Die reduktive Spaltung von Azofarbstoffen durch Dihydrochinoxaline

Abstract

Abstract The kinetics of the reduction of substituted azobenzenes by dihydroquinoxalines have been measured. Azo‐compounds with electron‐donating substituents in o ‐ or p ‐position are reduced to the amines in neutral or acid solution, whereby two molecules of the reducing agent are consumed. If such substituents are absent, the reaction stops at the hydrazo stage. This is also the case with cleaveable azo‐compounds in strongly alcaline solution. Our kinetic measurements of the reductive cleavage reaction of methylorange are in agreement with only one mechanism, which is discussed in detail. The very high reactivity of protonated methylorange may be attributed to the formation of a mesomeric quinoid structure. Knowing the pH‐dependence of the reaction rate allows the determination of the p K ‐value of the extremely unstable hydrazo stage of methylorange. The structure of the dihydroquinoxaline has a strong influence on Δ H ‡ and Δ S ‡, and therefore on the reaction rate.