Solute-solvent interactions in acid-base dissociation: Seven protonated nitrogen bases in water-N-methyl-2-pyrrolidinone solvents

Acidic dissociation constants of seven protonated nitrogen bases, namely Bis-tris, 2-aminopyridine, Tris, morpholine, 2-amino-2-methyl-1,3-propanediol,n-butylamine, andt-butylamine, have been determined at 15, 25, and 35° C in mixtures of water andN-methyl-2-pyrrolidinone (NM2Py). The mole fractions of cosolvent were 0.2, 0.4, 0.6, and 0.8. Glass and silver-silver chloride electrodes were used in a cell without liquid junction, and both the ionic strength and molality of chloride ion remained substantially constant in all of the measurements. The pK_aof all the protonated bases decreases initially and passes through a minimum as the aqueous solvent is enriched with NM2Py. The order of acidic strengths is the same as in the H₂O-dimethyl sulfoxide (DMSO) solvent system, but the pK_avalues are all lower by up to 2 units. NM2Py appears to be a more basic solvent than DMSO or H₂O.On leave 1973–1975 from the University of Gdask, Poland.