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.
Structure and bonding
[edit | edit source]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.
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On the other hand, some compounds depicted as isothioureas and in fact thioureas, one example being mercaptobenzimidazole.[2]
Synthesis
[edit | edit source]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.
Reactions
[edit | edit source]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
[edit | edit source]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
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Further reading
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