Factors influencing the condensation of 4-aminoantipyrine with derivatives of hydroxybenzene—II. Influence of hydronium ion concentration on absorptivity

Abstract

The oxidative coupling of 4-aminoantipyrine with several hydroxy derivatives of benzene has been examined at pH values of 8.0 and 10.0. A reaction mechanism has been proposed. It appears that 4-aminoantipyrine exerts an electrophilic attack upon the para position of the phenol. [H 3 +O] influences this attack in two ways: (a) greater reactivity is observed at pH 8.0 as the phenol exhibits greater dissociation to a phenoxide ion. This is ascribed to more favorable reaction kinetics offered by the stronger electron-releasing properties of the −0 −, and (b) lesser reactivity is observed at pH 10.0 that is ascribed to a reduction of the electrophilic characteristics of the 4-aminoantipyrine. Substitution in the para position of the benzene ring influences the reaction in two ways: (a) oxidative coupling is blocked apparently by such substituents as CH 3-, C 2H 5-, C 6H 5-, and (b) oxidative coupling is altered by such substituents as Cl-, Br-, NH 2-, CH 3-, and C 2H 5O-. The latter may be due to electrophilic displacement and migration of the substituent or to formation of azo-type structures. The data offered herein very strongly shows the limitations of the analytical application of 4-aminoantipyrine for determination of phenols in water and waste water. Some advantage may be gained by conducting the reaction at a pH value of 8.0 where kinetics are more favorable.