Thiourea is an organosulfur compound with the formula SC2. It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad class of compounds with the general structure C=S. Thioureas are related to thioamides, e.g. RCNR2, where R is methyl, ethyl, etc.
Structure and bonding
Thiourea is a planar molecule. The C=S bond distance is 1.71 Å. The C-N distances average 1.33 Å. The weakening of the C-S bond by C-N pi-bonding is indicated by the short C=S bond in thiobenzophenone, which is 1.63 Å. Thiourea occurs in two tautomeric forms, of which the thione form predominates in aqueous solutions. The equilibrium constant has been calculated as Keq is. The thiol form, which is also known as an isothiourea, can be encountered in substituted compounds such as isothiouronium salts.
Production
The global annual production of thiourea is around 10,000 tonnes. About 40% is produced in Germany, another 40% in China, and 20% in Japan. Thiourea can be produced from ammonium thiocyanate, but more commonly it is produced by the reaction of hydrogen sulfide with calcium cyanamide in the presence of carbon dioxide.
Other industrial uses of thiourea include production of flame retardant resins, and vulcanization accelerators. Thiourea is used as an auxiliary agent in diazo paper, light-sensitive photocopy paper and almost all other types of copy paper. It is also used to tone silver-gelatin photographic prints. Thiourea is used in the Clifton-Phillips and Beaver bright and semi-bright electroplating processes. It is also used in a solution with tin chloride as an electroless tin plating solution for copper printed circuit boards.
Reactions
The material has the unusual property of changing to ammonium thiocyanate upon heating above. Upon cooling, the ammonium salt converts back to thiourea.
Reductant
Thiourea reduces peroxides to the corresponding diols. The intermediate of the reaction is an unstable endoperoxide. Thiourea is also used in the reductive workup of ozonolysis to give carbonyl compounds. Dimethyl sulfide is also an effective reagent for this reaction, but it is highly volatile and has an obnoxious odor whereas thiourea is odorless and conveniently non-volatile.
Precursor to heterocycles
Thioureas are building blocks to pyrimidine derivatives. Thus thioureas condense with β-dicarbonyl compounds. The amino group on the thiourea initially condenses with a carbonyl, followed by cyclization and tautomerization. Desulfurization delivers the pyrimidine. Similarly, aminothiazoles can be synthesized by the reaction of α-haloketones and thiourea. The pharmaceuticals thiobarbituric acid and sulfathiazole are prepared using thiourea. 4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole is prepared by the reaction of thiourea and hydrazine.
Silver polishing
According to the label on the consumer product, the liquid silver cleaning productTarnX contains thiourea, a detergent, and sulfamic acid. A lixiviant for gold and silver leaching can be created by selectively oxidizing thiourea, bypassing the steps of cyanide use and smelting.
Safety
The for thiourea is for rats. A goitrogenic effect has been reported for chronic exposure, reflecting the ability of thiourea to interfere with iodide uptake.
