Equilibria Involving Organic Anions in Dimethyl Sulfoxide and N-Methylpyrrolidin-2-one: Acidities, Ion Pairing, and Hydrogen Bonding

Article abstract of DOI:10.1021/jo01304a034

Four new indicators, 5-nitrobarbituric acid, 2,4-dinitro-1-naphthol, 4-chloro-2,6-dinitrophenol, and 2,4-dinitrophenol, have been used to complete the anchoring of the Me2SO acidity scale.Two of these indicators plus 2-methyl-4,6-dinitrophenol were used to anchor a new acidity scale in the dipolar nonhydroxylic solvent N-methylpyrrolidin-2-one (NMP).Overlapping indicators and standard acids are now available to cover the pK_a range 0 to 32 in both solvents.Absolute acidities were found to be about 1 pK_a unit lower in NMP than in Me2SO for most weak acids.This is attributed to a slightly lower basicity for NMP.For acids in which the negative charge in the anion is localized on oxygen the acidities were about 2 pK_a units lower in NMP.Homohydrogen bonding ("homoconjugation") constants for carboxylic acids and phenols were found to be about an order of magnitude larger in NMP than in Me2SO.Ion association (ion pairing) constants were also found to be about an order of magnitude larger in NMP than in Me2SO.The larger association constants for NMP are consistent with the more positive free energies of single-ion transfer from water to NMP than from water to Me2SO that have been observed for small anions.Acidity constants for 48 weak acids for the pK_a range 2 to 31 in NMP are reported.Homohydrogen bonding constants for four phenols and two carboxylic acids and association constants for the anions of 10 weak acids with K⁺ are given.It is suggested that solvents of the type NMP and Me2SO be classified as "dipolar nonhydroxylic", rather than as "dipolar aprotic".