Basicity of some aliphatic amines in mixed H2O?CH3CN solvents

Abstract

The values of pKams (Kams represents ionization constant of conjugate acid of amine base in mixed water–acetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1–0.4 pK units) with the increase in CH3CN content from 2 to ∼60% v/v. However, the pKams values at 70% v/v CH3CN become nearly equal or slightly larger (by ≤0.7 pK units) than the corresponding pKams at 2% v/v CH3CN for all neutral and charged amines. The values of pKams for phenol increase from 10.17 to 13.38 with the increase in the content of CH3CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, ρ*, obtained from the plots of pKams against ∑σ* for primary and secondary amines decrease by ca. 0.8 ρ* units with the increase in the CH3CN content from 2 to 70% v/v. The values of pKams show an empirical linear relationship with the corresponding values of pKaw (where pKaw represents the pKa obtained in aqueous solvent containing 2% v/v CH3CN), which allows the estimation of a pKa in mixed H2OCH3CN solvents from that in water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 146–152, 2000