WikiPortallus: Bromine.html

Balard found bromide salts 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 found a new element and named it bromine.⁵

Carl Jacob Löwig isolated bromine from mineral water spring from his home town 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.⁶

Bromine was not produced in quantity until 1860. The French chemist and physicist Joseph-Louis Gay-Lussac suggested the name bromine due to the characteristic smell of the vapors. Some also suggest that it may have been discovered by Bernard Courtois, the man who discovered iodine.⁸

Characteristics

Bromine is the only liquid nonmetallic element at room temperature, and one of only six elements on the periodic table that are liquid at or close to room temperature. The pure chemical element has the physical form of a diatomic molecule, Br₂. It is a dense, mobile, reddish-brown liquid, that evaporates easily at standard temperature and pressures to give a red vapor (its color resembles nitrogen dioxide) that has a strong disagreeable odor resembling that of chlorine. Bromine is a halogen, and is less reactive than chlorine and more reactive than iodine. Bromine is slightly soluble in water, and highly soluble in carbon disulfide, aliphatic alcohols (such as methanol), and acetic acid. It bonds easily with many elements and has a strong bleaching action. Bromine, like chlorine, is also used in maintenance of swimming pools.

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 mentiones several organobromine compounds for this phase out.

Bromine is a powerful oxidizing agent. It reacts vigorously with metals, especially in the presence of water, as well as most organic compounds, especially upon illumination.

Isotopes

Bromine has 2 stable isotopes: ⁷⁹Br (50.69 %) and ⁸¹Br (49.31%). And at least another 23¹⁰ 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.¹¹

Occurrence and production

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),¹² 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).¹³¹⁴

Approximately 556,000 metric tons (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.¹⁵¹⁶¹⁷ Bromine production has increased sixfold since the 1960s. The largest bromine reserve in the United States is located in Columbia and Union County, Arkansas, U.S.¹⁸ 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 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.

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).

Compounds

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 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.

Inorganic chemistry

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

Bromine will also oxidize metals and metaloids 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.

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.⁴

Illustrative of the addition reaction¹⁹ is the preparation of 1,2-Dibromoethane, the organobromine compound produced in the largest amounts:

Flame retardant

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.²³ Ethylene bromide is also used as a fumigant, but again this application is declining.¹⁷

Pesticide

Other Use

Biological role

Organobromine compounds do occur naturally. Marine organisms are the main source of organiobromine compounds. In 1999 over 1600 compounds were identified. The most abundant one is methyl bromide with a estimate amount of 56000 metric tonnes produced by marine algea.²⁹ The esential oil of the Hawaiian alga Asparagopsis taxiformis consists out of 80% of methyl bromide.³⁰ A famous example for a bromine containing organic compound which is used by human for a long time being Tyrian purple.³¹²⁹ 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.³² Most organobromine compounds in nature arise via the action of vanadium bromoperoxidase.³³

Safety

Elemental bromine is toxic and causes burns. As an oxidizing agent, it is incompatible with most organic and inorganic compounds. Care needs to 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), they will form a pale brown cloud of bromine gas. This gas smells like bleach and is very irritating to the mucus membranes. Upon exposure, one should move to fresh air immediately. If symptoms arise, medical attention is needed.

References

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