge distribution as a mixture of the two forms with their localized charges.
    The major (most stable) resonance form is the one you show as "resonance" in your drawing; a less stable resonance form is the one labeled "my changes".
    The proper way to show this (my opinion as practicing chemist) is to write the two forms with a double-headed arrow between them, showing that both contribute. In this case, most of the charge will be on the Nitrogen atom, much less on the Carbon atom
     
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