Thiourea

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Thiourea
IUPAC name

Thiourea

Other names

Thiocarbamide

Identifiers
CAS number 62-56-6 YesY
PubChem 2723790
ChemSpider 2005981 YesY
UNII GYV9AM2QAG YesY
UN number 2811
KEGG C14415 YesY
ChEBI CHEBI:36946 YesY
ChEMBL CHEMBL260876 YesY
RTECS number YU2800000
Jmol-3D images Image 1
Properties
Molecular formula CH4N2S
Molar mass 76.12 g/mol
Appearance white solid
Density 1.405 g/ml
Melting point 182 °C; 360 °F; 455 K
Solubility in water 14.2 g/100ml (25°C)
Hazards
EU Index 612-082-00-0
EU classification Carc. Cat. 3
Repr. Cat. 3
Harmful (Xn)
Dangerous for the environment (N)
R-phrases R22, R40, R51/53, R63
S-phrases (S2), S36/37, S61
NFPA 704
1
3
0
Related compounds
Related compounds Urea
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Thiourea is an organosulfur compound with the formula SC(NH2)2 . 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 (R1R2N)(R3R4N)C=S. Thioureas are related to thioamides, e.g. RC(S)NR2, where R is methyl, ethyl, etc.

General chemical structure of a thiourea

Structure and bonding

Thiourea is a planar molecule. The C=S bond distance is 1.60±0.1 Å for thiourea (as well as many of its derivatives). The material has the unusual property of changing to ammonium thiocyanate upon heating above 130 °C. Upon cooling, the ammonium salt converts back to thiourea.

Thiourea occurs in two tautomeric forms. In aqueous solution, the thione shown on the left below predominates:

Production

The global annual production of thiourea is around 10,000 tons.[1] 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.[1]

Synthesis of substituted thioureas

Many derivatives of thiourea derivatives are useful in organocatalysis. N,N-unsubstituted thioureas can be generally prepared by treating the corresponding cyanamide with "LiAlHSH" in the presence of 1 N HCl in anhydrous diethyl ether. The "LiAlHSH" is prepared by treating lithium aluminium hydride with elemental sulfur.[2]

Alternatively, N,N' disubstituted thioureas can be prepared by coupling two amines with thiophosgene:[3]

R2NH + R'2NH + CSCl2 + 2 pyridine → (R2N)(R'2N)CS + 2 [Hpyridine]Cl

Amines also condense with thiocyanates to give thioureas:[4]

R2NH + R'NCS → (R2N)(R'(H)N)CS

Applications

The main application of thiourea is in textile processing.[1]

Organic synthesis

Thiourea reduces peroxides to the corresponding diols.[5] The intermediate of the reaction is an unstable epidioxide which can only be identified at −100 °C. Epidioxide is similar to epoxide except with two oxygen atoms. This intermediate reduces to diol by thiourea.

Thiourea is also used in the reductive workup of ozonolysis to give carbonyl compounds.[6] Dimethyl sulfide is also an effective reagent for this reaction, but it is highly volatile (b.p. 37 °C) and has an obnoxious odor whereas thiourea is odorless and conveniently non-volatile (reflecting its polarity).

Source of sulfide

Thiourea is commonly employed as a source of sulfide, e.g. for converting alkyl halides to thiols. Such reactions proceed via the intermediacy of isothiuronium salts. The reaction capitalizes on the high nucleophilicity of the sulfur center and easy hydrolysis of the intermediate isothiouronium salt:

CS(NH2)2 + RX → RSC(NH2)2+X-
RSC(NH2)2+X- + 2 NaOH → RSNa + OC(NH2)2 + NaX
RSNa + HCl → RSH + NaCl

In this example, ethane-1,2-dithiol is prepared from 1,2-dibromoethane:[7]

C2H4Br2 + 2 SC(NH2)2 → [ C2H4(SC(NH2)2)2]Br2
[ C2H4(SC(NH2)2)2]Br2 + 2 KOH → C2H4(SH)2 + 2 OC(NH2)2 + 2 KBr

Like thioamides, thiourea can serve as a source of sulfide upon reaction with soft metal ions. For example, mercury sulfide forms when mercuric salts in aqueous solution are treated with thiourea:

Hg2+ + SC(NH2)2 + H2O → HgS + OC(NH2)2 + 2 H+

Precursor to heterocycles

Thioureas are used a building blocks to pyrimidine derivatives. Thus thioureas condense with β-dicarbonyl compounds.[8] 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 alpha-halo ketones and thiourea.[9]

The pharmaceuticals thiobarbituric acid and sulfathiazole is prepared using thiourea.[citation needed]

Silver polishing

According to the label on the consumer product, the liquid silver cleaning product TarnX 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.[10]

Organocatalysis

Substituted thioureas are useful catalysts for organic synthesis. The phenomenon is called thiourea organocatalysis.[11]

Other uses

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.

Safety

The LD50 for thiourea is 125 mg/kg for rats (oral).[12] A goitrogenic effect (enlargement of the thyroid gland) has been reported for chronic exposure, reflecting the ability of thiourea to interfere with iodide uptake.[1]

References

  1. ↑ 1.0 1.1 1.2 1.3 Bernd Mertschenk, Ferdinand Beck, Wolfgang Bauer "Thiourea and Thiourea Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2002 by Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved. doi:10.1002/14356007.a26_803
  2. ↑ Koketsu, Mamoru; Kobayashi, Chikashi; Ishihara, Hideharu (2003). "Synthesis ofN-arylS-alkylthiocarbamates". Heteroatom Chemistry 14 (4): 374. doi:10.1002/hc.10163. 
  3. ↑ Yi-Bo Huang, Wen-Bin Yi, and Chun Cai "Thiourea Based Fluorous Organocatalyst" Top Curr Chem 2012, vol. 308, p. 191–212. doi:10.1007/128_2011_248
  4. ↑ Miyabe, H.; Takemoto, Y. "Discovery and application of asymmetric reaction by multifunctional thioureas" Bull Chem Soc Jpn 2008, vol. 81, p785ff.
  5. ↑ C. Kaneko, A. Sugimoro, and S. Tanaka (1974). "A facile one-step synthesis of cis-2-cyclopentene and cis-2-cyclohexene-1,4-diols from the corresponding cyclodienes". Synthesis 1974 (12): 876. doi:10.1055/s-1974-23462. 
  6. ↑ Gupta, D., Soman, G., and Dev, S. (1982). "Thiourea, a convenient reagent for the reductive cleavage of olefin ozonolysis products". Tetrahedron 38 (20): 3013. doi:10.1016/0040-4020(82)80187-7. 
  7. ↑ Speziale, A. J. (1963), "Ethanedithiol", Org. Synth. ; Coll. Vol. 4: 401 
  8. ↑ Foster, H. M., and Snyder, H. R. (1963), "4-Methyl-6-hydroxypyrimidine", Org. Synth. ; Coll. Vol. 4: 638 
  9. ↑ Dodson, R. M., and King, L. C. (1945). "The reaction of ketones with halogens and thiourea". J. Am. Chem. Soc. 67 (12): 2242. doi:10.1021/ja01228a059. PMID 21005695. 
  10. ↑ Anthony Esposito. "Peñoles, UAM unveil pilot thiourea Au-Ag leaching plant - Mexico". Business News Americas (July 13, 2007).
  11. ↑ Peter R. Schreiner, "Metal-free organocatalysis through explicit hydrogen bonding interactions" Chem. Soc. Rev., 2003, vol. 32, 289-296.
  12. ↑ http://gis.dep.wv.gov/tri/cheminfo/msds1385.txt

Further reading

  • The Chemistry of double-bonded functional groups edited by S. Patai. pp 1355–1496. John Wiley & Sons. New York, NY, 1977. ISBN 0-471-92493-8.

External links

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