Лако-красочные материалы — производство

2.1.1 Introduction

The azo dyes are by far the most important class, accounting for over 50% of all commercial dyes, and having been studied more than any other class. Azo dyes contain at least one azo group (—N=N—) but can contain two (disazo), three (trisazo), or, more rarely, four (tetrakisazo) or more (polyazo) azo groups. The azo group is attached to two groups, of which at least one, but more usually both, are aromatic. They exist in the trans form 1 in which the bond angle is ca. 120°, the nitrogen atoms are sp2 hybridized, and the designation of A and E groups is consistent with C. I. usage [1].

A.—N

"n—E

1

In monoazo dyes, the most important type, the A group often contains elec­tron-accepting substituents, and the E group contains electron-donating substitu­ents, particularly hydroxyl and amino groups. If the dyes contain only aromatic groups, such as benzene and naphthalene, they are known as carbocyclic azo dyes. If they contain one or more heterocyclic groups, the dyes are known as het­erocyclic azo dyes. Examples of various azo dyes are shown in Figure 2.1. These illustrate the enormous structural variety possible in azo dyes, particularly with polyazo dyes.