Azo chemistry is one of the most widely used methods of making synthetic dyes and pigments. The term azo refers to a nitrogen double bond within the molecule. The discovery of nitrogen is attributed to Daniel Rutherford who was the first to isolate the gas and demonstrated that it was an asphyxiant and did not support combustion. Azo is abbreviation for azote,
the name proposed by French chemist Antoine Lavoisier for the element nitrogen. Azote derives from the Greek a (not) and zoe (to live).
William Henry Perkin is credited with the discovery, albeit accidental, of the first synthetic organic dye called mauvine in 1856. The key to synthesis of mauvine was analine, an aromatic amine. In the 1800s, the burning of coal became an important source of heat and light in many European and American cities. Burning of coal leaves behind a viscous, brown liquid that was called coal tar because it resembled the pine tar.
In failed experiments aimed at making quinine, Perkin created the bright purple dye from coal tar derived benzene. During a time when most fabric dyes were based on natural sources that were expensive to produce and highly variable in quality, Perkin patented the dye making process and promptly reigned from college in order to set up the business of manufacturing the dye.
Many new dyes were synthesized from aniline, eventually leading to the development of azo dyes. Synthetic organic pigments are sometimes referred to as coal tar dyes today, but the term is outdated as modern cosmetic dyes are prepared from petroleum distillates. Azo pigments account for most of the red, orange, and yellow pigments used in cosmetics including Red 6 and 7 Lakes.
A quick look at the FDA’s reports on total pounds certified reveals Red 6 and 7 lakes are the most widely used. This is due to their bold hues, excellent economic value, and good stability properties. They are the two main colorants used to create bright red lipsticks and nail polish with Red 6 being a yellow-shade red and Red 7 a blue-shade.