Chemistry:Thioureas
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. For the parent thiourea, the thione form predominates in aqueous solutions.[2] The thiol form, known as an isothiourea, can be encountered in substituted compounds such as isothiouronium salts.
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On the other hand, some compounds depicted as isothioureas and in fact thioureas, one example being mercaptobenzimidazole.[3]
Synthesis
N,N′-unsubstituted thioureas can be prepared by treating the corresponding cyanamide with hydrogen sulfide or similar sulfide sources.[4] Organic ammonium salts react with potassium thiocyanate as the source of the thiocarbonyl (C=S).[5]
Alternatively, N,N′-disubstituted thioureas can be prepared by coupling two amines with thiophosgene:[6]
- HNR
2 + S=CCl
2 → 2 S=C(NR
2)
2 + 2 HCl
Amines also condense with organic thiocyanates to give thioureas:[7]
- 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.[8] In some cases, thioureas can be prepared by thiation of ureas using phosphorus pentasulfide. thumb|138px|[[Ethylene thiourea is an accelerant of vulcanization of neoprene and polychloroprene rubbers.]]
Applications
Agrichemicals that feature the thiourea functional group include methimazole, carbimazole (converted in vivo to methimazole), and propylthiouracil.
Catalysis
Some thioureas are vulcanization accelerators. Thioureas are also used in a research theme called thiourea organocatalysis.[9]
References
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ Koketsu, Mamoru; Kobayashi, Chikashi; Ishihara, Hideharu (2003). "Synthesis of N-aryl-S-alkylthiocarbamates". Heteroatom Chemistry 14 (4): 374–378. doi:10.1002/hc.10163.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ C. F. H. Allen; C. O. Edens; James VanAllan. "Ethylene Thiourea". Org. Syntheses 26: 34. doi:10.15227/orgsyn.026.0034.
- ↑ 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.
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.
External links
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