Titrations in non-aqueous media. Part VI. Effects of substituents on basicity or acidity of N-salicylidene-2-hydroxyaniline

Abstract

Fifteen Schiff bases were synthesised from N-salicylidene-2-hydroxyaniline, which was chosen as the model base, by substituting methyl, hydroxy, chlorine, bromine and nitro groups for the hydrogen atoms, which are at the para positions with respect to the hydroxy groups that are already present on the model Schiff base. From each substituent, three derivatives of the model base were prepared stepwise. The effects of each substituent on the basicity or acidity of the model base, which is an amphoteric compound, were investigated; they were titrated potentiometrically either in nitrobenzene as the base or in pyridine as the acid. The following results were obtained. The increase in basicity due to the methyl group is approximately the same whether the latter is on the aldehyde or on the amine component of the model base. The sum of these effects is approximately equal to the separate effects of the methyl groups, when one is on the aldehyde and the other is on the amine component. Except for the basifying strength, the hydroxy group exerts the same effect on the basicity of the model as does the methyl group. A chlorine atom decreases the basicity of the model base more when it is on the aldehyde component than when it is on the amine component. However, the effect is again approximately equal to the sum of the effects of the two chlorine atoms when one is on the aldehyde and the other is on the amine component. Except for the basicity-decreasing strengths, the bromine and nitro functions show the same behaviour as the chlorine atom. Further, the bromine atom has a greater basicity-decreasing effect than the chlorine atom. The basicity-decreasing power decreases in the order NO₂ > Br > Cl > H. In pyridine, the hydroxy group is acidifying when it is on either the aldehyde or amine component, but basifying when it is on both of the components. In pyridine, the chlorine, bromine and nitro functions are all acidifying when they are on the aldehyde or on the amine components or both. The acidifying effects of chlorine and bromine are approximately the same. Moreover, the effect of each substituent is additive.