Dissociation and liquid–vapour equilibria of some amines in water at elevated temperatures

Abstract

Base dissociation constants and steam–water partition coefficients for the amines cyclohexylamine, piperidine, 2-amino-2-methylpropan-1-ol (AMP), and quinuclidine have been determined in the temperature range 100–300 °C from liquid–vapour equilibrium measurements. It has been shown that the temperature dependence of the dissociation constants of the conjugate acids (K_a) between 0 and 300 °C can be expressed, within experimental error, by an equation of the form: ln K_a=a₁+a₂/T+a₃ ln T where a₁, a₂, and a₃ are independent of temperature and pressure. A similar equation involving steam density can be applied to partition coefficients.

Better concordance between low- and high-temperature dissociation data, particularly ΔC_p° values at 298 K, is obtained by using the four-term equation: ln K_a=a₁+a₂/T+a₃ ln T+a₄/T²

The rate of decrease of base strength with temperature is less for quinuclidine than for the other bases, although the order of base strengths, quinuclidine > piperidine > cyclohexylamine > AMP, is similar at 25 and 250 °C. There does not appear to be a correlation between volatilities at 100–300 °C, where piperidine ≈ cyclohexylamine > quinuclidine > AMP, and basicities over the same temperature range.