AN ACIDITY FUNCTION FOR CARBON BASES IN AQUEOUS SULFURIC AND PERCHLORIC ACIDS

The degree of equilibrium protonation of eleven carbon-protonating bases (substituted azulenes, 1,1-diarylethylenes, and aromatic polyethers) has been determined as a function of the concentrations of aqueous sulfuric and perchloric acids. Plots of log ([BH+]/[B]) against —Ho for all bases are nearly parallel in regions of overlap, with slopes near 1.6 for sulfuric acid and 1.7 for perchloric acid. The data were used to determine the acidity constants of the indicator cations and to establish a new Hammett-type acidity function, Hc. In the concentration range from 1 to 14 moles/1., sulfuric acid is 0.2-0.9 unit less acidic on the Hc scale than on the HR′ scale. We believe that Hc is more appropriate than HR′- to the protonation of carbon bases, and we obtained activity coefficient data to support this view. © 1969, American Chemical Society. All rights reserved.