Bromine

Bromine (pronounced /ˈbroʊmiːn/ BROH-meen or /ˈbroʊmɨn/ BROH-min; from Greek: βρῶμος, brómos, meaning "stench (of he-goats)"),^[2] is a chemical element with the symbol Br and atomic number 35. It is in the halogen element group. Bromine vapors are corrosive and toxic. Approximately 556,000 metric tons were produced in 2007.^[3] The main applications for bromine are in fire retardants and manufacture of chemicals.

Characteristics

Physical

Elemental bromine exists as a diatomic molecule, Br₂. It is a dense, mobile, slightly transparent reddish-brown liquid, that evaporates easily at standard temperature and pressures to give a red vapor (its color resembles nitrogen dioxide) that has a strongly disagreeable odor resembling that of chlorine. It is the only nonmetallic element that is a liquid at room temperature, and one of only two elements on the periodic table that are liquids at room temperature (mercury is the other, but caesium, rubidium, francium and gallium are quite close.).

At a pressure of 55 GPa bromine converts to a metal. At 75 GPa it converts to a face centered orthorhombic structure. At 100 GPa it converts to a body centered orthorhombic monoatomic form.^[4]

Chemical characteristics

Being less reactive than chlorine but more reactive than iodine, bromine reacts vigorously with metals, especially in the presence of water to give bromide salts. It is also reactive toward most organic compounds, especially upon illumination, conditions that favor the dissociation of the diatomic molecule into atomic bromine:

Br₂ $\overrightarrow{\leftarrow}$ 2 Br

It bonds easily with many elements and has a strong bleaching action.

Bromine is slightly soluble in water, but it is highly soluble in organic solvents such as carbon disulfide, aliphatic alcohols , and acetic acid.

Occurrence

World bromine production trend

View of salt evaporation pans on the Dead Sea, where Jordan (right) and Israel (left) produce salt and bromine

The diatomic element Br₂ does not occur naturally. Instead, bromine exists exclusively as bromide salts in diffuse amounts in crustal rock. Due to leaching, bromide salts have accumulated in sea water (65 ppm),^[5] but at a lower concentration than chloride. Bromine may be economically recovered from bromide-rich brine wells and from the Dead Sea waters (up to 50000 ppm).^[6]^[7] A large number of organobromine compounds are found in small amounts in nature.

China's bromine reserves are located in the Shandong Province and Israel's bromine reserves are contained in the waters of the Dead Sea. The largest bromine reserve in the United States is located in Columbia and Union County, Arkansas, U.S.^[8]

Mapping of industrial releases in the United States

One tool that maps the most recent release information of bromine [1] to particular locations in the United States^[9] and also provides additional information about such releases is TOXMAP. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the United States National Library of Medicine (NLM) that uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency's (EPA) Toxics Release Inventory and Superfund Basic Research Programs. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET)^[10] and PubMed, and from other authoritative sources.

Isotopes

Bromine has two stable isotopes, ⁷⁹Br (50.69 %) and ⁸¹Br (49.31%). At least another 23 radioisotopes are known to exist.^[11] Many of the bromine isotopes are fission products. Several of the heavier bromine isotopes from fission are delayed neutron emitters. All of the radioactive bromine isotopes are relatively short lived. The longest half life is the neutron deficient ⁷⁷Br at 2.376 days. The longest half life on the neutron rich side is ⁸²Br at 1.471 days. A number of the bromine isotopes exhibit metastable isomers. Stable ⁷⁹Br exhibits a radioactive isomer, with a half life of 4.86 seconds. It decays by isomeric transition to the stable ground state.^[12]

Compounds and chemistry

Organic chemistry

N-Bromosuccinimide

Organic compounds are brominated by either addition or substitution reactions. Bromine undergoes electrophilic addition to the double-bonds of alkenes, via a cyclic bromonium intermediate. In non-aqueous solvents such as carbon disulfide, this affords the di-bromo product. For example, reaction with ethylene will produce 1,2-dibromoethane. Bromine also undergoes electrophilic addition to phenols and anilines. When used as bromine water, a small amount of the corresponding bromohydrin is formed as well as the dibromo compound. So reliable is the reactivity of bromine that bromine water is employed as a reagent to test for the presence of alkenes, phenols, and anilines. Like the other halogens, bromine participates in free radical reactions. For example hydrocarbons are brominated upon treatment with bromine in the presence of light.

Bromine, sometimes with a catalytic amount of phosphorus, easily brominates carboxylic acids at the α-position. This method, the Hell-Volhard-Zelinsky reaction, is the basis of the commercial route to bromoacetic acid. N-Bromosuccinimide is commonly used as a substitute for elemental bromine, being easier to handle, and reacting more mildly and thus more selectively. Organic bromides are often preferable relative to the less reactive chlorides and more expensive iodide-containing reagents. Thus, Grignard and organolithium compound are most often generated from the corresponding bromides.

Certain bromine-related compounds have been evaluated to have an ozone depletion potential or bioaccumulate in living organisms. As a result, many industrial bromine compounds are no longer manufactured, are being restricted, or scheduled for phasing out. The Montreal Protocol mentions several organobromine compounds for this phase out.

Inorganic chemistry

Inorganic bromine compounds adopt a variety of oxidation states from -1 to +7.^[13]

Oxidation states of bromine
−1	HBr
0	Br₂
+1	BrCl
+3	BrF₃
+5	BrF₅
+5	BrO−3
+7	BrO−4

Bromine is an oxidizer, and it will oxidize iodide ions to iodine, being itself reduced to bromide:

Br₂ + 2 I⁻ → 2 Br⁻ + I₂

Bromine will also oxidize metals and metalloids to the corresponding bromides. Anhydrous bromine is less reactive toward many metals than hydrated bromine, however. Dry bromine reacts vigorously with aluminium, titanium, mercury as well as alkaline earths and alkali metals.

Dissolving bromine in alkaline solution gives a mixture of bromide and hypobromite:

Br₂ + 2 OH^- → Br⁻ + OBr⁻ + H₂O

This hypobromite is responsible for the bleaching abilities of bromide solutions. Warming of these solutions causes the disproportion reaction of the hypobromite to give bromate, a strong oxidising agent very similar to chlorate.

3 BrO⁻ → BrO−3 + 2 Br⁻

In contrast to the route to perchlorates, perbromates are not accessible through electrolysis but only by reacting bromate solutions with fluorine or ozone.

BrO₃⁻ + H₂O + F₂ → BrO−4 + 2 HF

BrO₃⁻ + O₃ → BrO−4 + O₂

History

Illustrative and secure bromine sample for teaching

Bromine was discovered independently by two chemists Antoine Balard^[14] and Carl Jacob Löwig^[15] in 1825 and 1826.^[16]

Balard found bromide chemicals in the ash of sea weed from the salt marshes of Montpellier in 1826. The sea weed was used to produce iodine, but also contained bromine. Balard distilled the bromine from a solution of seaweed ash saturated with chlorine. The properties of the resulting substance resembled that of an intermediate of chlorine and iodine; with those results he tried to prove that the substance was iodine monochloride (ICl), but after failing to do so he was sure that he had found a new element and named it muride, derived from the Latin word muria for brine.^[14]

Carl Jacob Löwig isolated bromine from a mineral water spring from his hometown Bad Kreuznach in 1825. Löwig used a solution of the mineral salt saturated with chlorine and extracted the bromine with diethylether. After evaporation of the ether a brown liquid remained. With this liquid as a sample for his work he applied for a position in the laboratory of Leopold Gmelin in Heidelberg. The publication of the results was delayed and Balard published his results first.^[15]

After the French chemists Louis Nicolas Vauquelin, Louis Jacques Thénard, and Joseph-Louis Gay-Lussac approved the experiments of the young pharmacist Balard, the results were presented at a lecture of the Académie des Sciences and published in Annales de Chimie et Physique.^[17] In his publication Balard states that he changed the name from muride to brôme on the proposal of M. Anglada. Other sources claim that the French chemist and physicist Joseph-Louis Gay-Lussac suggested the name brôme for the characteristic smell of the vapors.^[18] Bromine was not produced in large quantities until 1860.

The first commercial use, besides some minor medical applications, was the use of bromine for the daguerreotype. In 1840 it was discovered that bromine had some advantages over the previously used iodine vapor to create the light sensitive silver halide layer used for daguerreotypy.^[19]

Potassium bromide and sodium bromide were used as anticonvulsants and sedatives in the late 19th and early 20th centuries, until they were gradually superseded by chloral hydrate and then the barbiturates.^[20]

Production

Approximately 556,000 metric tonnes (worth around US$2.5 billion) of bromine are produced per year (2007) worldwide with the United States, Israel, and China being the primary producers.^[21]^[22]^[23] Bromine production has increased sixfold since the 1960s.

Bromide-rich brines are treated with chlorine gas, flushing through with air. In this treatment, bromide anions are oxidized to bromine by the chlorine gas.

2 Br⁻ + Cl₂ → 2 Cl⁻ + Br₂

Laboratory methods

In the laboratory, because of its commercial availability and long shelf-life, bromine is not typically prepared. Small amounts of bromine can however be generated through the reaction of solid sodium bromide with concentrated sulfuric acid (H₂SO₄). The first stage is formation of hydrogen bromide (HBr), which is a gas, but under the reaction conditions some of the HBr is oxidized further by the sulfuric acid to form bromine (Br₂) and sulfur dioxide (SO₂).

NaBr (s) + H₂SO₄ (aq) → HBr (aq) + NaHSO₄ (aq)

2 HBr (aq) + H₂SO₄ (aq) → Br₂ (g) + SO₂ (g) + 2 H₂O (l)

Similar alternatives, such as the use of dilute hydrochloric acid with sodium hypobromite, are also available. The most important thing is that the anion of the acid (in the above examples, sulfate and chloride, respectively) be more electronegative than bromine, allowing the substitution reaction to occur.

Reaction involving a strong oxidizing agent, such as potassium permanganate, on bromide ions in the presence of an acid also gives bromine. An acidic solution of bromate ions and bromide ions will also comproportionate slowly to give bromine.

Bromine is only slightly soluble in water. But the solubility can be increased by the presence of bromide ions. However, concentrated solutions of bromine are rarely prepared in the lab because of hazards. Sodium thiosulphate is an excellent reagent for getting rid of bromine completely including the stains and odor.

Applications

A wide variety of organobromine compounds are used in industry. Some are prepared from bromine and others are prepared from hydrogen bromide, which is obtained by burning hydrogen in bromine.^[3]

Illustrative of the addition reaction^[24] is the preparation of 1,2-dibromoethane, the organobromine compound produced in the largest amounts:

C₂H₄ + Br₂ → CH₂BrCH₂Br

Flame retardant

Tetrabromobisphenol A

Brominated flame retardants represent a commodity of growing importance. If the material burns the flame retardants produce hydrobromic acid which interferes in the radical chain reaction of the oxidation reaction of the fire. The highly reactive hydrogen oxygen and hydroxy radicals react with hydrobromic acid and form less reactive bromine radicals.^[25]^[26] The bromine-containing compounds can be placed in the polymers either during polymerization if a small amount of brominated monomer is added or the bromine containing compound is added after polymerization. Tetrabromobisphenol A can be added to produce polyesters or epoxy resins. Epoxy used in printed circuit boards (PCB) are normally made from flame retardant resins, indicated by the FR in the abbreviation of the products (FR-4 and FR-2. Vinyl bromide can be used in the production of polyethylene, polyvinylchloride or polypropylene. Decabromodiphenyl ether can be added to the final polymers.^[27]

Gasoline additive

Ethylene bromide was an additive in gasolines containing lead anti-engine knocking agents. It scavenges lead by forming volatile lead bromide, which is exhausted from the engine. This application accounted for 77% of the bromine uses in 1966 in the US. This application has declined since the 1970s due to environmental regulations.^[28] Ethylene bromide is also used as a fumigant, but again this application is declining.^[23]

Pesticide

Methyl bromide (bromomethane)

Methyl bromide was widely used as pesticide to fumigate soil. The Montreal Protocol on Substances that Deplete the Ozone scheduled the phase out for the ozone depleting chemical until 2005. In 1991, an estimated 35,000 metric tonnes of the chemical were used to control nematodes, fungi, weeds and other soil-borne diseases.^[29]^[30]

Medical and veterinary

Bromide compounds, especially potassium bromide, were frequently used as sedatives in the 19th and early 20th century. Bromides in the form of simple salts are still used as anticonvulsants in both veterinary and human medicine.

Other uses

Orange fluoresces of DNA Ethidium bromide intercalate

The bromides of calcium, sodium, and zinc account for a sizable part of the bromine market. These salts form dense solutions in water that are used as drilling fluids sometimes called clear brine fluids.^[23]^[31]
Bromine is also used in the production of brominated vegetable oil, which is used as an emulsifier in many citrus-flavored soft drinks (e.g. Mountain Dew). After the introduction in the 1940s the compound was extensively used until the UK and the US limited its use in the mid 1970s and alternative emulsifiers were developed.^[32]
Soft drinks containing brominated vegetable oil are still sold in the US (2009).^[33]

Tralomethrin

Several dyes, agrichemicals, and pharmaceuticals are organobromine compounds. 1-Bromo-3-chloropropane, 1-bromoethylbenzene, and 1-bromoalkanes are prepared by the antimarkovnikov addition of HBr to alkenes. Ethidium bromide, EtBr, is used as a DNA stain in gel electrophoresis.
High refractive index compounds
Bromine, like chlorine, is used in maintenance of swimming pools.
Water purification compounds, disinfectants and insecticides, such as tralomethrin (C₂₂H₁₉Br₄NO₃).^[23]
Potassium bromide is used in some photographic developers to inhibit the formation of fog (undesired reduction of silver).
Vapor is used as the second step in sensitizing daguerreotype plates to be developed under Mercury (Hg) vapor. Bromine acts as an accelerator to the light sensitivity of the previously iodized plate.
Bromine is also used to reduce mercury pollution from coal-fired power plants. This can be achieved either by treating activated carbon with bromine or by injecting bromine compounds onto the coal prior to combustion.

Biological role

Tyrian purple

Bromine has no known essential role in human or mammalian health, but inorganic bromine and organobromine compounds do occur naturally, and some may be of use to higher organisms in dealing with parasites. For example, in the presence of H₂O₂ formed by the eosinophil, and either chloride or bromide ions, eosinophil peroxidase provides a potent mechanism by which eosinophils kill multicellular parasites (such as, for example, the nematode worms involved in filariasis); and also certain bacteria (such as tuberculosis bacteria). Eosinophil peroxidase is a haloperoxidase that preferentially uses bromide over chloride for this purpose, generating hypobromite (hypobromous acid).^[34]

Marine organisms are the main source of organobromine compounds. Over 1600 compounds were identified by 1999. The most abundant one is methyl bromide with estimated 56,000 metric tonnes produced by marine algae.^[35] The essential oil of the Hawaiian alga Asparagopsis taxiformis consists of 80% methyl bromide.^[36] A famous example of a bromine-containing organic compound that has been used by humans for a long time is Tyrian purple.^[35]^[37] The brominated indigo is produced by a medium-sized predatory sea snail, the marine gastropod Murex brandaris. It took until 1909 before the organobromine nature of the compound was discovered by Paul Friedländer.^[38] Most organobromine compounds in nature arise via the action of vanadium bromoperoxidase.^[39]

Safety

Elemental bromine is toxic and causes burns. As an oxidizing agent, it is incompatible with most organic and inorganic compounds. Care needs to be taken when transporting bromine; it is commonly carried in steel tanks lined with lead, supported by strong metal frames.

When certain ionic compounds containing bromine are mixed with potassium permanganate (KMnO₄) and an acidic substance, they will form a pale brown cloud of bromine gas. This gas smells like bleach and is very irritating to the mucous membranes. Upon exposure, one should move to fresh air immediately. If symptoms of bromine poisoning arise, medical attention is needed.

References

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