Synergetic Effect of Thiolacids on the Oxidation of Thiourea by Methylene Blue in Acidic Medium

Abstract

Thiourea (TU) and methylene blue (MB) react in a molar ratio of 2:1 in acidic medium to give the corresponding disulphide and dihydromethylene blue. The rate of the reaction is considerably enhanced in the presence of 2-mercaptonicotinic acid (MNA) and o-mercaptobenzoic acid (TSA) due to synergetic effect. The order in thiourea is nearly unity in both the cases. The reaction follows a first order kinetics in MB in TU–MNA–MB reaction whereas in TU–TSA–MB reaction, the order in MB varies between zero and one. The rate of reaction increases linearly on increasing [H+] in case of TU–MNA–MB reaction while an insignificant influence has been noticed in TU–TSA–MB reaction. The rate increases linearly on increasing the concentration of total sulphydryl group. The synergetic effect of thiolacids is found to depend appreciably on their structure and it is attributed to the participation of a persulphide-like intermediate or a dithiol as revealed by ESI–MS, 1H-NMR and FT-IR techniques. These studies suggest that the inhibitory effect of thiourea on thyroxine biosynthesis may be minimized through its in situ oxidation assisted by synergetic effect of the thiolacids.