Sapling Week 7/8 #18

[Aaina 2D]

Hi! For Week 8 Sapling Number 18, why is it that the hydrogen atoms are in the same plane for H2CCH2 and H2CCCCH2 but not for H2CCCH2?

[Kyle Walsh 2J]

So as I understand it, the hydrogen atoms in H2CCCH2 are actually perpendicular, as the pi bonds from the double bonds on each side of the central C are perpendicular to each other. This is contrasted with H2CCH2, which only has one pi bond, so it has to lie in the same plane. Then for H2CCCCH2, the inner pi bond and outer pi bonds are in different planes, but since all the hydrogens are bound to outer pi bonds, they are in the same plane. Hopefully this made sense and helped a bit!

[Jayasree Peri 2J]

I'm not exactly sure if this is the correct answer, but the way I was thinking about it was that, with since H₂CCCH₂ has an odd number of Cs, the extra carbon in the middle offsets the shape of the molecule, making it so that the bonds are now essentially perpendicular (meaning the hydrogen atoms would be in perpendicular planes). This is also similar to the explanation in Sapling, but I am not completely sure if this is correct.