Carbon 10/10/21

Science with Richard Bleil

Carbon is a fascinating element.  Previously I know I’ve written on the difference between organic (carbon containing) compounds and inorganic compounds (everything else), and yet, even in its elemental state, carbon is very interesting. 

You know carbon.  You’ve seen it and touched it.  There are two major allotropes of carbon.  I guess, before we move on we should discuss what an “allotrope” is.  By Dalton’s atomic theory, a compound is created by the combination of two or more different elements in fixed whole-number ratios, like carbon monoxide (carbon and oxygen in a 1:1 ratio) and carbon dioxide (carbon and oxygen in a 1:2 ratio).  Change either the elements (like carbon disulfide, carbon and sulfur in a 1:2 ratio) or the ratios (as shown above) and you have a very different compound.  But what if you have just one element, like oxygen, but bonded differently?  Oxygen is typically diatomic (two oxygen atoms) and ozone (three oxygen atoms).  The number of atoms bonded together varies, but these are not compounds, because both are made from just oxygen, not two (or more) different elements.  These are not compounds, and yet this bonding makes them behave very differently.  We depend on diatomic oxygen to survive, but triatomic oxygen is actually toxic in great amounts.  You’ve smelled ozone; it’s that “whiff” you get when there is an electrical spark, and the clean smelling air during an electrical storm, but in large cities, it’s also a major component of smog that is choking and can be very dangerous.  Because these both contain just oxygen, they are just the element oxygen.  Because they are different forms of the element, but bonded differently, we call them “allotropes”.

There are two major allotropes of carbon with which you are no doubt familiar, namely graphite and diamond.  There are actually other forms as well which I’ll touch on in a bit.  On an atomic level, diamond has each carbon bonded (with a single bond) to four surrounding carbons in a three-sided pyramidal shape that we call “tetrahedral”, while in graphite each carbon is bonded to three surrounding carbons (with a bond order of approximately one and a half) creating a hexagonal shape.  In diamond, the tetrahedral crystal lattice extends in all three directions with no real limit on its size, while with graphite the structure extends only in two directions (again without limit) creating basically a flat sheet of carbon.  The diamond grows to be as large as any diamond that you’ve seen and is the hardest substance on earth.  It is used in drill bits and cuts glass with ease, while sheets of graphite layer along the z-axis (assuming the sheets are oriented in the xy plane).  The attraction for these sheets is quite weak making it very easy to rub layers off.  We see this in graphite pencils when we use it to leave black streaks on paper, and it is why graphite is actually a very good lubricant.

Imagine that.  One element that is both the hardest material on earth and a great lubricant, and it’s not even a compound.

As charcoal, a form of graphite, it’s a highly porous material.  It’s often cleaned (chemically to remove impurities such as sulfur and nitrogen) and ground into a very fine powder and used by chemists as a cleaning step.  It’s called a “decolorizing agent”, an odd name since it’s very black, but what happens is that the pours are a variety of sizes and tend to trap chemicals that cause a chemists’ product in an organic reaction to be discolored (usually yellowish).  By adding this decolorizing agent, it traps those chemicals causing the discoloration (which are impurities, after all).  After filtering out the charcoal, the product is often much cleaner.

A relatively newly discovered form of carbon are referred to as “buckey-balls”.  These are three-dimensional carbon clusters, the earliest of which that were discovered were shaped kind of like soccer balls.  Having been structurally identified in the UK, this carbon class was referred to as “Buckminsterfullerenes”, after Buckminster and the favorite team of the scientists who discovered them.  These spheres can have clusters as small as thirteen or pretty much as large as you like.  In fact, one of these clusters (I believe the sixty-carbon sphere) is just the right size to get trapped in the virus that is responsible for HIV.  I know it has been studied as a treatment for this terrible disease, but I’m not sure of the current status.  One of my (relatively few) publications in chemistry ( https://www.sciencedirect.com/science/article/abs/pii/0009261494010838 ), in fact, was on the 13 carbon cluster based on computer modeling I did while I was a visiting scientist at Harvard (how’s THAT for a name drop?).  The debate was whether the C-13 structure was a line of carbons, or a ring.  My studies suggested it was temperature dependent, with higher temperatures favoring the straight-chain form.

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