Dissociation Constants (pKa) of Tertiary and Cyclic Amines: Structural and Temperature Dependences

Abstract

The dissociation constants of the conjugate acids of 14 amines (diethylethanolamine, monoethanolamine, n-butyldiethanolamine, t-butyldiethanolamine, n,n-dimethylpropanolamine, methyl-diethanolamine, ethyldiethanolamine, monoethylethanolamine, n,n-dimethylisopropanolamine, triethanolamine, 4-methylpiperazine-1-amine, 3-morpholino propylamine, 4,2-hydroxylethylmorpholine, and triethylamine) were measured over a temperature range between 293.15 and 333.15 K using the potentiometric titration method. The change in standard state thermodynamic properties was derived from the van’t Hoff equation. The influence of the steric hindrance, number of −OH groups, and length of alkyl chain on the dissociation constants was identified. Of the studied amines, few sterically hindered derivatives of piperazine, a secondary amine monoethylethanolamine, and a tertiary amine n,n-dimethylpropanolamine have high pKa values but lower standard enthalpy than those of the benchmark amine, monoethanolamine (MEA), and thus were deemed promising for CO2 capture technology. Monoethylethanolamine (MEEA) was found to have the highest basicity (pKa) with the lowest standard state enthalpy (ΔH°/kJ·mol–1).