(570ad) Development of An Acidity Scale for Brønsted Acidic Ionic Liquids

Ionic liquids and the technologies they enable are poised to initiate a significant change in the way chemical processes and consumer products are designed. The ionic nature of these species imparts them with vanishingly-low vapor pressures, which significantly limits the risk of fugitive emissions as compared to volatile organic solvents. Also, the organic nature of the cation, and often the anion, allows the properties of ionic liquids to be tuned at the molecular level using the broad palette of organic chemistry. One key manifestation of their potential is their use as catalysts. The ionic liquids group at South Alabama has arguably played a seminal role in the development of ionic liquids which are intrinsically strong Brønsted acids ? the first (to our knowledge) strong liquid acids essentially devoid of corrosive fumes or vapors. In many cases, it is desirable to have the strongest available acid, however, there are also situations where a moderately strong acid is preferable to promote the desired reaction and to minimize by-product formation. However, there is no general scale to describe relative acidity in an ionic liquids solution akin to pKa in aqueous solutions. In this project we are developing a methodology to characterize the relative acid strength of acidic ionic liquids in an number of different chemical environments include the pure ionic liquid, dilute in a neutral ionic liquids and dilute in aqueous solution (pKa). Additionally, we will use this acidity scale to probe the structure/property relationship to understand how to tune the acidity of different species. To develop the scale, we are using the relative reactivity of the acidic ionic liquids with diazodiphenylmethane (a bright red dye) as the basis for the scale, a method employed by Shorter and Chapman in describing acidities for organic acids in non-aqueous media. The rate of reaction is controlled by the acid strength and can be measured via UV-Vis spectroscopy, allowing for facile data collection. In this work we will present our methods and initial results.