by Preston MacDougall
June 22, 2005
The shiny, dark grey surface of crystalline graphite confused early chemists, who thought it was a lighter form of lead. Lead is in the same column of the periodic table as carbon, but it is four rows down, in the band of heavy metal toxic elements, with mercury and thallium. There never has been any lead in pencils.
The 18th century Swedish chemist, Carl Scheele, reacted both diamond and graphite with pure oxygen, which he called "fire air", essentially burning them, but under very controlled circumstances. So doing, he observed production of equivalent amounts of the same gas in both cases.
Scheele and his contemporaries were a long way from assigning formulas to chemical compounds (the gas produced was carbon dioxide), but he knew that he had unequivocally demonstrated that graphite and diamond have the same make-up - they were both elementally carbon.
Although the process is geologically slow, the conversion of diamond, the hardest substance known to man, into graphite, which smudges your fingers when you rub it, proceeds naturally. Naturally, that is, as long as the temperature and pressure do not reach the ultrahigh levels found deep beneath the Earth's crust, where the relative stabilities are reversed, and bling results.
It is hard to imagine two solids that are more different than diamond and graphite, yet, when pure, each is nothing but trillions upon trillions of inter-connected carbon atoms. There are actually bound to be some impurities, but far from erasing their distinction, in diamonds these add subtle color, and in graphite the metallic character is increased. So what makes them different in the first place?
The answer lies in how the atoms are connected to one another. This, in essence, is the molecular structure hypothesis - a 19th century idea so daring that chemists who were quick to believe in it were thought to be "tilting at windmills."
Consider the following appraisal, from a leading organic chemist in Germany, Hermann Kolbe, who was the editor of the Journal for Practical Chemistry at the time: "the fancy trifles in it are totally devoid of any factual reality and are completely incomprehensible to any clear-minded researcher."
This quotation, as well as the particularly salacious analogies that bracket it, are well-known to molecular researchers, as they appear in numerous organic chemistry textbooks. And thanks to Google, you no longer have to be a chemistry major to know just how vitriolic scientific criticism can be.
The idea being attacked was the proposition that the chemical and physical properties of organic compounds, such as whether they react with hydrogen gas to produce single or multiple products, are imparted not just by the atomic constituency of the compound, but also by how the atoms are arranged in space. The concept of molecular structure was born in a twelve-page pamphlet that accompanied the doctoral dissertation of Jacobus van't Hoff, which he defended in 1874.
The dissertation itself was a conventional one, discussing the chemical properties of naturally occurring organic acids. His revolutionary proposal was kept separate, and published later, presumably as a precaution against possible attack, which could have resulted in denial of his doctoral degree. Smart move.
I should add that a Frenchman, Joseph Le Bel, independently proposed a tetrahedral arrangement of atoms bonded to carbon in certain organic compounds. But it was van't Hoff's work that proved more influential, and, as already recounted, drew more scorn. Van't Hoff was later rewarded, in 1901, with the very first Nobel Prize in Chemistry.
The transformation of organic chemistry, from an art into a science, spanned much of the 19th century, and the molecular structure hypothesis can be seen as a sort of climax. This history is engagingly told, for a general audience, in John Buckingham's recent book "Chasing the Molecule." Drama, serendipity, betrayal, self-doubt, redemption, it's all there.
Recurring publicity and the
bawdy nature of Kolbe's attack are not the only reasons it so
widely recalled by chemists. I believe that they stoke the rebel
that lies within anyone whose life ambition is to master the
atomic domain, conquering molecular challenges that have withstood
the talents of generations of scientists.