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 13C 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 I3C in the reactants in the concentration range studied is very low, I3C-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 sulfinate 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. Ther? is a vast amount of information available on the reactions of thiourea with oxidizing agents [4, 5 , 18, 20, 2 1, 23, 241. The action of hydrogen peroxide on thiourea (Th, Figure 1 ) was investigated as long ago as 19 10, when Bamett [ 41 concluded that the resultant product depends very much on the pH and conditions of the reaction medium. Unless special precautions are taken, hydrogen peroxide decomposes thiourea into ammonia, carbon dioxide, sulfur, and sulfuric acid [ 13, 161. Under closely controlled neutral conditions, Barnett [ 41 and others [ 6 , 9, 16, 171 produced formamidinesulfinic acid (thiourea dioxide, ThDO, Figure 1 ) when they added finely powdered thiourea to an aqueous solution of hydrogen peroxide at 0-10°C under different compositions and conditions. Other workers, however, obtained formamidine disulfide (FDS, Figure 1 ) dihydrochloride by oxidation of thi(hydrochTbric acid, pH < I ) conditions [lo, 19, 211. Oiiisz -&h hjdiGg€ii peroxide under stiijngly X ~ & C Marshall [ 181 reported that the salts of formamidine disulfide are not very stable, and their solutions decompose, especially on warming, with formation of sulfur, thiourea, cyanamide, and free acid. A corresponding decomposition results immediately with the addition of alkali, and this constitutes a very characteristic reaction for these salts. Grigorova and Wright [ 121 reported that under strong oxidizing conditions, formamidine disulfide oxidizes to cyanamide, hydroH2N \ , NH2 C-SH ,c-s s-c HZN ,