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Wöhler synthesis

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Wöhler synthesis

The Wöhler synthesis is the conversion of salt appears only as an (unstable) intermediate. Wöhler demonstrated the reaction in his original publication with different sets of reactants: a combination of cyanic acid and ammonia, a combination of silver cyanate and ammonium chloride, a combination of lead cyanate and ammonia and finally from a combination of mercury cyanate and cyanatic ammonia (which is again cyanic acid with ammonia).

The reaction can be demonstrated by starting with solutions of potassium cyanate and ammonium chloride which are mixed, heated and cooled again. An additional proof of the chemical transformation is obtained by adding a solution of oxalic acid which forms urea oxalate as a white precipitate.

Alternatively the reaction can be carried out with lead cyanate and ammonia. The actual reaction taking place is a double displacement reaction to form ammonium cyanate:

\mathrm{Pb(NCO)_2 + 2NH_3 + 2H_2O \rightarrow Pb(OH)_2 + 2NH_4(NCO)}

Ammonium cyanate decomposes to ammonia and cyanic acid which in turn react to produce urea in a nucleophilic addition followed by tautomeric isomerization:

\mathrm{NH_4(NCO) \rightarrow NH_3 + HNCO \leftrightarrow (NH_2)_2CO}

Complexation with oxalic acid helps drive this chemical equilibrium to completion.

The Wöhler synthesis is of great historical significance because for the first time an biological sources such as urine. Wöhler reported to his mentor Berzelius

"I cannot, so to say, hold my chemical water and must tell you that I can make urea without thereby needing to have kidneys, or anyhow, an animal, be it human or dog".

It is argued that organic chemistry did not actually start with this discovery in 1828 but 4 years earlier with the synthesis of carbon disulfide to acetic acid before vitalism started to lose supporters in serious numbers.

References

  1. ^  
  2. ^ Wöhler's Synthesis of Urea: How Do the Textbooks Report It? Paul S. Cohen, Stephen M. Cohen J. Chem. Educ. 1996 73 883 Abstract
  3. ^ A Demonstration of Wöhler's Experiment: Preparation of Urea from Ammonium Chloride and Potassium Cyanate Zoltán Tóth. J. Chem. Educ. 1996 73 539. Abstract
  4. ^ Recreation of Wöhler's Synthesis of Urea: An Undergraduate Organic Laboratory Exercise James D. Batchelor, Everett E. Carpenter, Grant N. Holder, Cassandra T. Eagle, Jon Fielder, Jared Cummings The Chemical Educator 1/Vol .3,NO.6 1998 ISSN 1430-4171 Online article
  • P. Walden (1928). "Die Bedeutung der Wöhlerschen Harnstoff-Synthese". Naturwissenschaften 16 (45–47): 835–849.  
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