Protolytic properties of certain alkyl and aryl derivatives of imidazole

Abstract

Acidity constants of certain alkyl and aryl derivatives of imidazole in water and in acetone were determined by an extrapolation method. Certain solvents were evaluated in terms of their leveling or differentiating effect on the acidic properties of the imidazoles.Imidazoles substituted in position 2 have found extensive applications as catalysts for curing epoxy resins [1][2][3] and also in the determination of hydroxyl groups in polymers by the phthaloylation method [4, 5].We can assume that the curing rate of epoxy resins will be determined by the nucleophilicity of the catalyst and that the protonation constants of imidazoles can be regarded as a quantitative characteristic of their nucleophilicity. Since imidazoles are used in reactions that take place in nonaqueous solvents or mixtures, information is needed on their protolytic characteristics in such solvents.Data on the acidity of imidazoles [6] are limited and somewhat ambiguous. The work reported in the present communication was aimed at investigating the protolytic characteristics of certain imidazole derivatives in water, water/acetone solutions, and nonaqueous solvents.In this work, we used the following imidazole derivatives: 2-phenyl (I), 2-undecyl (II), 2-heptyl (III), 2-hexyl (IV), 2-ethyl (V), 2-methyl (VI), 2,4-dimethyl (VII), and 2-ethyl-4-methyl (VIII).Most of the imidazole derivatives are insoluble in water; therefore, we attempted to determine protonation constants by a method based on successive dilution of acetone solutions with water to obtain extrapolation relationships PKai = aC a + b, where C a is the mass fraction of acetone in the water/acetone solution; a is the slope of the linear relationship; b is the intercept of the extrapolation line on the ordinate axis.These linear relationships are shown in Fig. 1, and the results of a statistical workup by the least squares method [7] are presented in Table 1. The protonation constants of compounds I, V, and VI for aqueous solutions are in good agreement with values reported in the literature. The refined value of pK a for the imidazole VII is 7.96 (an approximate value of 8.5 has been reported in the literature).The acidic properties of the imidazoles in water increase in the following order: VIII < V < VII <IV < I][I <VI < II < I.With C a = I, the values of pK a obtained from the extrapolation equations are such that this series can be l'epresented as follows: VIII < VI < V < IV < HI < VII < II < I.

Phthalic anhydride (PA): a valuable substrate in organic transformations

“…Gitis group in 2000 reported the reaction of PA with 2-methylimidazole that formed amide. 91 The authors also examined the reaction of maleic anhydride that leads to the formation of molecular complexes.…”