Bond angle comparison in sp2 hybridised carbon atoms

[D_Tr]

1. Homework Statement

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. Homework Equations

None..

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 :)

 

[nate808]

Thank you for bringing this question to my attention. It is always important to critically evaluate our answers and seek out evidence to support our claims.

In this case, you are correct in your reasoning that the repulsion between the larger electron cloud of the fluorine atoms would result in a larger bond angle in F-C-F compared to H-C-H. However, as you have discovered, the experimental data shows that the bond angle in H-C-H is actually greater than in F-C-F.

This can be explained by the presence of the double bond in both molecules. The double bond in ethene and 1,1,2,2-tetrafluoroethene causes the atoms on either side of the bond to be locked in a specific position, resulting in a decrease in bond angle compared to a single bond. In ethene, the double bond is shorter than a single bond, causing a greater decrease in bond angle compared to 1,1,2,2-tetrafluoroethene where the double bond is longer.

Additionally, the electronegativity of the fluorine atoms in 1,1,2,2-tetrafluoroethene also plays a role. The greater electronegativity of fluorine compared to hydrogen results in stronger bonding between the fluorine atoms, causing a slight decrease in bond angle.

In conclusion, while your reasoning was sound, the presence of the double bond and electronegativity differences in these molecules ultimately determine the bond angle. I hope this clarifies the issue for you. Keep up the critical thinking and curiosity in your studies!