Kinetics of iodination of 4-methylimidazole and 2-methylimidazole

Abstract

Since the rate law for the diiodination of imidazole provides information on the iodination of C4(5) but not C2 in the imidazole ring,3,4 we have investigated the kinetics of iodination of the two positions separately in 2-methylimidazole and 4-methylimidazole, respectively. The observed rate laws for the methylimidazoles exhibited hydrogen ion dependencies that differed from that of imidazole; the iodinations of all three substrates were base catalyzed, but like imidazole, 2-methylimidazole undergoes uncatalyzed iodination, while 4-methylimidazole does not. The overall rate of iodination of 4-methylimidazole was faster than that of 2-methylimidazole which was faster than that of imidazole.

Carcinogenic 4(5)-Methylimidazole Found in Beverages, Sauces, and Caramel Colors: Chemical Properties, Analysis, and Biological Activities

“…A lipid oxidation study indicated that 4(5)-MI exhibited significant prooxidative activity . Additionally, a kinetic study on the iodination of imidazole demonstrated the role of a methyl substituent in the rate of iodination, which was, in decreasing order, 4(5)-MI > 2-methylimidazole > imidazole …”

“…26 Additionally, a kinetic study on the iodination of imidazole demonstrated the role of a methyl substituent in the rate of iodination, which was, in decreasing order, 4(5)-MI > 2-methylimidazole > imidazole. 27 There are three methylimidazole isomers: 2-, 4-, or 5methylimidazole. Among those, 4-and 5-methylimidazoles are present as tautomers in an aqueous solution at neutral-to-basic pH as shown in Figure 3.…”

Kinetics of the Rapid Base Catalyzed Iodination of Imidazole and 2‐Methylimidazole by Molecular Iodine in Aqueous Medium: pH Effect

“…The value of the rate constant for the iodination of II found to be lower than that of iodination of I using molecular iodine as an iodinating agent. This could be because of the following: The methyl group decreases the acidity of the N 1 position, which reduces the equilibrium concentration of the conjugate base . The methyl group shows a very minor inductive (+I) effect, which does not markedly enhance the specific reaction rate, when compared with I . The methyl group is sandwiched between N 1 and N 3 , which are strong electron‐withdrawing groups suppressing its electron‐donating effect. As compared to I , in II the position two is blocked by the methyl group.…”

“…1. The methyl group decreases the acidity of the N 1 position, which reduces the equilibrium concentration of the conjugate base [6]. 2.…”

“…Earlier investigators have studied the iodinations of five‐member ring substrates using iodine solutions containing I − ions and HClO 4 aqueous solutions. In the present studies, the solutions are totally devoid of I − ions and HClO 4 hence the formation of ions by molecular iodine is prevented.…”