Ureas

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 the specific chemical compound urea ((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.

Ureas can be prepared many methods, but rarely by direct carbonation, which is the route to urea itself. Instead, methods can be classified according those that assemble the urea functionality and those that start with preformed urea.

Phosgenation entails the reaction of amines with phosgene, proceeding via the isocyanate (or carbamoyl chloride) as an intermediate:

Overall reaction:

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:

Unsymmetrical ureas are generated by condensation of isocyanates with amines:

Analogously, unsymmetrical primary ureas are generated by condensation of ammonium salts and alkali metal cyanates:

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.