The idle ramblings of a Jack of some trades, Master of none

Feb 6, 2010

Cyanate or Fulminate

By the early 19th century, chemists were in a position to create several compounds on demand, compounds that hadn’t previously existed in the world, compounds that could be synthesised using mixtures of reagents in certain quantities. Such materials as steel and synthetic dyes had been produced largely by trial and error or happy accident. But scientists’ understanding of exactly how the basic elements combined and could be controlled was still hazy.

In a bid to master the elements, one German chemist, Justus von Liebig, became obsessed with creating explosive combinations. His passion was sparked as a child in Darmstadt when he saw a peddlar letting off fireworks. (The story goes that was apprenticed to a chemist as a youth, and was thrown out of his job when he concocted a massive explosion one day. The more prosaic truth appears to be that his father could no longer afford the fees of the apprenticeship and withdrew him). The fireworks were powered by fulminate of silver, the same chemical that is found today in bangers. Liebig had found his vocation.

But it was as much his personality as his love for explosives that powered his great breakthrough. It was said that he was arrogant, irascible, pugnacious, and pigheaded. Not a man to cross, you might think. So when another German chemist, Friedrich Wöhler, got an angry letter from Liebig in 1825, you can imagine Wöhler’s heart sank. Liebig had read a paper written by Wöhler about a compound he had made, called silver cyanate. This was the formula of silver cyanate: AgCNO, made of equal parts of silver (Ag), carbon (C), nitrogen (N), and oxygen (O). Wöhler described it as harmless and stable. Liebig saw silver, carbon, nitrogen and oxygen, and exploded, because this was exactly the composition of his own silver fulminate.

How could two substances that were made from exactly the same amounts of the same elements behave so differently? True to character, Liebig decided that there was only one answer, and that was that Wöhler was wrong. He dashed off a furious letter to Wöhler, slamming him as a hopeless analyst.

Well, Wöhler wasn’t having any of that. He challenged Liebig to make silver cyanate and test it for himself.

At the time, conventional wisdom held that the only thing that counted was what was in your material, what its composition was. When Liebig attempted to mix silver, carbon, nitrogen and oxygen in the proportions that Wöhler had indicated, he fully expected to recreate silver fulminate. Instead, he was shocked to find that he ended up with a harmless white powder – just as Wöhler had.

Liebig and Wöhler had discovered a fundamental characteristic of the elements. They had stumbled on what would later come to be known as ‘isomerism’ – what made their compounds different was the way in which the elements were connected. The study of this led to a fundamental reappraisal of chemistry, eventually involving the physics of the atom.

So how did the elements combine to create silver fulminate? The fulminate radical looked, in effect, like this:


The silver (Ag) bonded with the oxygen end of the radical to create silver fulminate: Ag+ ONC-

But the cyanate radical looked like this:


And the silver combined again with the oxygen end: Ag+ OCN-

These came to be known as 'structural isomers', and an explanation for this dichotomy came from Wöhler's teacher, Jöns Jakob Berzelius. But that is another story.

An aside: despite the combative first encounter between Wöhler and Liebig, they became very close life-long friends, and collaborated heavily in their ensuing chemical research. Who said rivals can’t be buddies?

[Most of the text comes from Jim Al-Khalili’s recent programme on BBC4: Chemistry – A Volatile History]


Nikhil Narayanan said...

Thanks for this bit.

Fëanor said...

You're welcome, old chap.

km said...

Fantastic. I didn't know this was the origin of "isomerism"

Space Bar said...

damn it. i also want to watch this series. ( can only explain my fascination with chemistry - undeterred by any real understanding of it - because my grandfather, a chemist himself, made such a bloody romance of it).

Fëanor said...

SB, do you like to blow things up? That's the main romance of chemistry, no?

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