Chemistry:Thioureas

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In organic chemistry, thioureas are members of a family of organosulfur compounds with the formula S=C(NR
2)
2 and structure R
2N–C(=S)–NR
2. The parent member of this class of compounds is thiourea (S=C(NH
2)
2). Substituted thioureas are found in several commercial chemicals.

Structure and bonding

Thioureas have a trigonal planar molecular geometry of the N
2C=S core. The C=S bond distance is near 1.71 Å, which is 0.1 Å longer than in normal ketones (R
2C=O). The C–N bond distances are short.[1] Thioureas occurs in two tautomeric forms.

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On the other hand, some compounds depicted as isothioureas and in fact thioureas, one example being mercaptobenzimidazole.[2]

Synthesis

N,N′-unsubstituted thioureas can be prepared by treating the corresponding cyanamide with hydrogen sulfide or similar sulfide sources.[3] Organic ammonium salts react with potassium thiocyanate as the source of the thiocarbonyl (C=S).[4]

Alternatively, N,N′-disubstituted thioureas can be prepared by coupling two amines with thiophosgene:[5]

HNR
2 + S=CCl
2 → 2 S=C(NR
2)
2 + 2 HCl

Amines also condense with organic thiocyanates to give thioureas:[6]

HNR
2 + S=C=NR' → S=C(NR
2)(NHR')

Cyclic thioureas are prepared by transamidation of thiourea with diamines. Ethylene thioureais synthesized by treating ethylenediamine with carbon disulfide.[7] In some cases, thioureas can be prepared by thiation of ureas using phosphorus pentasulfide.

Ethylene thiourea is an accelerant of vulcanization of neoprene and polychloroprene rubbers.

Reactions

Thioureas are susceptible to tautomerization. For the parent thiourea, the thione tautomer predominates in aqueous solutions.[8] The thiol form, known as an isothiourea, can be encountered in substituted compounds such as isothiouronium salts.

Thioureas are nucleophilic at sulfur. When they contain a pair of N-H substituents, thioureas engage in hydrogen bonding. This interaction is the basis of a research theme called thiourea organocatalysis.[9]Thioureas are often found to be stronger hydrogen-bond donors (i.e., more acidic) than ureas.[10][11]

Applications and occurrence

Agrichemicals that feature the thiourea functional group include diafenthiuron, methimazole, carbimazole (converted in vivo to methimazole), and propylthiouracil.[12] α-Naphthylthiourea is a commercial rodenticide.

Some thioureas are vulcanization accelerators.

Ergothioneine, which is derived from histidine, is a rare example of a thiourea found in nature.

The cyclic of thiourea called thiamazole is used to treat overactive thyroid

References

  1. ↑ D. Mullen; E. Hellner (1978). "A Simple Refinement of Density Distributions of Bonding Electrons. IX. Bond Electron Density Distribution in Thiourea, C=S(NH2)2, at 123K". Acta Crystallogr. B34 (9): 2789–2794. doi:10.1107/S0567740878009243. 
  2. ↑ Form, G. R.; Raper, E. S.; Downie, T. C. (1976). "The crystal and molecular structure of 2-mercaptobenzimidazole". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry 32 (2): 345–348. doi:10.1107/S0567740876003026. 
  3. ↑ Koketsu, Mamoru; Kobayashi, Chikashi; Ishihara, Hideharu (2003). "Synthesis of N-aryl-S-alkylthiocarbamates". Heteroatom Chemistry 14 (4): 374–378. doi:10.1002/hc.10163. 
  4. ↑ Herr, R. J.; Kuhler, L.; Meckler, H.; Opalka, C. J. (2000). "A Convenient Method for the Preparation of Primary and Symmetrical N,N′-Disubstituted Thioureas". Synthesis 2000 (11): 1569–1574. doi:10.1055/s-2000-7607. 
  5. ↑ Yi-Bo Huang; Wen-Bin Yi; Chun Cai (2012). "Thiourea Based Fluorous Organocatalyst". Topics in Current Chemistry 308: 191–212. doi:10.1007/128_2011_248. ISBN 978-3-642-25233-4. PMID 21972024. 
  6. ↑ Miyabe, H.; Takemoto, Y. (2008). "Discovery and Application of Asymmetric Reaction by Multifunctional Thioureas". Bull Chem Soc Jpn 81 (7): 785. doi:10.1246/bcsj.81.785. 
  7. ↑ C. F. H. Allen; C. O. Edens; James VanAllan. "Ethylene Thiourea". Org. Syntheses 26: 34. doi:10.15227/orgsyn.026.0034. 
  8. ↑ 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. 
  9. ↑ R. Schreiner, Peter (2003). "Metal-free organocatalysis through explicit hydrogen bonding interactions". Chem. Soc. Rev. 32 (5): 289–296. doi:10.1039/b107298f. PMID 14518182. 
  10. ↑ Jakab, Gergely; Tancon, Carlo; Zhang, Zhiguo; Lippert, Katharina M.; Schreiner, Peter R. (2012). "(Thio)urea Organocatalyst Equilibrium Acidities in DMSO". Organic Letters 14 (7): 1724–1727. doi:10.1021/ol300307c. PMID 22435999. 
  11. ↑ Nieuwland, Celine; Fonseca Guerra, Célia (2022). "How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides". Chemistry – A European Journal 28 (31). doi:10.1002/chem.202200755. PMID 35322485. 
  12. ↑ Yi, Qi-Qi; Sun, Ping; Zhang, Xinyi; Wang, Hao; Wu, Jian (2025). "Thiourea Derivatives in Agrochemical Discovery and Development". Journal of Agricultural and Food Chemistry 73 (15): 8756–8774. doi:10.1021/acs.jafc.5c00430. PMID 40190191. 

Further reading

  • 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. 



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