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.

General chemical structure of thioureas

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.

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.

Reactions

Main article: Thiourea § 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