Bromine in Chemical Element
Bromine (Br), fuming red-brown liquid at room temperature is a chemical element of Group 17 (Group VIIA) or halogen family of the periodic table, widely used in organic chemistry for the preparation of 1,2-dibromoethane (ethylene dibromide. It is the third-lightest element of the halogen series and the properties of the element intermediate between chlorine and iodine. The non-metallic chemical element, bromine has chemical symbol Br, chemical formula Br2, atomic number 35, atomic weight 79.904, melting point -7.2 °C, boiling point 58.8 °C, density 3.11 gm/cm3 (liquid), electron configuration [Ar] 3d10 4s2 4p5. Due to the presence of seven valence electrons and vacant d-orbital, bromine exhibits oxidation number or state -1, +1, +3, +5, +6, +7.
Fluorine, chlorine, bromine, and iodine are the non-metals of the Group-17 or halogen family. In learning chemistry, fluorine is the most reactive among all the chemical elements. The reactivity of halogens decreases from chorine to bromine to iodine. For example, iodine reacts with hydrogen when heated in presence of a chemical catalyst like platinum. The exceptionally high reactivity of fluorine due to very low bond energy (159 kJ mol-1) but the bond energy of other halogens like chlorine = 243 kJ mol-1, bromine = 193 159 kJ mol-1, iodine = 151 159 kJ mol-1 gradually decreases. This fact, suggests that the chemical reactivity of bromine is intermediate between chlorine and iodine and acts as an oxidizing agent.
Abundance and Discovery
Bromine is a much less abundant element than fluorine and chlorine, only to the extent of 2.5 ppm in earth crustal rocks (46th order). The largest natural sources of bromine are the oceans which contain about 6.5 mg/liter in ocean water. Some salt lakes and seas have a much higher concentration of bromide ions like Searle’s lake in California (850 mg/liter) and the Dead sea (4 gm/liter). The United States and Israel are the two main sources of bromine in the world which are transported through large-capacity metal drums for industrial applications. Like chlorine, bromine has also two stable isotopes like 79Br (50.7 percent) and 81Br (49.3 percent).
The chemical element bromine was discovered in 1826 by French chemist Antoine-Jerome Balard from the ash of seaweed from the salt marshes of Montpellier. He prepared the element by chlorinating concentration solutions of magnesium bromide around the pH scale 3.5. The material is distilled with magnesium oxide and sulfuric acid to produce bromine vapor, which is condensed and purified to a dark liquid. According to the French Science Academy, the name of the bromine is derived from the Greek word bromos which means bad smell or stench.
Chemistry and Chemical Compounds
Bromine is just one electron short of the next noble gas (krypton) configuration and the formation of bromide ion indicated the large electron affinity of the element. The bromide ion combines with electropositive elements through chemical bonding to form the ionic crystal lattice. But when the formation of an ionic lattice energetically favorable, covalent bonding is formed. Except for fluorine, all the halogen exhibits positive oxidation states in their oxides, oxoacids, and interhalogen compounds. Bromine is slightly soluble in water, saturates hydrocarbon, carbon tetrachloride, etc. In water, it forms a hydrated Br2 molecule together with HOBr.
Almost all elements of the periodic table except helium, neon, and argon form binary compounds with bromine to give a wide range of halides. The elements like oxygen, nitrogen, fluorine, and chlorine have electronegativity higher than that of bromine. Therefore, the binary compounds of such elements are not called bromides, they are called oxides, nitrides, fluorides, and chlorides. Most metal bromides have ionic compounds but the partial covalent character is developed depending upon the charge and oxidation state of the metal.
All the halogen reacts with hydrogen to form volatile covalent hydrides. The reactivity towards hydrogen gradually falls from fluorine to iodine. The hydrogen bromide is obtained by treating a mixture of water and red phosphorus with bromine. The hydrides HCl, HBr, and HI are gases at ordinary temperature but HF is liquid because of the association through hydrogen bonding. All the hydrides ionize in an aqueous solution due to solvation.
Oxides and Oxoacids of Bromine
Oxides of halogen are unstable compounds and the higher oxides are more stable than the lower ones. The stability of iodine oxides is higher than the chlorine oxides, while the oxides of bromine are least stable with no practical importance. Dibromine monoxide (Br2O) is prepared by reaction Br2 with HgO or low-temperature decomposition of BrO2 in a vacuum. It is a dark brown liquid with a melting point of -175 °C. The pale yellow crystalline solid, bromine dioxide (BrO2) is prepared by oxidizing bromine with ozone in CF3Cl at -78 °C.
There are four oxoacids like hypobromous acid (HOBr), bromous acid (HOBrO), bromic acid (HOBrO2), and perbromic acid (HOBrO3) are known well due to their stability in aqueous solution. Bromic acid can be obtained by reacting bromate with dilute sulfuric acid. Bromates are usually prepared by oxidizing bromides with hypochlorites (Cl2 + alkali) or reaction of bromine with hot concentrated alkali.
Uses of Bromine
World production of bromine is much less than that of chlorine nearly 0.4 million tonnes per year used to prepare a wide variety of organobromine compounds. Bromine is largely consumed to prepare 1, 2-dibromide, or ethylene dibromide which was added to petrol to remove lead. In 1970 the process is decline due to lead pollution of the environment. Ethylene dibromide is still produced in large quantities for the production of pesticides. For the production of polymers and plastics, we used vinyl bromide during the polymerization process. Bromine is used in the production of several bromocompounds which are effectively used in pesticides and flame retardants in fabrics. For example, tris-(dibromopropyl) phosphate has remarkable properties of flame-retarding.
In pharmacology, bromine compounds and salts are used in both veterinary and human medicine, but the use of such medicine is not permitted in all the countries of the world due to its side effect. For example, the U.S. Food and Drug Administration (FDA) does not approve bromide for the treatment of any disease and it is removed from sedative products like Bromo-Seltzer. Bromine is used in making various inorganic chemicals like hydrogen bromide, silver bromide in the photo films industry, and organic bromocompounds used in high-density drilling fluids and dyes.