# What should be the shape around each carbon atom?

Cubane is a hydrocarbon with the formula, C8H8. It has a cubic shape,
as its name implies, with a carbon atom at each corner of the cube. This molecule
is very unstable and some researchers have been seriously injured when crystals
of the compound exploded while being scooped out of a bottle. Not surprisingly,
it has some uses as an explosive.
(a) According to VESPR theory, what should be the shape around each carbon
atom? Why?
(b) If we assume a ideal cubic shape, what would be the bond angles around the carbon?
(c) Explain how your answer to (a) and (b) suggest why this molecule is so unstable.

I don't know what the shape would be for 8 carbon, I only know up to 2 carbons.

dextercioby
Homework Helper
Can u make a drawing...?That is,can u picture the bonds (how they would be ideally)...?What hybridization type does this structure correspond to...?

Daniel.

anti_crank
a) Try to come up with the structure of this molecule. Determine to how many other atoms each carbon atom binds, and from that, infer what the geometry should be according to the Vespr model.

b) This is extremely simple to answer. You assume an ideal cubic shape: what are the angles between the edges of a cube? .

ShawnD
Roxy said:
It has a cubic shape, as its name implies, with a carbon atom at each corner of the cube.
(a) According to VESPR theory, what should be the shape around each carbon
atom? Why?
(b) If we assume a ideal cubic shape, what would be the bond angles around the carbon?
(c) Explain how your answer to (a) and (b) suggest why this molecule is so unstable.

a.
Try to visualize this thing. You have a cube; that's basically just 2 squares with space betweem them. Each carbon is bound to another carbon on each axis, so that leaves 1 hydrogen per carbon. That would give you the C8H8 formula you are looking for, so just assume it's the correct structure and that each carbon is $sp^3$ hybridized. What's the shape of $sp^3$? Tetrahedral if I'm not mistaken.

b.
If it's like any cube I've seen, the angles between the carbon bonds would be 90 degrees. As for the angle between the carbon bonds and the hydrogen bonds, I don't really know what that would be; my background in math is sketchy at best. I think I'll start a math thread asking that question.

c.
It will be unstable because the bonds are really bent out of shape. A tetrahedron should have 109.5 degrees between the bonds. This thing likely has 90 degrees between the bonds. You can only pull on something so hard before it snaps.