Bromine

Bromine ( or ; from , meaning "stench (of he-goats)"), is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists in 1825-26. Elemental bromine is a fuming red-brown liquid at room temperature, corrosive and toxic, with properties between those of chlorine and iodine. Free bromine does not occur in nature, but occurs as colorless soluble crystalline mineral halide salts, analogous to table salt.

Bromine is rarer than about three-quarters of elements in the Earth's crust, however the high solubility of bromide ion has caused its accumulation in the oceans, and commercially the element is easily extracted from brine pools, mostly in the United States, Israel, and China. About 556,000 metric tons were produced in 2007, an amount similar to the far more abundant element magnesium.

At high temperatures, organobromine compounds are easily converted to free bromine atoms, a process which acts to terminate free radical chemical chain reactions. This makes such compounds useful fire retardants and this is bromine's primary industrial use, consuming more than half of world production of the element. The same property allows volatile organobromine compounds, under the action of sunlight, to form free bromine atoms in the atmosphere which are highly effective in ozone depletion. This unwanted side-effect has caused many common volatile brominated organics like methyl bromide, a pesticide that was formerly a large industrial bromine consumer, to be abandoned. Remaining uses of bromine compounds are in well-drilling fluids, as an intermediate in manufacture of organic chemicals, and in film photography.

Bromine has no essential function in mammals, though it is preferentially used over chloride by one antiparasitic enzyme in the human immune system. Organobromides are needed and produced enzymatically from bromide by some lower life forms in the sea, particularly algae. As a pharmaceutical, simple bromide ion, Br^-, has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative, before being replaced by shorter-acting drugs. They retain niche uses as antiepileptics.

Characteristics

Physical

bromine sample for teaching]] 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 an orange 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, although caesium, gallium, and rubidium melt just above room temperature).

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.

Chemical

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 bromine radicals:

: Br₂ $\backslash overrightarrow\{\backslash leftarrow\}$ 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.

Isotopes

Bromine has two stable isotopes, ⁷⁹Br (50.69 %) and ⁸¹Br (49.31%). At least another 23 radioisotopes are known to exist. 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.

History

Bromine was discovered independently by two chemists Antoine Balard and Carl Jacob Löwig in 1826 and 1825, respectively.

Balard found bromide chemicals in the ash of seaweed from the salt marshes of Montpellier in 1826. The seaweed 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.) Other sources claim that the French chemist and physicist Joseph-Louis Gay-Lussac suggested the name brôme for the characteristic smell of the vapors. 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.

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.

Occurrence

The diatomic element Br₂ does not occur naturally. Instead, bromine exists exclusively as bromide salts in diffuse amounts in crustal rock. Owing to leaching, bromide salts have accumulated in sea water (65 ppm), 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). It exists in the Earth's crust at 0.4 ppm, making it the 62nd most abundant element. The bromine concentration in soils varies normally between 5-40 ppm, but some volcanic soils can contain up to 500 ppm. The concentration of bromine in the atmosphere is extremely low, at only a few ppt. 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 County and Union County, Arkansas, U.S.

Production

Approximately 556,000 metric tons (worth around US $2.5 billion) of bromine is produced per year (2007) worldwide, with the United States, Israel, and China producing more than 80% of the world's supply, and Jordan and Japan producing much of the rest. 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 of production

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.

Reactions 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.

Like iodine, bromine is soluble in chloroform but only slightly soluble in water. In water, the solubility can be increased by the presence of bromide ions. Concentrated solutions of bromine are rarely prepared in the lab because of hazards. Also as with iodine sodium thiosulphate (or any soluble thiosulphate) is an effective reagent for reducing bromine to colorless odorless bromide, thus dealing with stains and odor from the element in unwanted places. For the same reason, thiosulfate is used in photography to deal with free bromine in emulsions.

Compounds and chemistry

Organic chemistry

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. 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 → + 2

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 → + 2 HF : BrO₃⁻ + O₃ → + O₂

Bromine reacts violently and explosively with aluminium metal, forming aluminium bromide:

:2 Al + 3 Br₂ → 2 AlBr₃

Bromine reacts with hydrogen in gaseous form and gives hydrogen bromide:

:H₂ + Br₂ → 2HBr

Bromine reacts with alkali metal iodides in a displacement reaction. This reaction forms alkali metal bromides and produces elemental iodine:

:2 NaI + Br₂ → 2 NaBr + I₂ :2 KI + Br₂ → 2 KBr + I₂

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. is the preparation of 1,2-dibromoethane, the organobromine compound produced in the largest amounts: :C₂H₄ + Br₂ → CH₂BrCH₂Br

Flame retardant

Brominated flame retardants represent a commodity of growing importance, and represent the largest use of bromine. When the brominated material burns, the flame retardant produces hydrobromic acid which interferes in the radical chain reaction of the oxidation reaction of the fire. The mechanism is that the highly reactive hydrogen oxygen and hydroxy radicals react with hydrobromic acid and form less reactive bromine radicals (free bromine atoms). These also react with radicals in the first to help terminate the reaction.

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.

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 use in 1966 in the US. This application has declined since the 1970s due to environmental regulations (see below).

Pesticide

Poisonous methyl bromide was widely used as pesticide to fumigate soil and to fumagate housing, by the tenting method. Ethylene bromide was similarly used.

Medical and veterinary

Bromide compounds, especially potassium bromide, were frequently used as general 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, although the latter use varies from country to country. For example, the U.S. Food and Drug Administration (FDA) does not approve bromide for the treatment of any disease, and it was removed from over-the-counter sedative products like Bromo-Seltzer, in 1975. Thus, bromide levels are not routinely measured by medical laboratories in the U.S. However, U.S. veterinary medical diagnostic testing laboratories will measure blood bromide levels on request, as an aid to treatment of epilepsy in dogs. Long-term use of potassium bromide can lead to bromism.

Other uses

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. Bromine is also used in the production of brominated vegetable oil, which is used as an emulsifier in many citrus-flavored soft drinks (for example, 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.Soft drinks containing brominated vegetable oil are still sold in the US (2011).

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, especially spas (hot tubs), where it is generated in situ from a bromide plus hydrogen peroxide. In spas, the high water temperatures render chlorinated water purification and buffering compounds unstable, and bromine compounds may improve the life of the free-halogen antimicrobial.

Water purification compounds, disinfectants and insecticides, such as tralomethrin (C₂₂H₁₉Br₄NO₃).

Marine organisms are the main source of organobromine compounds. Over 1600 compounds were identified by 1999. The most abundant one is methyl bromide (CH₃Br) with an estimated 56,000 metric tonnes produced by marine algae each year. The essential oil of the Hawaiian alga Asparagopsis taxiformis consists of 80% methyl bromide. In fact, most organobromine compounds in nature arise in the sea, via the action of a unique algal enzyme, vanadium bromoperoxidase. Though this enzyme is the most prolific creator of organic bromides by living organisms, other bromoperoxidases exist in nature that do not use vanadium.

A famous example of a bromine-containing organic compound that has been used by humans since ancient times is the fabric dye Tyrian purple. The brominated indole indigo dye is produced by a medium-sized predatory sea snail, the marine gastropod Murex brandaris. The organobromine nature of the compound was not discovered until 1909 (see Paul Friedländer).

Bromine can also be substituted for the methyl substituent in the nitrogenous base thymine of DNA, creating the base analog 5-bromouracil. When this base is incorporated into DNA its different hydrogen bonding properties may cause mutation at the site of that base pair.

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.

: 6Br^- + 2MnO₄^- + 8H⁺ → 3Br₂ + 2MnO₂ + 4H₂O

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

External links

WebElements.com – Bromine

Theodoregray.com – Bromine

USGS Minerals Information: Bromine

Bromine Science and Environmental Forum (BSEF)

Chemistry in its element podcast (MP3) from the Royal Society of Chemistry's Chemistry World: Bromine

Category:Chemical elements Category:Halogens * Category:Oxidizing agents