Synthesis, spectroscopic and molecular structures investigations of some carboxylated schiff bases

A series of nine carboxylated Schiff bases (five of them are newly prepared viz. compounds 5–9), are prepared and characterized by various physico-chemical techniques. The molecular structures of synthesized Schiff bases are investigated by IR, UV–Visible, molar conductivities at different concentrations in two different solvents and by their pH values in ethanolic solutions. The IR spectra show absorptions due to ZN 4 H– stretching and –N–H bending vibrations, the UV–Visible spectra indicates absorptions are due to protonated species. The molar conductivities, 0.1–0.6 UK1 cm2 molK1, prove that these compounds are weak electrolytes and are even weaker than tyrosine and phenylalanine, 2.5–13 UK1 cm2 molK1. The melting points and pH values of Schiff bases are compared with those of some a-aminoacids and some related Schiff bases that have no COOH group in their structures. On the bases of these data, it was concluded that carboxylated Schiff bases exist in two forms, the ionized and the free base where the later is predominant. The ionized form is similar to the zwitterion of the a-aminoacid, in which a proton is transferred from COOH to the azomethine (–CHaN–) group. q 2004 Published by Elsevier B.V.