Thermodynamic quantities for the solution and protonation of four C6-amines in water over a wide temperature range

The enthalpies of solution ofn-hexyl- and cyclohexylamine have been determined at temperatures up to 125°C, of di-n-propylamine up to 100°C, and of triethylamine up to 75°C from reaction-solution calorimetric experiments. Quadratic polynomials were found to give good representations of the experimental values, implying a linear variation with temperature of the heatcapacity changes. The enthalpies of protonation of the aqueous amines have been determined in the same temperature range. Linear expressions give satisfactory representations of the protonation enthalpies of the hexylamines and dipropylamine, whereas a quadratic expression is required to represent the triethylamine values. The calorimetric results are used together with reported pK_a values at 25°C to calculate the dissociation constants of the corresponding ammonium ions between 0 and 200°C. At 0°C triethylamine was found to be the weakest base, the relative base strengths being triethyln-hexyl