Valence Bond Theory

[Seiryuu]

So today I learned about sigma and pi bonds in my chemistry lecture today. As I'm doing some of the exercises, I'm stuck in the bonding of allene ([ce]C3H4[/ce]). Is it possible to have orbitals share pi bonds? If I'm doing this correctly, there should still be p orbitals on all carbons after the sp2 orbitals that don't mix with any other orbitals.

[hypervalent_iodine]

What you have to remember is that each carbon centre has 3 available p orbitals; one each on the x, y and z axis. You should recall that an alkyne bond utilises two of these p orbitals, let's say the p_x and p_z orbital, to form two pi bonds to the corresponding p_x and p_z orbital of the adjacent carbon, and one sigma bond along the y-axis, giving us a structure that looks like this:

 

 

(Image from here)

 

The pi bonding in allene is more or less the same concept and looks like this:

 

 

 

(Image from here)

 

You can see from this that the central carbon does still have two pi bonds, each using a different p-orbital, just the same as it would were it triply bonded to a single carbon in an alkyne.

 

Interestingly (and somewhat irrelevant), the structure that allene is forced to adopt, which is non-planar (the hydrogens on one end are in the plane, where the two on the opposite end stick out of the plane), means that allenes can actually be chiral, despite not having a stereogenic centre. If you imagine that you had four different substituents attached to the terminal ends of the molecule, you can perhaps see that the mirror image of the new compound will be non-superimposable on itself, which is the definition of a chiral molecule. In fact, it doesn't even require all four substituents to be different; so long as the two substituents on either end differ from one another, it is chiral.

 

Hope that clears things up.

Edited November 17, 2011 by hypervalent_iodine

[Seiryuu]

I think I understand now. So pi bonds along the same plane of two atoms will only form a pi bond between them and not any other ones? And this is why allene has a sort of "twist" in its shape; so that the carbons on the ends don't have p orbitals along the same plane?