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Here’s a neat addendum to the “Compounds Discovered Entirely by Mistake” post, courtesy of my new favourite non-fiction book “Eurekas and Euphorias: The Oxford Book of Scientific Anecdotes.”

At the turn of the 1900’s synthetic organic chemistry was beginning to coalesce into a serious art.  Infrared and NMR spectroscopy were both more than half a century away, but combustion analysis was allowing relatively facile assignment of molecular formulae (molecular structure was a bit of a guessing game).

IndigoOf great interest at the time was the production of new synthetic dyes, as well as synthesis of dyes found in nature.  Of key concern was indigo, isolated from plants of the genus Indigofera.  Since ancient times indigo had been produced in India and sold to Europe, and over the past hundred years the East India Company had helped transform the humble dye into a major cash crop for the British [1].  Locked out of India and frustrated by the British prices, German companies began to produce the dye synthetically.

Given the limited techniques available, indigo was a challenging target for early chemists.  Much of the early work was completed by Alfred Baeyer, who over twenty years determined the true structure and developed an early, impractical synthesis (this work in part earned him the 1905 Nobel Prize in Chemistry).  One of the more promising commercial syntheses began with the readily available napthalene, which was oxidized to phthalic anhydride in fuming sulfuric acid (H2SO4/SO3).  The reaction was slow but effective, and the low cost of the starting material helped make the the final synthetic indigo competitive with natural sources (if barely).Mercury SulfateThe breakthrough came when a worker at BASF’s indigo plant, Eugene Sapper, decided that one batch of oxidizing napthalene was in need of stirring.  Lacking a tool appropriate to the task he decide to use a mercury thermometer, which of course broke under the strain.  In the acidic, oxidizing environment the mercury was quickly converted to mercury (II) sulfate, a hitherto unknown but very effective catalyst for the oxidation of napthalene.

Other workers at BASF, notably Karl Heumann, were quick to capitalize on the discovery, and seven years later large quantities of synthetic indigo began to flood the market.  Fifteen years after that production in India had all but ceased, never to recover.

[1] The Indian farmers did not benefit significantly from the profits collected by the EIC.