Chemical Properties
Colorless, very deliquescent crystals;
bitter taste.
Soluble in water; slightly soluble in alcohol.
Physical properties
The anhydrous MgBr2 is a white crystalline substance; hexagonal crystals; deliquescent; density 3.72 g/cm3; melts at 700°C; highly soluble in water (101.5g/100mL at 20°C); moderately soluble in methanol and ethanol (21.8 and 6.9 g/mL at 20°C, respectively).
The hexahydrate, MgBr2.6H2O consists of colorless monoclinic crystals; bitter taste; hygroscopic; fluoresce in x-rays; density 2.07 g/cm3; melts at 172.4°C; intensely soluble in water, 316 g/100 mL at 0°C; dissolves in methanol and ethanol; slightly soluble in ammonia solution.
Occurrence
Magnesium bromide is found in seawater, some mineral springs, natural brines, inland seas and lakes such as the Dead Sea and the Great Salt Lake, and salt deposits such as the Stassfurt deposits. In seawater, it is the primary source of bromine. By the action of chlorine gas upon seawater or seawater bitterns, bromine is formed.
Application
The major use of Magnesium bromide has been in the commercial production of bromine. The Dow process is an electrolytic method of bromine extraction from brine, and was Herbert Dow’s second revolutionary process for generating bromine commercially (1889).
Magnesium bromide has also been used in organic syntheses. An efficient and environmentally friendly procedure for the one-pot synthesis of tetrahydropyrimidines from aldehydes, a-diketones and urea/thiourea by using magnesium bromide as an inexpensive and easily available catalyst under solvent-free conditions has been described. Compared with the classical Biginelli reaction conditions, this new method has the advantage of good to excellent yields (74%–94%) and short reaction times (45–90 min). The structure of the Biginelli reaction product from a-diketone, salicylaldehyde and urea has been proposed to possess an oxygen bridge by cyclization (intramolecular Michael addition).
Preparation
Magnesium bromide is prepared by treating MgO
with hydrobromic acid and then crystallization
above 0.0°C in solution. The product is the hexahydrate
salt:
MgO＋2HBr→MgBr2＋H2O→MgBr2·6H2O
The anhydrous form may also be prepared by heating
the hexahydrate with dry HBr gas. This compound can
also be formed directly from the elements:
Mg＋Br2→MgBr2
Purification Methods
Crystallise it from EtOH or H2O (3.3g/mL). Dry it in a vacuum at ~150o, or heat the hydrate in a stream of HCl. It is very deliquescent. [Ehrlich in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I p 909 1963.]
Synthesis
Magnesium bromide is prepared by treating magnesium oxide with hydrobromic acid and subsequent crystallization above 0°C. The product is hexahydrate, MgBr2.6H2O:
MgO + 2HBr → MgBr2 + H2O
The anhydrous MgBr2 may be obtained by heating the hexahydrate with dry hydrogen bromide gas. Magnesium bromide also can be made from its elements. Heating magnesium metal with bromine vapor yields the salt:
Mg + Br2 → MgBr2
Magnesium bromide, like the chloride salt, is obtained from sea water (see Magnesium and Magnesium chloride). In this process, magnesium hydroxide precipitated from sea water is neutralized with hydrobromic acid, and MgBr2 is obtained by crystallization.
