Ammonium Cyanate to Urea

From WikidChem

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Slide 26

The antibonding orbital on ammonium ion (LUMO - antibonding of N-H) interacts with the unshared pair on the N (HOMO – atomic p orbital). This moves the unshared pair into a bond (N-H) and leaves ammonia. The HOMO on ammonia is the unshared pair, and this interacts with the LUMO of cyanate (pi*). This leaves a molecule with separated formal charges. To get rid of these charges (lower energy), the unshared pair on the bottom N (negative charge) interacts with the LUMO (antibonding orbital) on the top N (positive charge), which results in Urea. Because the overlap in this last interaction is not very good, however, this step is most likely between two idential molecules and not intramolecule.

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Slide 27

Liebig and Woehler ultimately were able to prepare ammonium cyanate from dry ammonia and cyanic acid, but it converts to urea so easily when moist, that for more than 150 years no one had ever grown proper crystals of the salt. Finally in 1998 it was possible to determine the structure by X-ray diffraction using a powder sample. This slide shows the crystal structure of ammonium cyanate confirming that it is indeed an isomer of urea.