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HOME > PAST ISSUE > July-August 2002 > Article Detail

MARGINALIA

Carbides

Roald Hoffmann

Ask an American chemistry graduate student, "Tell me what you know about carbides," and here's what I guess you'd get as an answer: "There's calcium carbide, CaC2, and I've heard of those long chains of carbon in John Gladysz's organometallic molecules, and, yes, some transition metal carbide clusters, for instance the iron carbide carbonyl Fe5C(CO)15 [Figure 1]."

The realities: Calcium carbide, CaC2, remains a major commercial chemical, though I suspect that few connect that molecule to the name of the one-time chemical giant, Union Carbide Corporation, now a subsidiary of Dow; WC—no, not the loo but tungsten carbide—in drilling bits and in "cemented" carbides is a major industry. Looming above these is steel, of weapons and at the core of large constructions. Steel is hardly uncommon, but I'd bet there is nary a word about this alloy of iron and carbon in the education of our students. Steel is a solid solution of carbon in iron mixed with compounds such as iron carbide, Fe3C, and Hägg's carbide, Fe5C2. Besides these three major players—CaC2, WC and the carbides in steel—many other metal carbides have been made, more for curiosity than for profit.

Figure 1. This carbide made of iron, carbon and oxygen . . .Click to Enlarge Image

The lacunae in the student's answer—and professors will do no better—tell us much about education and fashions in science. I see in the answer (a) the triumph of molecular chemistry in the past century, so, for example, more people know the later and rarer discrete Fe5C(CO)15 cluster than the extended Fe3C structure, and (b) the love affair people—subspecies chemists—have with simplicity, so both teachers and students would rather deal with a simple discrete molecule rather than the difficult to explain lanthanum nickel carbide, La2Ni5C3, or, God forbid, messy steel.




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