The Tautomeric Equilibria of 4-(Dialkylamino)azobenzene Derivatives

Abstract

Abstract The tautomeric equilibrium constants, Kt=[ammonium ion]/[azonium ion], of the first conjugate acids of 4-dialkylaminoazobenzene derivatives were estimated by the spectrophotometric method. The Kt values increase in this order: pyrrolidino- <dimethylamino- <diethylaminoazobenzenes. The effects of N-alkyl groups on the base strength of the amino and azo nitrogens were examined in order to explain the effects on the tautomeric equilibrium. The base strength of azo nitrogens is governed by the degree of the resonance interaction between the amino group and the rest of the molecule. For dimethylamino- and pyrrolidinoazobenzenes, the base strength of the amino nitrogen can also be explained in terms of the resonance effects, but the amino nitrogen of diethylaminoazobenzenes exhibits an anomalously high base strength, arising from the steric inhibition of the hydrogen bonding in the free base. The above order of Kt can be explained in terms of these cumulative resonance and steric effects.