Liquid ammonia solutions. IX. An acidity scale for liquid ammonia

Abstract

LIQUID AMMONIA SOLUTIONS. IX. AN ACIDITY SCALE FOR LIQUID AMMONIA J. H. Takemoto and J. J. Lagowski Department of Chemistry, The University of Texas at Austin Austin, Texas, 78712 ~Reeeived 30 J a n ~ 1970) Since the work of Conant and Wheland (i) and McEwen (2), the subject of the acidities of weak hydrocarbon and nitrogen acids in various nonaqueous solvent systems has been an area of expanding interest (3). There have been numerous qualitative examples of the use of liquid ammonia as a reaction medium in which extremely basic species could exist (4) as well as attempts to establish relative scales of acidities involving a variety of compounds (5-8). Quantitative estimates of the extent of ionization of weak acids in ammonia have been made using potentiometric (9, I0) and spectrophotometric (ii, 12) data. The purpose of this communication is to suggest a method of linking the quantitative results recently obtained on carbon acids using spectroscopic data (ii) to the relative acidities of an extensive series of substituted anilines obtained from nmr measurements (5). As was pointed out earlier (5), the lower acidity limit of the nmr method occurs for compounds with acidities that lie between toluene and diphenylmethane. The spectroscopic method (ii) which depends upon establishing the concentrations of the weak acid, its conjugate base, and amide ion from a quantitative analysis of the spectra of mixtures of these species is not amenable to acids as strong as the anilines for which the nmr method was used. Acids which undergo partial ionization in pure liquid ammonia have been studied spectroscopically (12), however these are much more acidic than the anilines. Thus, the relative acidities of the