Solution Chemistry of Proton Complexes

Bencini, A., Bianchi, A., Garcia-Espana, E., Micheloni, M., Ramirez, J. A., Proton coordination by polyamine compounds in aqueous solution, Coord. Chem. Rev. 1999, 188, 97-156. 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMes NHMe2 NH2Me NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 

Note it is important to specify temperature, solvent (usually water) and solution ionic strength, /. Basicity constants are related to the acid dissociation constants (KJ of the base s conjugate acid via the dissociation constant of water, = 10 at 25 °C. Thus K xK = K . 

Unfavourable intramolecular repulsions between NH+ groups are highly dependent on the distance between the protonation sites and thus the introduction of a longer, propylenic spacer generally increases the second and subsequent basicity constants as well as influencing the first by increased electron donation. Compare entries 3 and 4 in Table 3.16. The effect is also evident in the linear 

A-methylation changes secondary amines into tertiary amines. While inductive effects might mean an increase in the amine gas phase basicity, the low solvation of tertiary amines generally means that methylation reduces solution basicity (compare entries 1 and 7). In addition to solvation effects, it is thought that steric effects may favour the out conformer. In the case of tetramethylcyclam (entry 8) 

---