Ambiguous Bromine

in your element Ambiguous bromine Many chemical elements behave quite differently depending on the compound they are found in, but Matt Rattley argues that bromine does so in a particularly striking manner. very element is a little bit schizophrenic textiles to prevent, or slow down, the spread in its own way. Carbon’s split personality of fire should they be placed near an open Eis best exemplified through two well- flame. These compounds may, however, known allotropes, graphite and diamond; have a harmful effect on human health — transition metals transform from bland the extent of which remains unclear — and grey blocks to wonderfully colourful salts; some polybrominated diphenyl ethers and even notoriously inert noble gases, for have recently stopped being used as flame example xenon, can be coaxed into reacting retardants in various countries; whether with other members of the periodic table to alternative compounds now in use are produce exotic species including fluorides harmless is still an open question4. and oxides. And so it is with bromine too. LIBRARY PHOTO © ISM/SCIENCE Although a somewhat volatile and Sitting quietly in the middle of the p-block, unpredictable element, bromine has played it is one of the most deadly elements, yet one bromide. A not-too-distant cousin of table an important part in advancing the field with intriguing and useful properties. salt, this deceptively simple compound of molecular biology in the form of the It is one of bromine’s less pleasant was used for much of the nineteenth and polybrominated dye eosin, in which this characteristics — a more biting, sour smell twentieth centuries as an anticonvulsant heavy atom ensures efficient absorption than that of chlorine gas — that gave it its and a sedative. It exhibits chronic toxicity — of light. Alternative dyes do exist, but name, from the Greek βρώμος (brómos) abuse leads to a condition called bromism, in tandem with another small organic meaning ‘stench’ or ‘foul odour’1. Despite characterized by seizures, vomiting, compound, haemotoxylin, eosin is used being a liquid at room temperature, it is highly psychosis, skin lesions and delirium — almost universally to stain the cytoplasm volatile and produces a thick vapour layer. but when suitably administered it works in a cell and numerous other protein-based The only redeeming feature of the vapour remarkably well by inhibiting the central structures (although, notably, the nucleus is its distinctive orange colour, which means nervous system. Although for human use is preferentially stained by haematoxylin). that you can readily avoid it. And avoid it has now been replaced by alternative The resulting cell images are rather striking, it you should. Bromine is highly toxic, compounds such as pregabalin3 that act with rich purple colours arising from eosin especially in sunlight; radicals generated on more rapidly and with fewer side effects, (pictured for Hepatitis C cells). ultraviolet irradiation will wreak havoc on it still fulfils this sedative role today in The diverse reactivity that bromine brings anything and everything they encounter, veterinary medicine. goes far beyond simply acting as a relatively including lung tissue. Bromine atoms are Other attempts to tame and use sizeable anion to balance charge, taking part also found in some astonishingly toxic brominated compounds have not been so in cross-coupling reactions, or forming the natural products synthesized in some successful. This is no recent trend — and basis of a good partner in an SN2 reaction. marine sponges and corals, which can perhaps something the women of ancient We may yet find other ways to make the exhibit lethal doses more than 1,000 times Egypt might have preferred to know. They most of brominated species, owing to, or in more potent than aqueous arsenic. often added small amounts of bromine spite of, their frequent toxic side effects. ❐ It may sound surprising, mannite (a derivative of polyalcohol therefore, that anyone should want mannitol found in plants) to a mixture This essay was selected as a winning entry in our to go near this noxious element. O used to colour their lips to writing competition, see http://go.nature.com/oi187X Yet brominated molecules F Cl obtain a rich red-brown colour. have attracted much attention. F N Unfortunately, even the small MATT RATTLEY is an undergraduate F The widely used insecticide amount of residual bromine in the student at the Department of Chemistry, chlorfenapyr2, for example, is an Br mixture was likely to be enough University of Oxford, South Parks Road, unusual species containing three N to accidentally kill both women Oxford OX1 3QR, UK. different halogens (fluorine, chlorine Chlorfenapyr wearing it and men kissing them. e-mail: [email protected] and bromine). The ambiguous nature of bromine Perhaps nothing shows bromine’s also shines through in uses that were References equivocal character quite like potassium initially intended to keep us safe. A variety 1. Emsley, J. Nature’s Building Blocks (Oxford Univ. Press, 2003) of compounds, such as the polybrominated 2. Raghavendra, K. et al. Malaria J. 10, 16–22 (2011). 3. Mackey, C. Nature Rev. Drug Discov. 9, 265–266 (2010). aromatic compound tetrabromobisphenol A, 4. Cressey, D. http://www.nature.com/news/2011/110520/full/ have been incorporated in clothes and other news.2011.311.html Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te 512 NATURE CHEMISTRY | VOL 4 | JUNE 2012 | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved.

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