March 2014

# Chance discovery favours the prepared mind

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Michael P Jansen, Crescent School, Toronto ON

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The title paraphrases1 the quotation attributed to Louis Pasteur (1822 – 1895), a French microbiologist credited with developing the germ theory of disease. No one-trick pony, Pasteur discovered stereochemistry and invented Pasteurization,2 probably in his spare time.

I heard this pearl of wisdom in an organic chemistry video produced by TV Ontario in the 1980s. The video described how Friedrich Wöhler (1800 – 1882), a German chemist, was the first to prepare an organic compound (urea) without the use of a plant or animal.3

Before moving to chemistry, Wöhler studied medicine. He spent time studying urea (H2NCONH2) obtained from urine; he knew its properties: melting point, appearance of its crystals, water solubility — and I’m guessing — given the lack of anything even remotely to do with health and safety at the time, how it tasted.

Wöhler prepared what he hoped was ammonium cyanate, NH4OCN, in the double displacement reaction:

AgOCN(aq) + NH4Cl(aq) → AgCl(s) + NH4OCN(aq)

He filtered the silver chloride, gently dried the filtrate and characterized the crystals. A lesser chemist would have assigned these properties to the newly prepared ammonium cyanate.

But not Wöhler.

He noticed that the properties of what he thought was ammonium cyanate were identical to those of urea. The upshot: Wöhler synthesized urea, an organic compound, without a kidney. He shattered the theory of vitalism;4 he connected organic and inorganic chemistry.

Aside from being a great story, the message is clear. If Wöhler hadn’t remembered the properties of urea, it may have taken years for this to be discovered.

I mention this frequently; the quotation is displayed in our lab.

As part of our study of redox chemistry in September of AP Chemistry, we examined the oxidation states of manganese in a demonstration.5

Several months later, we carried out an activity that included the addition of basic KMnO4(aq) to olive oil, which is poly-unsaturated.6 A positive test for a C–C pi bond is evidenced by the disappearance of the purple colour of the permanganate ion and the appearance of a brown precipitate of MnO2. One group of students noticed that the solution turned green before the brown precipitate appeared. Someone shouted, “Sir, we see manganese (VI)!”

oxidation state of Mn | colour | compound/ion observed | comments
+7 | purple | MnO4–(aq), permanganate ion |
+6 | dark green | MnO42–(aq), manganate ion | basic solution
+4 | brown ppt | MnO2 (s), manganese (IV) oxide | basic solution
+2 | clear, virtually colourless | Mn2+(aq) | acidic solution

This was one of the sweetest moments of my career. We followed with a discussion of how Mn6+ was a reaction intermediate. When I asked if anyone had a comment, a boy with a grin who said “Gotcha”, explained: “Chance discovery favours the prepared mind, Sir.”

## References (websites accessed January 2014)

1. The full quotation is something like: “In the fields of observation chance favours only the prepared mind.”
2. https://en.wikiquote.org/wiki/Louis_Pasteur
3. https://en.wikipedia.org/wiki/W%C3%B6hler_synthesis
4. https://en.wikipedia.org/wiki/Vitalism
5. L.R. Summerlin, J.L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: 1985; Volume. 1, pages 95-97 and R.S. Pearson, Journal of Chemical Education, 1988, 65, page 451.
6. https://en.wikipedia.org/wiki/Baeyer%27s_reagent

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