Graphite is essentially large sheets of benzene rings, with the sheets stacked on top of each other. The fact that the sheets slide nicely across each
other gives it its slipperiness and use as a lubricant, and the fact that all the pi-electrons can be delocalized across the entire sheet gives it its
electrical conductivity. Under relatively normal conditions of heat and pressure, this is the most stable form that pure carbon can adopt. If the
pressure and temperature are extremely high, carbon can adopt a denser configuration, the adamantane (or diamond) structure, in which each carbon is
bonded to four other carbons at the points of a tetrahedron, rather than the benzene ring structure.
If you take any hydrocarbon and subject it to heat and pressure in the absence of oxygen, it will gradually adopt the graphite structure. This is
pyrolysis - oxygen gets eliminated as water, hydrogen gets eliminated either as water or methane, and so on. Typically structures such as starches,
sugars, resins, and cellulose, which give plant materials so much of their structure, are already laid out in mostly five and six membered rings, but
the hydrogens and hydroxyl groups take up a lot of space. As you compress and heat wood (excluding oxygen), you can remove all of the volatile
material and compress the remaining material into soft coal, and as the process continues and more hydrogen and oxygen are removed, the material will
get denser and denser until only the carbons remain and the material is all graphite.
I say "only the carbon", but that isn't quite true. Nitrogen and sulfur, in particular, can exist inside the graphite structure with only a small
energy cost, and most coal contains at least some nitrogen and sulfur. The amount of sulfur in the original organic material determines the amount of
sulfur in the final coal, and some coals are much sweeter than others due to the source of the original organic material. |
