Thioureas

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General chemical structure of thioureas Thiourea functional group.svg
General chemical structure of thioureas

In organic chemistry, thioureas are members of a family of organosulfur compounds with the formula S=C(NR2)2 and structure R2N−C(=S)−NR2. The parent member of this class of compounds is thiourea (S=C(NH2)2). Substituted thioureas are found in several commercial chemicals.

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Structure and bonding

Thioureas have a trigonal planar molecular geometry of the N2C=S core. The C=S bond distance is near 1.71  Å, which is 0.1 Å longer than in normal ketones (R2C=O). The C–N bond distances are short. [1] Thioureas occurs in two tautomeric forms.

Thiourea tautomers (cropped).png

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]

HNR2 + S=CCl2 → 2 S=C(NR2)2 + 2 HCl

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

HNR2 + S=C=NR' → S=C(NR2)(NHR')

Cyclic thioureas are prepared by transamidation of thiourea with diamines. Ethylene thiourea is 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. Ethylene thiourea.svg
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): e202200755. doi:10.1002/chem.202200755. PMC   9324920 . 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.

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