A Brief Chemical History of Purple Dyes

Visiting classrooms and observing the outstanding teachers at LCHS is always a learning experience for me.

Recently, I visited Dr. Joel Penning's Western-civilization class where I learned about the ancient fascination with purple dye.  Tyrian purple, also known as royal purple, was a rare and expensive dye of antiquity.  It was first produced around 1600-1750 BC, primarily in the Phoenician city-state of Tyre, which was located in what is now modern-day Lebanon.  It was obtained from Mediterranean murex sea snails, and it smelled so bad its production was restricted to certain locations.

Interestingly, "Phoenicia" is derived from the Greek word phoinix meaning "purple."  Tyrian purple was described by Aristotle in 350 BC and by Pliny the Elder in 77 AD. 

Tyrian purple was replaced by plant-based indigo dye after the mid 1400s. Because indigo is blue, it was mixed with red dyes in order to produce the desired purple color.  Aniline purple, a synthetic coal-tar derivative, was used to replace natural Tyrian purple.  It was first obtained accidentally by William Henry Perkin in 1856 while he was attempting to synthesize quinine. 

In 1909, Paul Friedlander determined the empirical formula (C₁₆H₈Br₂N₂O₂) and deduced the molecular structure of Tyrian purple as 6,6'-dibromoindigo.  Thus, blue indigo becomes purple when two bromine atoms are attached to each indigo molecule.  Friedlander produced 1.4 grams of Tyrian purple from processing 12,000 sea snails.

In the early 1900s, unsuccessful attempts were made to synthesize Tyrian purple via direct bromination of indigo. It was reported in 2012 that Tyrian purple can be used as an organic semiconductor material.  Conagen, Inc., a biotechnology company located near Boston, MA, has been producing Tyrian purple since 2023 using a patented microbial biosynthesis and fermentation process that eliminates the labor-intensive harvesting and processing of sea snails, but efficiently produces large quantities of Tyrian purple at reasonable cost.