On the mechanism of protonation of triamines

The protonation behaviour of several triamines having the general formula H2N[CH2]mNH[CH2]nNH2 with m⩽n has been studied by 13C and 1H n.m.r. techniques. In all cases, in the first two protonation steps, only primary amino-groups are involved. The protonation of the secondary amino-group starts only after the protonation of the primary ones is complete.In the case of symmetrical triamines, in the first step, the proton is shared by the two primary nitrogens to an equal extent. Unsymmetrical triamines having m < 3 are initially protonated on the primary nitrogen atom attached to the longer aliphatic chain. However the unsymmetrical triamine spermidine, having m= 3 and n= 4. behaves like the symmetrical ones.