page_title: Ureas
section_title: Assembly of N-substituted urea functionality
hierarchical_section_title: Ureas / Synthesis / Assembly of N-substituted urea functionality
caption_attribution_description: English: chem structure of IBDU
context_page_description: In chemistry, ureas are a class of organic compounds with the formula (R₂N)₂CO where R = H, alkyl, aryl, etc. Thus, in addition to describing a specific chemical compound (H₂N)₂CO, urea is the name of a functional group that is found in many compounds and materials of both practical and theoretical interest. Generally ureas are colorless crystalline solids, which, owing to the presence of fewer hydrogen bonds, exhibit melting points lower than that of urea itself.
context_section_description: Phosgenation entails the reaction of amines with phosgene, proceeding via the isocyanate (or carbamoyl chloride) as an intermediate:
  COCl₂  +  R₂NH   →  R₂NC(O)Cl +  HCl
  COCl₂  +  RNH₂   →  RNCO + 2 HCl
  Overall reaction:
  COCl₂  +  2 R₂NH   →    (R₂N)₂CO  +  2 HCl
  Tetramethylurea is prepared in this way and in general secondary amines give reliable results. Reactions using primary amines must be carefully controlled as the isocyanate intermediate can react with the urea to form a biuret:
  R₂NC(O)N(H)R' + R"NCO   →  R₂NC(O)NR'C(O)NHR"
  Unsymmetrical ureas are generated by condensation of isocyanates with amines:
  RNCO  +  R'₂NH  →   (R'₂N)(R(H)N)CO
  Analogously, unsymmetrical primary ureas are generated by condensation of ammonium salts and alkali metal cyanates:
  Na⁺NCO⁻  +  [R₂NH₂]Cl  →    (R’₂N)(H₂N)CO  +  KCl
  The artificial sweetener dulcin is produced by the condensation of ethoxyaniline with potassium cyanate.  Si(NCO)₄ is also used as a precursor to such unsymmetrical ureas.
  The very high toxicities of compounds such as phosgene and isocyanates makes them unappealing to work with and there has been a drive towards safer reagents. These have traditionally been more expensive and hence mostly been limited to laboratory-scale work.
caption: Isobutylidenediurea.
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