Chemical of the Week 14 – Tyrian Purple

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This post was inspired by staff room chat a couple of weeks ago. Bemoaning the end of summer our attention turned to the lovely colours some of the female staff where wearing. The bright colours brought me back to A level Latin class and our translation of The Aeneid. You see, this topic of conversation is not a new one as even back about 30BC Virgil waxed lyrical about the Tyrian purple cloak that Dido had made for Aeneas heading into battle, he even remained cloaked in it on his funeral pyre !  It was also the colour of choice for the Roman ruling classes.  The status of the individual was reflected in the width of the purple stripe on their toga with the Emperor having a ceremonial toga of Tyrian purple.

Natural dyes are derived from plants, invertebrates, or minerals. The majority of natural dyes are vegetable dyes from plant sources and other organic sources such as fungi and lichens. Tyrian purple has been around since approximately 1600BC and is obtained from the hypobranchial gland of various species of marine molluscs. It was original extracted and marketed as a dye in the city of Tyre, now the fourth largest city in Lebanon but back in the ancient world a major commercial centre. It took 12,000 molluscs to produce approximately 1 1/2 grams of this dye so it was considered as valuable as silver. The molluscs were crushed and boiled up to release the colourful chemical required for dying. The chemical, 6,6-dibromoindigo, that is responsible for Tyrian Purple was identified at the turn of the last century by the chemist Paul Friedlander.  He was able to isolate 1g of the dye from the molluscs and analyse it to obtain an empirical formula ( using the structure below can you work it out !)   The presence of bromine in the molecule was unexpected and it is believed that it is what causes the reddish tinge to an indigo based molecule.

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The background to the production of synthetic dyes is interesting as it took off when dyes started to be synthesised from coal tar. Coal tar contains various aromatic hydrocarbons like toluene, xylene, benzene and is obtained from the distillation of bituminous coal. Most organic compounds are colourless but certain organic molecules with an extensively delocalised electron system will absorb in the visible range of the spectrum. Synthetic dyes have the advantage of being less costly and more reliable to produce. The famous chemist William Perkin made the first synthetic dye Maurvin in 1856. His legacy includes the Perkin Medal which is widely acknowledged as the highest honour in American industrial chemistry and also the RSC journal Perkin Transactions (1997-2002). By the end of the 1800s indigo and alizarin had also been made synthetically and so began our colourful journey. Today there are over 3600 different types of synthetic dye and 500,000 tonnes are produced each year. Chemists now tailor the molecules so that they do not present any environmental problems and that they adsorb on to the desired substrate eg cotton, polyester. Environmentally the high water to dye ratio in the dying process has resulted in major pollution problems with waterless dyeing being the holy grail in the textiles industry. We’ve come a long way from the marine molluscs but interestingly natural dyes are all the rage again ! Batik scarf anyone ?

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