Titrations in non-aqueous media. Part XVII. Potentiometric investigation of effects of several electron-donating and -withdrawing substituents on the basicity of the azomethine group of salicylideneaniline in nitrobenzene

Abstract

Methyl, hydroxy, methoxy, carboxy, nitro, chloro, bromo and iodo substituted salicylideneaniline (Schiff base) have been prepared to investigate the effects of the substituents on the basicity of salicylideneaniline (standard). For the sake of clarity, substitutions were made only on the amine component (aniline) of salicylideneaniline. For each substituent three isomers, ortho, meta and para, have been synthesised. Investigations have been carried out potentiometrically with perchloric acid in nitrobenzene. All the compounds gave well shaped titration curves (sigmoids), from which half-neutralisation potentials were calculated. These potentials were compared with the half-neutralisation potential of salicylideneaniline itself. The methyl, methoxy and hydroxy groups all increased the basicity of the standard irrespective of their positions on the amine component. The carboxy group increased the basicity only when in the ortho position. The nitro, chloro, bromo, and iodo groups decreased the basicity at any position on the amine component. The ortho-hydroxy, ortho-methoxy, ortho-carboxy and ortho-nitro substituted derivatives were more basic than their meta or para isomers. Contrary to general belief, this is explained by the formation of intramolecular nine- or ten-membered hydrogen-bonded rings. The basicity order, including the standard, was found to be o-OMe > o-OH > o-CO₂H > standard > o-NO₂. On the contrary, all the ortho-halogen derivatives were less basic than their meta or para isomers. The basicity order was standard > o-Cl > o-Br > o-l. The basicity order of the halogens (excluding fluorine) was in the opposite direction to the electronegativity order.