Influence of the distance between ionizable groups on the protonation behavior of various hexaamines

A synthesis route for N,N,N′,N′-tetraaminoethyl-1,2-ethylenediamine, N,N,N′,N′-tetraaminopropyl-1,2-ethylenediamine, N,N,N′,N′-tetraaminopropyl-1,3-propylenediamine and N,N,N′,N′-tetraaminopropyl-1,4-butylenediamine is presented. These molecules differ from each other in the number of carbon atoms between the six amino groups. This results in different protonation behavior. Potentiometric titrations are performed in 0.1 M and 1.0 M KCl, and the six macroscopic protonation constants are obtained from these curves. An Ising model with a limited number of microscopic protonation constants and short-ranged pair interactions describes the protonation behavior quantitatively. The results are compared to those of other, similar molecules. The advantage of the Ising model over empirical relations such as the Taft equations is the more systematic approach with which the titration curves of more complex molecules can be described. The values for the Ising model parameters obtained here can be used to predict the protonation behavior of more complex, in particular larger, polyamines.