Targeted and Systematic Approach to the Study of pK_a Values of Imidazolium Salts in Dimethyl Sulfoxide

A range of more than 25 imidazolium salts, chosen for their differing steric and electronic features, were prepared, and their pK_a values were determined using the bracketing indicator method. Through the systematic change in the structure of the imidazolium cation, the effect of varying substituents at each position on the heterocyclic ring was determined; particularly, the transmission of electronic effects was quantified using Hammett parameters. These new data give an indication of the strength of base required for deprotonation and the potential to correlate these data with the nucleophilicity of the corresponding carbenes.