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Homework Help: Bond angle comparison in sp2 hybridised carbon atoms

  1. Feb 3, 2012 #1
    1. The problem statement, all variables and given/known data

    This was a question on an exam I took a week ago in inorganic chemistry. The problem statement was like: "We have two molecules, H2CCH2 (ethene) and F2CCF2 (1,1,2,2-tetrafluoroethene). Which bond angle is greater? H-C-H in ethene or F-C-F in 1,1,2,2-tetrafluoroethene?"

    2. Relevant equations


    3. The attempt at a solution

    I was quick to answer that the bond angle will be greater in F-C-F. There is a double C=C bond in both molecules which would tend to cause both angles to be slightly less than 120 degrees. However, since the F atoms have a larger electron cloud, the repulsion etween the two F atoms will be greater than the repulsion between two H atoms, so the F-C-F angle will be greater than the H-C-H angle. The problem is, I searched for experimental data and found that the opposite is the case.. Wikipedia says 117.4 degrees for H-C-H and here http://cccbdb.nist.gov/exp2.asp?casno=116143 the experimental value for F-C-F is 112.4 degrees. So does it turn out that I am wrong? Why? Do the double bond electrons come into play?

    Thanks for your time people :)
  2. jcsd
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