Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide-Bromate Substrate-Depletive Clock Reaction.

György Csekő,Attila K Horváth,Attila Takács,Qingyu Gao

doi:10.1021/acs.jpca.9b02025

Abstract

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H2PO4- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

Highlights

Chemical compounds including thioureido group are one of the most biologically active moieties
Under our experimental conditions, thiourea dioxide (TDO) does not have a pronounced effect on the Landolt time; it has only a subtle role, though it seems to be well established that increase of TDO concentration slightly increases the Landolt time under our experimental conditions
It has been illustrated that the title system is a substrate-depletive clock reaction and a relatively simple kinetic model is able to explain the main kinetic features found experimentally

Summary

Introduction

Chemical compounds including thioureido group are one of the most biologically active moieties. The derivative of thiourea, such as N,N′dimethylthiourea (DMTU), is, for example, a well-known and effective scavenger of toxic oxygen metabolites, removing rapidly in vitro hydrogen peroxide, hydroxyl radicals, and hypochlorous acid.[4−8] Another medically important derivative of thiourea is guanylthiourea (GTU) which is generally used as a stimulator of intestinal peristalsis.[9] S-allyl-GTU has been tested as a promising immunostimulant and tumor inhibitor.[10,11] In addition to that GTU is extensively used in industry because of its special structure It enables to be used in the synthesis of anion-caged supramolecular compounds[12] and in vulcanization of natural rubber.[13,14] As seen, these species are extensively involved in biological and technological processes and they are considered as reactive agents; a firm knowledge of the fundamental background of their redox transformations is eagerly expected. Because all of the above-mentioned reactions display clock behavior,[20] it looks to be crucial to understand the kinetic role of the intermediates formed during the oxidation of substituted thioureas

Methods

Results

Discussion

Conclusion