Strongly basic systems. VIII. The H− function for dimethyl sulfoxide – water – tetramethylammonium hydroxide

Abstract

An H− acidity function based on the ionization of 24 substituted anilines and diphenylamines has been established in the system dimethyl sulfoxide – water – tetramethylammonium hydroxide. The H− of a 0.011 M solution of tetramethylammonium hydroxide ranges from 12 in water to 26.2 in 99.6 mole % dimethyl sulfoxide – water, an increase in basicity of 14 powers of 10. The increase in basicity is due to the increased activity of the hydroxide ion brought about by the reduction in its solvation in the poor anion-solvating solvent dimethyl sulfoxide, and indicates the extensive solvation enjoyed by the hydroxide ion in water.The pKHA values of the indicator acids vary from 13.9 for 2,4-dinitrodiphenylamine to 25.6 for 3-chloroaniline. From a plot of log KHA versus the Hammett substituent constants (σ) for six monosubstituted diphenylamines, a ρ value of 4.07 is found. Similar results are obtained for the anilines. The acidities of all the substituted diphenylamines do not follow the above-mentioned correlation with the Hammett σ constants; the pKHA values of 4-amino-, 4-methoxy-, 4-methylsulfonyl-, and 4-nitro-diphenylamine are all less than expected from the Hammett σ constants for the substituents in these compounds. The 4-nitro substituent exerts a particularly large acid-strengthening effect on the acidities of aniline and diphenylamine, the decreases in pKHA being approximately 8.4 and 6.8 pK units, respectively. An exalted substituent constant of +1.65 for p-NO2 is needed to account for the acidity of p-nitro-diphenylamine.