Reactions in strongly basic media. Part 8. Correlation of the rates of alkaline hydrolysis of 2,4-dinitroanisole and 2-methoxy-5-nitropyridine in aqueous dipolar aprotic solvents with acidity functions. An order of basicity for aqueous dipolar aprotic solvents

Abstract

The rate coefficients for the alkaline hydrolysis of 2,4-dinitroanisole and 2-methoxy-5-nitropyridine have been measured in a series of aqueous dipolar aprotic solvents. These rates have been correlated with the acidity function H_– for aqueous dimethyl sulphoxide, dimethylformamide, sulpholane, and pyridine, which were available, as well as for aqueous acetonitrile, tetramethylurea, and hexamethyl-phosphoramide, whose acidity function has been constructed. For aqueous dioxane, a satisfactory acidity function could not be constructed. The order of the solvents in increasing the basicity is: hexamethylphosphoramide dimethyl sulphoxide > tetramethylurea > dimethylformamide > pyridine > sulpholane acetonitrile > dioxane. The latter order appears to be related directly to the hydrogen-bonding capacity of the solvent. Except for pyridine, the slopes of the rate-acidity function correlations have been found to be solely a function of the substrate and independent of the dipolar aprotic solvent used. Pyridine catalyses the substitution reactions by direct nucleophilic catalysis.