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This recent question about the gauche effect led me to read this rather older answer. The problem is that I notice I am still confused.

To take the case of 1,2 difluoroethane, I drew these orbitals: orbitals of C2H4F2 in the gauche (1,2) and anti (3,4) confromations Figure1: orbitals of $\ce{C2H4F2}$ in the gauche (1,2) and anti (3,4) confromations

Now I understand that 1 (gauche) leads to more stabilization than 3 (anti). But 2 (gauche) leads to less stabilization than 4 (anti). I don't know which effect is strongest: the destabilization in 2 or the stabilisation in 1. So I don't know why the gauche conformation (1,2) leads to more stabilization than the anti (3,4).

Did I draw something wong? Did I make a mistake in reasoning? Did I miss something else? Is this one too many error-acknowledgement questions?

Community
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SCH
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  • The most important interaction in depicted in image 1. The energetic match of these oribtals is the best from all the listed. – EJC Aug 04 '16 at 15:41
  • Yes, but how do I know that it's enough to compensate for the loss of stabilization in 2 vs. 4? – SCH Aug 04 '16 at 15:55

1 Answers1

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In figures 2, it looks like you're saying the sigma of the C-F will be donating into the sigma* of the C-H, and that causes stabilization. However, a C-F sigma bond is a much poorer donor than a C-H due to the high electronegativity.

Since there is little to no donation actually occurring in 2 (or 4, for the matter) those interactions are much less relevant than depicted in 1/3 where there is significant donation for C-H sigma into C-F sigma*. Frankly I would never have even thought to draw 2/4 since there is so little electron density in the H lobe of the C-F sigma, but cool!

Bennie
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