Why are the two CH2 groups in allene perpendicular to each other?

Question

Allene, $\\ce{H2C=C=CH2}$, has two $\\ce{=CH2}$ groups perpendicular to each other. This is the same as hydrogen peroxide's $\\ce{H}$ atoms. Is there any reason why they adopt the perpendicular arrangement, when the VSEPR model allows both the perpendicular and planar arrangements for the groups on the ends? I don't think it's a steric effect, because the hydrogen atoms/methylene groups are facing opposite directions and far apart.

Answer 1

It's not a steric effect. It has to do with how the orbitals overlap in 3-D space. The best way to understand it is with a 3-D model that shows the orbitals, which you can see at this link: http://www.chemtube3d.com/orbitalsallene.htm

The reasoning requires the following logic:

1) The end carbons are sp2 hybridized, and the middle carbon is sp hybridized. In an sp2 hybridized carbon, the p orbital is perpendicular to the hybrid orbitals. An sp hybridized atom has two p orbitals, which are perpendicular to each other.

2) Pi bonds require parallel p orbital overlap.

The two rules above don't make much sense without the 3-D model though. You'll have to play around with it and click all the buttons that say "show p orbitals".