Abstract
N-Aminopyridinium iodide (NAPI) turned a bluish purple color upon the addition of NaOH. The NMR spectral data of the NAPI-NaOH system indicate that N-1-pyridinio amidate (NPA) is the species responsible for the coloration, which is probably due to intramolecular charge transfer transition. The intensity of the coloration increased remarkably with addition of benzaldehyde derivatives, especially p-nitrobenzaldehyde, to the NAPI-NaOH system. NAPI reacted with the benzaldehyde derivatives to form the corresponding Schiff bases. The mechanism of the coloration in the NAPI-NaOH-benzaldehyde derivative system is proposed to involve the formation of betaines, which can also be formed by adding NaOH to the isolated Schiff bases. Furthermore, there was a good correlation between the enhancing effects on the coloration and the redox potentials of the benzaldehyde derivatives. This indicates that benzaldehyde derivatives with an electron-withdrawing substituent group can enhance the coloration intensity much more effectively.
