Friday, 26 April 2013

Chemistry Classics: Foundation of Organic Chemistry

Day five of #RealTimeChem week, and the final post in my series about classic chemistry papers. These quick reads have covered topics varying from chromatography to crystallography, focusing on ubiquitous techniques and revolutionary discoveries.

Today I want to discuss a truly foundational paper which describes a single-step synthesis. The original paper is in German, so I am using the English translation at John Park's ChemTeam website.

Friedrich Wöhler, "On the artificial production of urea", Annalen der Physik und Chemie, 1828, 88, 253-256.

This paper hardly needs introducing; pretty much every undergraduate is told that this paper represents the death of vitalism and the birth of organic chemistry. The usual account runs thus: in the early 19th century, compounds were classified as organic and inorganic based on whether they were found in living or inanimate sources respectively. It was thought that organic matter held some sort of élan vital which was absent from inorganic matter, and was therefore fundamentally different. Under this scheme, transforming inorganic to organic matter would be impossible.

Here, Wöhler synthesised urea - a component of urine and hence an organic compound - from inorganic reagents, thus disproving the notion of vitalism once and for all. Every chemist worldwide read this paper and, good logical positivists they were, immediately abandoned vitalism and thus science was born.

That's not quite how it happened. I've provided some links to relevant historical material below if you want a more accurate picture.

What does Wöhler's paper actually say? It's wonderfully brief, and he gets to the point in the first paragraph: in his previous synthesis of oxalic acid he obtained an unidentified white solid, which turns out to be urea, "a fact that is noteworthy since it furnishes an example of the artificial production of an organic, indeed a so-called animal substance, from inorganic materials". This is the only reference to vitalism in the paper.

His synthesis is simple: decomposition of lead cyanate with liquid ammonia (if you're thinking of trying this, BRSM has some chemtips for you). "It was precipitated in colourless, transparent crystals, often more than an inch long". No fancy purification for Herr Wöhler, oh no.

Pb(CNO)2 + NH3 + H2 Pb(OH)2 + NH4CNO
NH4 ⇌ NH3 + HNCO ⇌ (NH2)2CO

Identification is by comparison of reactivity with authentic samples of urea, and known properties of other possible products such as cyanates. His description of these experiments is poetic in places, quite unlike a modern SI!

He concludes with discussion of the fact that the empirical formulae of urea and hydrated ammonium cyanate are the same, pointing to the existence of isomerism - which is, perhaps, far more important to chemists than the distinction between vitalism and reductionism!

Perhaps the title of this post is inaccurate, as would any attempt to pin the beginnings of organic chemistry to a single moment. Regardless, this experiment remains crucial to the development of organic synthesis and theory.

I hope you've enjoyed this short series of chemistry classics. If you have, you can subscribe to the RSS feed using the button at the top-right of the page, or follow me on twitter.

Some links:

A procedure for repeating this synthesis.
Did the urea synthesis destroy vitalism? 1, 2.
ChemistryWorld podcast about urea

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