No abstract

Despite the utility of imidazoles for a wide variety of chemical and biological applications as well as the growing research in imidazolium-based ionic liquids (ILs), synthetic studies and characterization data for N-functionalized imidazole derivatives with substituents present at the C(2) and/or C(4) and/or C(5) positions are generally unreported. Here, we modify our prior method for synthesizing monofunctionalized imidazoles and apply it to the production of a library of 30 di- and trifunctionalized alkylimidazoles using only commodity chemicals and avoiding anhydrous solvents or air/water-sensitive reagents. For all products, purities of >98% could be readily achieved, although yields were lower than in our prior work with imidazole, which may be due to mass transfer limitations and/or increased nucleophilicity of substituted imidazole products. Interestingly, we also observe that, when 4-methylimidazole or 2-ethyl-4-methylimidazole is used as a starting material, two regioisomers are inevitably formed. We employed electronic structural calculations to aid in identifying the chemical shifts and quantifying the relative presence of the regioisomers. In both series of compounds where regioisomers could be formed, the 4-methyl regioisomer was favored. Although the formation of similar regioisomers has been previously noted in the literature, it has perhaps not been fully considered in works related to imidazolium-based ILs.

In this work, the QCM and TGA methods were used concurrently to study the two alkoxy-substituted ionic liquid (IL) series: 1-[oligo(ethylene glycol)]-3-methylimidazolium bis(triflamide) ([P x mim][NTf 2 ]) and 1-[oligo(ethylene glycol)]-2,3-dimethylimidazolium bis(triflamide) ([P x mmim][NTf 2 ]). For comparison, enthalpies of vaporization measured at elevated temperatures were adjusted to the reference temperature 298 K and tested for consistency. It was found that the vaporization enthalpies of the alkoxy-substituted ILs are significantly lower than those of the analogous ILs with the alkyl-substituted cation. This is in contrast to molecular solvents, for which alkoxy groups are typically observed to increase vaporization enthalpy relative to those of the hydrocarbon analogues. Two useful group contributions for the quick estimation of vaporization enthalpies of various alkoxy-substituted IL cations (e.g., imidazolium, ammonium, pyridinium) are recommended based on the findings of this work.

The imidazole structure offers a versatile means of developing molecules with controlled/tunable physicochemical properties that have significant utility in many applications and can be further derivatized to form ionic liquids. In the literature, the vast majority of studies on structure−property relationships in these types of molecules are devoted to linear (e.g., n-alkyl) substituents. However, imidazoles with branched or cycloalkyl groups are equally accessible through convenient synthetic methods − yet there are essentially no reports on the physical properties of such compounds in the literature. Here, the absolute vapor pressures of branched and cycloalkyl derivatives of imidazole have been determined as a function of temperature by the transpiration method. The standard molar enthalpies of vaporization were derived from the temperature dependences of vapor pressures. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent species, and a group contribution method is put forth by which the vaporization enthalpies of imidazoles, and imidazolium-based ILs, with alkyl groups in any configuration can be rapidly predicted.