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The thermodynamic functions Δ G o n , Δ H o n and Δ S o n of the two steps of proton ionization of N,N′-dimethylpiperazine, 2,6-dimethylpiperazine, 2,5-dimethylpiperazine ( cis ) and 2,5-dimethylpiperazine ( trans ) are determined at 25°C in water and water—ethanol (52%, w/w) solvents, including 0.1 M KNO 3 as ionic strength. Free energies are calculated from the ionisation constants obtained by pH-metric method; enthalpies are determined from calorimetric measurements. The comparison of these functions and of the corresponding entropies is discussed in terms of the position of CH 3 groups in each molecule and takes account of previous results relative to piperazine and its monomethyl derivatives.

Stability constants of silver complexes with the substituted ligands 2-methyl-2,5-dimethyl-, 2,6-dimethyl-, 2-chloro- and 2-amino-pyrazines are determined by means of a potentiomeric method, using metal complex electrodes at 25°C and 0.1M KNO 3 as an ionic strength in aqueous medium. Standard enthalpies of the two steps of complexation observed are obtained by a direct calorimetric technique (except for the Ag-2-aminopyrazine system whose complexes are not sufficiently soluble) under the same experimental conditions. The corresponding standard entropies are calculated. The discussion of the thermodynamic functions takes into account the nature and the position of the substitutents to interprete the coordination ability of each amine.

Resume Les grandeurs des fonctions thermodynamiques standard (Δ H n o , Δ S n o et Δ G n o ) de protonation de la piperazine et de ses derives n-methylpiperazine, 2-methylpiperazine et n-phenylpiperazine ont ete determinees par deux voies experimentales distinctes: d'une part au moyen des constantes de protonation mesurees a diverses temperatures (20, 25, 30, 35, 40°C); d'autre part en utilisant des donnees calorimetriques directes obtenues au voisinage de 25°C. L'ensemble des experiences a ete effectue dans des conditions de milieu identiques employant KNO 3 0,1 M comme electrolyte support. Les resultats sont discutes en termes de nature et de position d'un groupement substituant dans un coordinat donne.