Acid–base and hydrogen-bonding equilibria in aliphatic amine and carboxylic acid systems in non-aqueous solutions Original Research Article

Acid–base and hydrogen-bonding equilibria in acetic acid – acetate, n-butylammonium cation – n-butylamine and acetic acid (acetate) – n-butylamine (n-butylammonium cation) systems were studied in five polar non-aqueous solvents with varying prototropic and dielectric properties: acetonitrile, acetone, dimethyl sulfoxide, methanol, and propylene carbonate using the potentiometric-titration technique. These systems were designed to model the acid–base and hydrogen-bonding phenomena that involve acid and basic amino-acid side chains in proteins. The obtained order of pKa values of acetic acid and n-butylamine is consistent with the variation of prototropic, basic, dielectric, and hydrogen-bonding properties of the solvents used. The homoconjugation and heteroconjugation constant values decrease with increasing basicity of the solvents; they turn out to be indeterminable in dimethyl sulfoxide (the most basic solvent) for cationic homoconjugation and in methanol (the second most basic solvent) for anionic homoconjugation and heteroconjugation.