Conversion of urea to hydrazine: Hofmann reaction or Favorskii analogue? Exploring reaction mechanism through isotopic labeling studies

The power of isotopic substitution for the elucidation of a reaction mechanism is illustrated with reactions named after Hofmann and Favorskii. These reactions have important roles in synthetic chemistry; therefore, a wide range of experiments involving isotopic labeling or kinetic isotope effects were employed to establish their mechanistic pathways. The concepts introduced by these investigations are drawn together with an isotopic labeling study of the oxidation of urea with hypohalites. The two mechanisms proposed for this reaction have been described as a Hofmann rearrangement and a nitrogen analogue of the Favorskii rearrangement.