Reaction of Thiourea with Hydrogen Peroxide: ¹³ C NMR Studies of an Oxidative/Reductive Bleaching Process

Thiourea's reaction with hydrogen peroxide in solution under bleaching conditions at three different pH values has been investigated using ¹³ C NMR spectroscopy. Since this reaction is fast and exothermic, it is essential that short total acquisition times be used to accumulate sufficient data to detect different species formed during the reaction. As the abundance of ¹³ C in the reactants in the concentration range studied is very low, ¹³ C-labeled thiourea was used as the starting material. Sufficient data were accumulated in short acquisition times (2–4 minutes) to identify different species formed during the reaction. The results showed that different intermediate products are formed during the reactions, depending on the pH of the medium and the molar ratio of the reactants. The reaction goes through a thiourea dioxide intermediate; this then hydrolyzes under heat and neutral or alkaline conditions to yield sulfonate anion and urea if the initial reaction of thiourea with hydrogen peroxide is allowed to take place in acidic/neutral medium (pH = 4.0–7.0). Because thiourea dioxide hydrolyzes in solution, there is a rapid change in redox potential from a positive value to a high negative value. The species causing the negative redox potential, and hence the species responsible for reductive bleaching, is believed to be the sulfinate anion. The reaction of thiourea with hydrogen peroxide in hydrochloric acid at pH < 1 results in the formation of formamidine disulfide dihydrochloride, which decomposes at pH values greater than 1.