Thiocarbamide, CS(NH₂)₂

Thiocarbamide or Thiourea, CS(NH₂)₂, is formed by heating ammonium thiocyanate to 140° C., when it undergoes an intramolecular rearrangement:

NH₄SCN ⇔ SC(NH₂)₂.

At the same time some guanidine thiocyanate, NH:C(NH₂)₂.HSCN, is produced, although a larger proportion of this substance is formed at 170° to 180° C. The thiourea is freed from guanidine thiocyanate and unchanged ammonium thiocyanate by treatment with cold water, in which thiourea is but sparingly soluble; it may then be purified by recrystallisation from water.

Another method of preparation consists in heating carbon disulphide with ammonium carbonate in a sealed tube. The other product of this reaction is ammonium thiocyanate. The yield of thiocarbamide by this method is stated to be much below the theoretical value.

Thiourea crystallises in rhombic prisms and is soluble in eleven times its weight of cold water. When heated with water to 140° C., ammonium thiocyanate is re-formed. It forms salts which are analogous to those of urea, the most characteristic being the nitrate, CS(NH₂)₂.HNO₃. It is oxidised in solution containing blood charcoal according to the equation:

2CS(NH₂)₂ + O₂ = C₂H₄N₄S + 2H₂O + S.

Thiourea is used with formaldehyde on a large scale for the production of synthetic resins. It can also be used to produce excellent mirrors of galena on glass.