Thiourea dioxide or thiox is an organosulfur compound that is used in the textile industry. It functions as a reducing agent. It is a white solid, and exhibits tautomerism in solution.
Crystalline and gaseous thiourea dioxide adopts a C<sub>2v</sub>-symmetric structure. Selected bond lengths: S-C = 186, C-N = 130, and S-O = 149 pm. The sulfur center is pyramidal. The C-S bond length is close to that of a single bond. For comparison, the C=S bond in thiourea is 171 pm. Instead the bonding is described with a significant contribution from a dipolar resonance structure with multiple bonding between C and N. One consequence of this bonding is the planarity of the nitrogen centers. In the presence of water or DMSO, thiourea dioxide converts to the tautomer, a sulfinic acid, , named formamidine sulfinic acid.
Thiourea dioxide was first prepared in 1910 by the English chemist Edward de Barry Barnett, through a method not dissimilar from the modern synthesis.
Thiourea dioxide is prepared by the oxidation of thiourea with hydrogen peroxide, while maintaining a pH between 3 and 5 and temperature below 10 ðC:
It can also be prepared by oxidation with chlorine dioxide. The quality of the product can be assessed by titration with indigo.
Below pH 6.5, thiourea dioxide hydrolyzes to sulfoxylic acid and urea. Further oxidation with a peracid gives the corresponding sulfonic acid.
Thiourea dioxide is used in reductive bleaching in textiles. Thiourea dioxide has also been used for the reduction of aromatic nitroaldehydes and nitroketones to nitroalcohols.