The effect of zinc–bromine interactions on electrophilic bromination in concentrated ZnBr2 solutions

Facile bromination can be carried out in concentrated solutions of zinc bromide (60–80% w/w ZnBr2 in water) [A. Ewenson, D. Itzhak, M. Freiberg-Bergstein, A. Shushan, B. Croitoru, D. Beneish, N. Faza, WO patent 99/19275, Bromine Compounds Ltd., Israel, 1999]. These brine solutions offer an alternative to expensive and harmful organic solvents. A correlation between the reactivity and the molecular ratio H2O:Zn has been found and quantified using UV-Vis spectroscopy [J.A. Duffy, G.L. Wood, J. Chem. Soc., Dalton Trans. (1987) 1485; E. Eyal, A. Treinin, J. Am. Chem. Soc. 86 (1964) 4287–4290; D. Meyerstein, A. Treinin, Trans. Faraday Soc. 59 (1963) 1114–1120]. Based on spectrophotometric and NMR results, it is proposed that these concentrated aqueous solutions, which behave similarly to anhydrous media, enhance the reactivity of molecular bromine by polarizing the Br–Br bond. Furthermore they affect the electron density distribution in the organic substrates.