Acid−Base Equilibria in Nonpolar Media. 2.¹ Self-Consistent Basicity Scale in THF Solution Ranging from 2-Methoxypyridine to EtP₁(pyrr) Phosphazene

Abstract

Relative ion-pair basicities Delta(pK)(ip) of 25 substituted aryl and alkyl iminophosphoranes (phosphazenes) and 20 other N-bases (various pyridines, amines, amidines) have been measured in THF medium using the UV-Vis and/or (13)C NMR methods. The Delta(pK)(ip) values were corrected for ion pairing using the Fuoss equation to obtain relative ionic basicities Delta(pK)(alpha). Based on the measurements, a basicity scale ranging from 2-methoxypyridine to EtP(1)(pyrr) and having a total span over 18 pK units has been created. The scale has been anchored to the pK(alpha) value of triethylamine (pK(alpha) = 12.5). The results are compared to pK(a) values in various other solvents and in the gas phase. The pK(alpha) values give better correlations than the pK(ip) values, thus indirectly validating the procedure of correction for ion pairing. The predictability of the basicity together with suitable spectral properties in the UV range make the phenylphosphazenes convenient neutral indicators in the high basicity range where the choice of neutral indicators is very limited.