Halogens readily form a variety of covalent compounds with non-metals, where a filled octet is achieved by sharing pairs of electrons.
As covalent molecules with non-polar bonds (e.g., Br2, Cl2, I2), the halogens dissolve rather poorly in water. This is especially the case for iodine, I2 (although in the presence of iodide ions, the solubility of iodine in water is enhanced by the formation of I3- species).
Halogens dissolve readily in organic solvents, such as carbon tetrachloride (tetrachloromethane), ether (CH3OCH3) and ethanol (C2H5OH).
On the other hand, alkali metal halides (and many other metal halides) are soluble in water. Additionally, the binary compounds with hydrogen, HX, e.g. hydrogen chloride, are covalent diatomic molecules that are gases at room temperature. The H-X bond is polar; thus in the presence of water, hydrogen halides dissolve readily and ionize to give strong hydrohalic acids (H3O+ + Cl-), e.g. hydrochloric acid.
Nonmetal halides are generally hydrolyzed to a hydrogen halide and to an oxy-acid
containing the other element.
When pure bromine is added to water, it forms hypobromous acid and hydrobromic acid (HBr):
Br2(l) + H2O(l) → HOBr(aq) + HBr(aq)
When bromine dissolves in water, it partially disproportionates.
Br2 +H2O <-----> HOBr + H+ + Br-
The equilibrium constant for this reaction at 25°C is 7.2×109 M2 . Light catalyzes the decomposition of hypobromous acid to hydrogen bromide and oxygen.
HOBr ----(hν)--> HBr + 1/2O2
In the dark, hypobromous acid decomposes to bromic acid and bromine. Bromic acid is relatively unstable and decomposes slowly to give bromine and oxygen.
5HOBr ------> HBrO2 + 2Br2 + 2H2O
4HBrO3 ---> 2Br2 + 5 O2 + 2H2O
In alkaline solution, bromine reacts rapidly to produce hypobromite.
Br2 + 2OH- <--------> Br- + BrO- + H2O K = 2 x 108
It is necessary to maintain this reaction below 0°C to minimize the disproportionation of hypobromite to bromate and bromide.
3BrO- ----> 2Br- + BrO3-
Kirk-Othmer Encyclopedia of Chemical Technology Copyright © 2001 by John Wiley & Sons, Inc. Baruch Grinbaum, IMI(TAMI) Institute for Research and Development Mira Freiberg, Dead Sea Bromine Group