Comparative quantitative kinetic study of rapid iodination of thiazole and its methyl derivatives in aqueous medium

The study provides a quantitative framework for the assessment of the relative reactivity of thiazole and its two methyl derivatives namely 2-methylthiazole and 4-methylthiazole towards iodination reaction. Thiazole and its derivatives have a wide range of industrial and medicinal applications attributed to the ability of thiazole to manifest substituents around the core scaffold. The kinetic study of the iodination reactions of these substrates helps in the above applications since kinetics is an important investigational tool in configuring the mechanism of the reaction. In the present work, molecular iodine (I-2) and iodine monochloride (ICl) were used as reagents for the iodination of thiazole, 2-methylthiazole, and 4-methylthiazole in aqueous medium. The iodination reactions are rapid electrophilic substitution reactions and hence special technique known as hydrodynamic voltammetry was employed for the kinetic study. Iodine is the only electro-reducible species in the reactions. As the solution is devoid of iodide ions, a decrease in the concentration of iodine is measured in terms of diffusion current using a rotating platinum electrode with a saturated calomel electrode (SCE) as a reference electrode. The reactivity correlation of substrates towards iodination reaction was quantitively assessed by determining the reaction rate constants and thermodynamic parameters. The study shows overall rate of iodination of 4-methylthiazole is faster than 2-methylthiazole which is faster than thiazole. The study helps to develop a mechanistic approach considering the effect of substituent present on the thiazole ring. The order of reactivity of the reagents was determined based on obtained rate constants and explained owing to the nature of the reagent.