The volume of a cubane molecule!?!?

I was just wondering how anyone would even go about aprroaching this question. My chemistry teacher told me to think about it because it's the type of thing they ask at interviews.

Essentially cubane is a cube made up of 8 hydrogen atoms with 8 hydrogen atoms coming off each corner of the carbon cube.

So the question is calculate (basically an estimate) the internal volume of the cube, and then would a cyclo-octane molecule fit inside?

Atoms are essentially space filling when bonded together. You need the bond lengths of the cubane molecule, which I imagine willl be similar to the bond lengths in cyclobutane.

Use these and the atomic radii (covalent) to work out the geometric dimensions.

There is no chance of cyclooctane fitting inside!

Ah thank you

So there would be no way of a rough estimate of working out the bond lengths of the C-C bonds without doing it experimentally?

Atoms are essentially space filling when bonded together. You need the bond lengths of the cubane molecule, which I imagine willl be similar to the bond lengths in cyclobutane.

Use these and the atomic radii (covalent) to work out the geometric dimensions.

There is no chance of cyclooctane fitting inside!

So the question is calculate (basically an estimate) the internal volume of the cube, and then would a cyclo-octane molecule fit inside?

X

Thanks for your great reply. Could you show me how you would answer it in an interview situation. So for example would you go about the question by saying if we assume that the bond lengths of the c-c bonds are 200nm (for example) and that the carbon atoms are infinitely small dots, this would give us an internal volume of (200^3)nm^3, would this be a viable way to approach it bearing in mind it's an A-level answer?

Also being that you're an Oxford Chemist Could I just ask you why Brady's reagent produces a precipitate when reacted with ketones and aldehydes but not with carboxylic acids or esters. I know it's something to do with electron density but no one can seem to answer me and I can't find a simple clear answer. Thanks alot for your help

X

Sorry for all the questions but why does the oxygen get a negative charge after it's donated a lone pair of electrons? In your diagram the oxygen goes from being double bonded to singly bonded, so does that mean it's lost a bond pair?

Sorry for all the questions but why does the oxygen get a negative charge after it's donated a lone pair of electrons? In your diagram the oxygen goes from being double bonded to singly bonded, so does that mean it's lost a bond pair?

With regards to cubane, could you just confirm if I was going about it the right way in the example of using the average radii?

I was just wondering how anyone would even go about aprroaching this question. My chemistry teacher told me to think about it because it's the type of thing they ask at interviews.

Essentially cubane is a cube made up of 8 hydrogen atoms with 8 hydrogen atoms coming off each corner of the carbon cube.

So the question is calculate (basically an estimate) the internal volume of the cube, and then would a cyclo-octane molecule fit inside?

X

X

Thats the genius of the written language, you can look it up! Not a simple technique to get bond lengths, you can get very rough estimates from techniques like IR, but that requires a calibration curve formed from similar species.
The most accurate techniques are Neutron and Electron diffraction techniques. As far as I know the only place in the UK where this is possible is at the Diamond light source in Oxfordshire. (So a very, well not necessarily expensive, but rarely used technique as you need to book a time with this lab waaaayyy in advance.)

Your question is merely an exercise in geometry! Should be a more comfortable field for pre-uni level chemists. It does highlight the different bond lengths, throwing out any pre-conceptions that all C-C bonds are equal, but the question, is essentially does an octagon fit in a cube. (If we boil it down to the absolute bare bones)

To take this question to a more advanced level you can then consider the fact that the carbons and H's are not infinitely small dots but occupy a significant space. If we take the radius of the carbon atom to be it's covalent radius, then for cubane we have 4 primitive cubic packed carbons. This has a packing efficiency of ~52%, so even if cyclooctane would fit in a cubane formed from infinitely small carbons, it won't fit in real cubane as the available space is actually much smaller than that of a cube of the same dimensions.

You could advance this even further by considering the actual conformation of cyclooctane. Cyclooctane is not an Octahedron! Check out the major conformers on the wiki page http://en.wikipedia.org/wiki/Cyclooctane
This makes the geometry of the problem more difficult as the cyclooctane is no longer a regular polyhedron but can probably be modeled quite reasonably as a sphere of radius equal to the dimensions of the conformer of cyclooctane.




Yes the bond lengths are very similar to cyclobutane!

X-ray crystallography might also be an option, cubane is a solid at SATP.

Is this because cubane has such an unusual shape? I didn't know X-ray crystallography would work with hydrocarbons.

Is this because cubane has such an unusual shape? I didn't know X-ray crystallography would work with hydrocarbons.