The Sceptical Chymist | Ambiguous bromine

Published in Chemistry

At first sight bromine seems to be ‘just another halogen’, a helpful counter-anion or leaving group in SN2 or cross-coupling reactions. Of course this isn’t the whole story, as Matt Rattley — a chemistry student at the University of Oxford and the author of the winning essay on bromine for last year’s contest — points out in his article (subscription required).

Bromine was isolated independently by Carl Jacob Löwig from a mineral water spring, and Antoine Balard from seaweed, in 1825 and 1826. Having identified that he’d obtained a substance between chlorine and iodine, Balard first thought it was idodine chloride before recognizing it as a new element. It seems unclear who exactly from Balard or Gay-Lussac thought of the name brôme but we know it comes from the Greek bromos, stench — a fair description of gaseous bromine. In addition to its rather unpleasant smell, bromine is also toxic — as Rattley puts it, bromine’s orange-brown colour is convenient because “avoid it you should”. In the sunlight, elemental bromine (Br2) splits into radicals that readily attack other species, including lung tissues.

Brominated compounds have been used throughout history for a variety of purposes, with varying degrees of success — find out in the article how one was dangerous (likely lethal, really) to ancient Egyptians in the seemingly mundane form of a lipstick. More successful applications include that of potassium bromide, which acts on the nervous system, as an efficient epilepsy remedy, an anticonvulsant and a sedative during the late-19th and 20th centuries. It still is used in veterinary medicine, but bromide’s chronic toxicity has since put a stop to human uses. Other instances have also exploited toxicity — Rattley mentions the insecticide chlorenapyr, whose rather peculiar structure comprises three different halogens — while in others no particular problems arose. A polybrominated dye for example has been widely used to stain various cell components for imaging purposes.

In light of such diversity, it certainly doesn’t seem unreasonable to think that bromine will continue to feature prominently both in research and practical applications.


Anne Pichon (Associate Editor, Nature Chemistry)

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