nanoCarbon Is this a Molecule? Frontiers between Molecules and Solids

Is this a Molecule ? Could that weird array of hexagons possibly represent a real molecule?. Could it be a tiny chunk of a solid ? . ... Maybe ... . In Nature there are many molecules that scientist represent with hexagons.

Benzene is the simplest one. Just one hexagonal array of 6 Carbon atoms, each bonded to two C neighbors and to one Hydrogen. The german chemist Friedrich August Kekule was the first one to realize its hexagonal structure. The following drawings are three different ways to represent a molecule of benzene:

If we take two of these hexagonal rings and fuse them together we get a new molecule: Naphthalene
Benzene is a liquid, but naphthalene is solid. It is the white flaky compound used in mothballs. Here's three ways to represent a molecule of naphthalene:

You can see how fusing two rings like this makes two carbon atoms to be shared by the two rings. These "inner" C atoms have no Hydrogen bonded to them.

And there are many other larger molecules built with hexagonal arrays of carbons. Look at this series of linearly fused rings. All of them represent REAL molecules

Benzene
Naphthalene
Anthracene
Naphthacene
Pentacene

And the series would continue ...

Probably you don't know the name of the following member of this series, a molecule with 6 fused rings.
It's all right, I don't know it either. But I bet you could guess its formula !. Could you ?
(Find the answer at the end of this page)

Well, that is not bad. Now, how about putting rings together not only in one dimension to make chains but in two dimensions ?. We might get molecules like the ones below. Again, they ARE ALL REAL molecules

Things are getting crowded now. The more rings we condense together the less hydrogen is there in relation to carbon. Check this out !, our last molecule, C₂₄H₁₂ has just half the number of H atoms than C atoms. The ring of benzene had 6C and 6H.

How far can we get with this trick ?.

Well, there is one natural product, probably not too far from you right now, which is made of just carbon atoms arranged in hexagonal patterns. It's graphite and you can find it in the core of your pencils.

A tiny grain of graphite is made of billions of carbon atoms arranged in layers like the one shown above. The layers in turn are stacked on top of each other but are not bonded together, which gives graphite its soft character.

By going from benzene (liquid) to naphthalene (soluble white solid) to graphite (insoluble black solid) we have come a long way. Graphite is an extended solid with properties radically different from those of molecular solids like naphthalene of anthracene.
For instance, graphite is an electrical conductor: electrons can go through that material by jumping from one C atom to another. Molecular solids, made of billions of single molecules like the ones shown before, are insulators. Benzene or naphthalene are compounds with single bonded units (molecules) a few atoms in size ( 10^-10 meters, Angstroms); graphite bonded units extend over much wider dimensions (10^-6 meters, microns). We jumped from one to the other very quickly, but... what is in between ?.

The nanometric dimension.

    Are there molecules with say 100 C atoms ? or 1000?.
    How small can we break a tiny particle of graphite?.
    Could we get a 100 000 atoms chunk?. Could we isolate a single layer ?
    How could we make a linear polymer of fused rings of formula C₄₀₀₀₂H₂₀₀₀₄ ?
    Would that polymer be an electronic conductor ?

All those are open questions.

The fact is that we are just beginning to appreciate the importance of molecules, compounds or solids of nanometric dimensions. The discovery of fullerenes is just one example of startling new discoveries in this direction (see the related story in Hot Molecules: Fullerenes). After all, scientists have only been playing with all the fancy compounds now found in Organic Chemistry textbooks for just five generations. There is still plenty to be done and plenty to be found... in the nanometric dimension.

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The sixth member of the series would be C₂₆H₁₆ .
In general, the n^th member of this linear series, with n rings, would be C_4n+2H_2n+4