Challenging the Limitations of the Streitwieser Lithium Indicator Acidity Scale with Cyclopentadiene Derivatives

Abstract

This dissertation focuses on linking the Streitwieser Lithium Indicator (SLI) acidity scale to the aqueous Bronsted-Lowry scale to give true pKa values to carbon acids measured on the SLI scale. To achieve this goal, we needed at least one carbon acid that is both water- and THF-soluble and that has an acidity appropriate to measurement on both scales. After considering various options, cyano-substituted cyclopentadiene derivatives were selected for our study. Because the published methods of synthesizing these compounds were not suited to a routinely equipped academic lab, we developed our own synthetic method based on electrophilic cyanation of cyclopentadiene derivatives using tosyl cyanide (TsCN) as the cyanizing reagent. Neutral cyanocyclopentadienes are thermally unstable (likely they polymerize), so we developed workup procedures that maintained these compounds in their anionic, conjugate-base forms (as potassium salts). Using this procedure, several potassium cyanocyclopentadienide derivatives were successfully synthesized in yields ranging from 44-64%. Of the salts synthesized, potassium 1,2-dicyanocyclopentadienide and its inseparable 1,3-isomer were subjected to acidity measurements using 1H and 19F NMR spectrometry in CD3CN solution, bracketing their acidities between two "indicator" acids that were previously established on the SLI scale. This process revealed pKMeCN, pKTHF (SLI acidity) and pKa ("true" aqueous acidity) values for each compound. These compounds were then used to offset the SLI scale against the Bronsted-Lowry (aqueous) scale. Assuming this offset to be constant, all the acids heretofore measured on the SLI acidity scale were finally assigned "true" pKa values.