Sapling #18

[Zach Richardson 2f]

Can someone explain to me why an odd number of carbons will cause the end hydrogen atoms to be on perpendicular planes? I understand that it has something to do with the pi-bonds, but why exactly?

[IreneSeo3F]

According to the solution, the pi bonds in C=C are perpendicular to one another in H2CCCH2 molecules, while pi bonds in terminal C=C bonds are parallel to one another in even number of C molecules. Thus, H atoms are coplanar when there are even number of C atoms.

[Isaac Wen]

If the pi bonds between the first two central carbon atoms are connected by two p orbitals in the x orientation, the pi bonds between the second and third central carbon atoms have to be connected by two p orbitals in either the y or z orientation. That is what accounts for the perpendicular plane. Subsequently, the hydrogen atoms connected to the third (or odd #) carbon atom won't be in the same plane as the hydrogen atoms connected to the first carbon attom.