The Wöhler synthesis is the conversion of ammonium cyanate into urea.[1]

OverviewEdit

This chemical reaction was discovered in 1828 by Friedrich Wöhler in an attempt to synthesize ammonium cyanate. It is considered the starting point of modern organic chemistry. Although the Wöhler reaction concerns the conversion of ammonium cyanate, this 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).[2]

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.[3]

Alternatively the reaction can be carried out with lead cyanate and ammonia.[4] The actual reaction taking place is a double displacement reaction to form ammonium cyanate:

 

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

 

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

Vitalism mythEdit

A widespread myth is that the Wöhler synthesis sparked the downfall of the theory of vitalism, which stated that organic matter possessed a certain vital force common to all living things. Supposedly, as the myth goes, a sharp distinction existed between organic and inorganic chemistry, shattered when the Wöhler synthesis produced urea, an organic compound, from inorganic matter. In fact, prior to the Wöhler synthesis, the work of John Dalton and Jöns Jacob Berzelius had already convinced chemists that organic and inorganic matter obey the same chemical laws, and furthermore, the Wöhler synthesis failed to refute vitalism because Wöhler had started with organic matter, derived inorganic elements from it, and used those to produce urea. It was argued, therefore, that a residue of the vital force could have been transmitted through the artificial synthesis. Furthermore, while versions of vitalism supposed that an immaterial the force pervading living systems was immaterial, many versions had developed that stated that the 'force' of vitalism was the product of a complex combination of physical and chemical laws. It took until 1845 when Kolbe repeated an inorganic – organic conversion of carbon disulfide to acetic acid before vitalism started to lose support.[5] A 2000 survey found that 90% of chemical textbooks repeat some version of the Wöhler myth.[6]

ReferencesEdit

  1. ^ Friedrich Wöhler (1828). "Ueber künstliche Bildung des Harnstoffs". Annalen der Physik und Chemie. 88 (2): 253–256
  2. ^ Wöhler's Synthesis of Urea: How Do the Textbooks Report It? Paul S. Cohen, Stephen M. Cohen J. Chem. Educ. 1996 73 883
  3. ^ A Demonstration of Wöhler's Experiment: Preparation of Urea from Ammonium Chloride and Potassium Cyanate Zoltán Tóth. J. Chem. Educ. 1996 73 539.
  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
  5. ^ Ramberg, Peter, "Myth 7. That Friedrich Wöhler’s Synthesis of Urea in 1828 Destroyed Vitalism and Gave Rise to Organic Chemistry" eds. Numbers, Ronald L., and Kostas Kampourakis, Newton's apple and other myths about science. Harvard university press, 2015, 59-66.
  6. ^ Ramberg, Peter J. "The death of vitalism and the birth of organic chemistry: Wohler's urea synthesis and the disciplinary identity of organic chemistry." Ambix 47.3 (2000): 170-195.