Azo dye
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Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C-N=N-C linkage.
Contents
General
Azo dyes are synthetic dyes and do not occur naturally. Most azo dyes contain only one azo group, but some dyes called "disazo dyes" contain two azo groups, some dyes called "trisazo dyes" contain three azo groups and are or more.
Many kinds of azo dyes are known, and several classification systems exist. Some classes include disperse dyes, metal-complex dyes, reactive dyes, and substantive dyes.
Azo dyes are solids. Most are salts, the colored component being the anion usually, although some cationic azo dyes are known. The anionic character of most dyes arises from the presence of 1-3 sulfonic acid groups, which are fully ionized at the pH of the dyed article. Some azo compounds, like methyl orange, are used as acid-base indicators.
Preparation
Most azo dyes are prepared by azo coupling, which entails an electrophilic substitution reaction of an aryl diazonium cation with another compound, the coupling partner.
Common azo dyes
- Acid orange 7
- Chrysoine resorcinol
- Congo red
- Eriochrome Black T
- Methyl orange
- Methyl red
- Methyl yellow
- Tartrazine
Uses
- Coloring dye
- pH indicator
- Colored smoke
