Acid-base equilibria in nonpolar media. 3. Expanding the spectrophotometric acidity scale in heptane

The UV-vis spectrophotometric ion-pair acidity scale in heptane has been significantly expanded: it includes now 21 bulky CH and NH indicator acids and spans for about 10 pK(ip) units. The phosphazene base t-Bu-P-4 was used for deprotonating. The correlations between acidities in heptane versus gas-phase acidities or acidities in DMSO or 1,2-dimethoxyethane have been made for some compounds. It was demonstrated that the substituent effects on the acidity of the studied CH acids are attenuated ca. 1.24 times when the gas phase is substituted for the nonpolar solvent, heptane. In its turn, for the series of NH acids, the latter is found to be a somewhat more differentiating solvent than DMSO.