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

Acid-base and hydrogen-bonding equilibria in acetic acid - acetate, n-butyl ammonium cation - n-butylamine and acetic acid (acetate)-n-butylamine (n-bu tylammonium 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 potentiome tric-titration technique. These systems were designed to model the acid-bas e and hydrogen-bonding phenomena that involve acid arid basic amino-acid si de chains in proteins. The obtained order of pK(a) values of acetic acid an d n-butylamine is consistent with the variation of prototropic, basic, diel ectric, and hydrogen-bonding properties of the solvents used. The homoconju gation and heteroconjugation constant values decrease with increasing basic ity of the solvents; they turn out to be indeterminable in dimethyl sulfoxi de (the most basic solvent) for cationic homoconjugation and in methanol (t he second most basic solvent) for anionic homoconjugation and heteroconjuga tion. (C) 1999 Elsevier Science B.V. All rights reserved.