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.