Thiourea

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Thiourea
Thiourea.png
Thiourea-3D-vdW.png
Names
Preferred IUPAC name
Thiourea[1]
Other names
Thiocarbamide
Identifiers
3D model (JSmol)
605327
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.494
1604
KEGG
RTECS number
  • YU2800000
UNII
UN number 2811
Properties
CH4N2S
Molar mass 76.12 g/mol
Appearance white solid
Density 1.405 g/ml
Melting point 182 °C (360 °F; 455 K)
142 g/l (25 °C)
−4.24×10−5 cm3/mol
Hazards
Carc. Cat. 3
Repr. Cat. 3
Harmful (Xn)
Dangerous for the environment (N)
R-phrases (outdated) R22, R40, R51/53, R63
S-phrases (outdated) (S2), S36/37, S61
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
1
3
0
Related compounds
Related compounds
Urea
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☑Y verify (what is ☑Y☒N ?)
Infobox references

Thiourea (/ˌθjʊəˈrə/)[2][3] 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[edit]

Thiourea is a planar molecule. The C=S bond distance is 1.71 Å. The C-N distances average 1.33 Å.[4] 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 Keq is 1.04×10−3.[5] The thiol form, which is also known as an isothiourea, can be encountered in substituted compounds such as isothiouronium salts.

Thiourea tautomers.png

Production[edit]

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.[6]

Applications[edit]

Thiourea per se has few applications. It is mainly consumed as a precursor to thiourea dioxide, which is a common reducing agent in textile processing.[6]

Other uses[edit]

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.[7] It is also used in a solution with tin(II) chloride as an electroless tin plating solution for copper printed circuit boards.

Reactions[edit]

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.[citation needed]

Reductant[edit]

Thiourea reduces peroxides to the corresponding diols.[8] The intermediate of the reaction is an unstable endoperoxide.

reduction of cyclic peroxide

Thiourea is also used in the reductive workup of ozonolysis to give carbonyl compounds.[9] Dimethyl sulfide is also an effective reagent for this reaction, but it is highly volatile (boiling point 37 °C) and has an obnoxious odor whereas thiourea is odorless and conveniently non-volatile (reflecting its polarity).

reduction cleavage of product from ozonolysis

Source of sulfide[edit]

Thiourea is employed as a source of sulfide, such as for converting alkyl halides to thiols. The reaction capitalizes on the high nucleophilicity of the sulfur center and easy hydrolysis of the intermediate isothiouronium salt:

CS(NH2)2 + RX → RSC(NH
2
)+
2
X
RSC(NH
2
)+
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:[10]

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 other thioamides, thiourea can serve as a source of sulfide upon reaction with 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+

These sulfiding reactions, which have been applied to the synthesis of many metal sulfides, require water and typically some heating.[11][12]

Precursor to heterocycles[edit]

Thioureas are building blocks to pyrimidine derivatives. Thus thioureas condense with β-dicarbonyl compounds.[13] The amino group on the thiourea initially condenses with a carbonyl, followed by cyclization and tautomerization. Desulfurization delivers the pyrimidine.

Pyrimidine.png

Similarly, aminothiazoles can be synthesized by the reaction of α-haloketones and thiourea.[14]

Aminothiazole.png

The pharmaceuticals thiobarbituric acid and sulfathiazole are prepared using thiourea.[6] 4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole is prepared by the reaction of thiourea and hydrazine.

Silver polishing[edit]

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.[15]

Safety[edit]

The LD50 for thiourea is 125 mg/kg for rats (oral).[16]

A goitrogenic effect (enlargement of the thyroid gland) has been reported for chronic exposure, reflecting the ability of thiourea to interfere with iodide uptake.[6]

See also[edit]

References[edit]

  1. ^ Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: Royal Society of Chemistry. 2014. pp. 98, 864. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
  2. ^ "Thiourea". Oxford Dictionaries. Oxford University Press. Retrieved 2016-01-21.
  3. ^ "Thiourea". Merriam-Webster Dictionary. Retrieved 2016-01-21.
  4. ^ D. Mullen; E. Hellner (1978). "A Simple Refinement of Density Distributions of Bonding Electrons. IX. Bond Electron Density Distribution in Thiourea, CS(NH2)2, at 123K". Acta Crystallogr. B34: 2789–2794. doi:10.1107/S0567740878009243.
  5. ^ Allegretti, P.E; Castro, E.A; Furlong, J.J.P (March 2000). "Tautomeric equilibrium of amides and related compounds: theoretical and spectral evidences". Journal of Molecular Structure: THEOCHEM. 499 (1–3): 121–126. doi:10.1016/S0166-1280(99)00294-8.
  6. ^ a b c d Bernd Mertschenk; Ferdinand Beck; Wolfgang Bauer (2002). "Thiourea and Thiourea Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a26_803.
  7. ^ 81st Universal Metal Finishing Guidebook. Metal Finishing Magazine. Fall 2013. p. 285. ISSN 0026-0576.
  8. ^ C. Kaneko; A. Sugimoro & 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): 876–877. doi:10.1055/s-1974-23462.
  9. ^ Gupta, D., Soman, G., and Dev, S. (1982). "Thiourea, a convenient reagent for the reductive cleavage of olefin ozonolysis products". Tetrahedron. 38 (20): 3013–3018. doi:10.1016/0040-4020(82)80187-7.CS1 maint: multiple names: authors list (link)
  10. ^ Speziale, A. J. (1963). "Ethanedithiol". Organic Syntheses.; Collective Volume, 4, p. 401
  11. ^ Liang, Y.; et, al. (2016). "An efficient precursor to synthesize various FeS2 nanostructures via a simple hydrothermal synthesis method". CrystEngComm. 18: 6262–6271. doi:10.1039/c6ce01203e.
  12. ^ Bao, N.; et al. (2007). "Facile Cd−Thiourea Complex Thermolysis Synthesis of Phase-Controlled CdS Nanocrystals for Photocatalytic Hydrogen Production under Visible Light". The Journal of Physical Chemistry C. 111: 17527–17534. doi:10.1021/jp076566s.
  13. ^ Foster, H. M., and Snyder, H. R. (1963). "4-Methyl-6-hydroxypyrimidine". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 4, p. 638
  14. ^ Dodson, R. M. & King, L. C. (1945). "The reaction of ketones with halogens and thiourea". J. Am. Chem. Soc. 67 (12): 2242–2243. doi:10.1021/ja01228a059. PMID 21005695.
  15. ^ Anthony Esposito. "Peñoles, UAM unveil pilot thiourea Au-Ag leaching plant - Mexico". Business News Americas (July 13, 2007).
  16. ^ http://gis.dep.wv.gov/tri/cheminfo/msds1385.txt

Further reading[edit]

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

External links[edit]