Azo compounds typically contain two benzene rings linked by a double-bonded nitrogen bridge. If the benzene rings and their substituents are represented as R, the basic formula for an azo compound is R-N=N-R. Despite some concerns about their use and possible effects, azo compounds are popular as dyes on account of their bright and appealing colours.
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The electrons in azo compounds become extensively delocalised over both the benzene rings and nitrogen atoms. This means the electrons are not associated with a single atom but are found in orbitals that spread out over multiple atoms. Compounds like these have more closely spaced energy levels available to their electrons, so they can often absorb specific wavelengths in the visible range. When white light falls on substances with these compounds, some wavelengths are absorbed while others are reflected or transmitted, which is why they appear so colourful.
Azo compounds are among the most popular synthetic dyes, especially in the clothing and fashion industry. Some paints also contain clay particles coloured with azo dyes. Congo red, aniline yellow, methyl orange and methyl red are all examples of azo dyes with brilliant red, orange or yellow colours. Typically, azo dyes used in clothing have the sulfonic acid groups, R-SO3- attached to one or more of the benzene rings. These groups allow the azo dye to become attached to charged groups on the polymers that make up the fabric. Azo dyes are also used in a variety of cosmetics.
Methyl orange and methyl red are two azo dyes that are often used for titrations in chemistry labs. Because these dyes change colour over a given pH range, chemists can observe the colour of a solution to tell when the pH passes a certain point. Methyl orange turns orange-red in acidic solutions and yellow in neutral or basic solutions, and methyl red exhibits a similar colour shift at a higher but still acidic pH.
In recent years, regulatory agencies have become concerned over the possible carcinogenicity of some azo compounds. The azo dye 4-dimethylaminoazobenzene, or Butter Yellow, was once used as a food colouring but is now considered a suspect carcinogen by the FDA. Other azo dyes derived from benzidine are carcinogens, as well. According to the European Commission, the azo compounds used in cosmetics and the sulphonated dyes used in clothing are not believed to be carcinogenic.
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