Determination of acid dissociation constants of some hydroxy schiff bases by pH-metric titration. application of the Hammett equation

摘要

Ten compounds hydroxybases Schiff were synthesized from salicylaldehyde and the substituted aniline in the ortho, meta and para methyl groups, chloro and nitro. To study the effect of substituents on their acidities, the acid dissociation constants( Ka) of salicylideneaniline and some of their derivatives were determined by pH-metric titration. All Schiff bases were titrated with NaOH in mixtures of 60% dioxane-water and 60% ethanol-water at a constant ionic strength and a temperature of 25 °C. The calculated acidity constants, pKa values were evaluated in protonation –deprotanation mechanisms. The Hammett relationship linear type was applied to quantify the effects of substituents on the acidity of hydroxybases Schiff, and therefore a reaction constant, ρ is calculated.