Mechanism of a Chemical Classic:  Quantum Chemical Investigation of the Autocatalyzed Reaction of the Serendipitous Wöhler Synthesis of Urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (Wöhler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH₃ with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of Wöhler's reaction in a vacuum involving the formation of “four-center” transition states. Modeling Wöhler's reaction in aqueous solution and in the solid state, it was found that the addition of NH₃ to both acids is assisted (autocatalyzed) by the active participation of extra H₂O and/or NH₃ molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of “six-center” or even “eight-center” transition states. The most energetically economic path of the rate-determining step of Wöhler's reaction is that of the addition of NH₃ to the CN double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the C⋮N triple bond of cyanic acid, yielding urea's tautomer HNC(OH)NH₂, seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.