Br.C([O:4][P:5]([C:10]1[CH:15]=[CH:14][C:13]([C:16]2[CH:21]=[CH:20][C:19]([P:22]([O:27]CC)(=[O:26])[O:23]CC)=[CH:18][CH:17]=2)=[CH:12][CH:11]=1)(=[O:9])[O:6]CC)C.O>C(O)(=O)C>[C:16]1([C:13]2[CH:14]=[CH:15][C:10]([P:5]([OH:6])(=[O:4])[OH:9])=[CH:11][CH:12]=2)[CH:17]=[CH:18][C:19]([P:22]([OH:26])(=[O:23])[OH:27])=[CH:20][CH:21]=166 ml of HBr at 33% by weight in acetic acid are added to 4.9 g (11.5 moles) of 4,4'-biphenyl-diphosphonic acid tetra-ethyl ester and the mixture is heated at 80° C. for 4 hours. After cooling to ambient temperature, 150 ml of H2O are added under agitation and the precipitate obtained is filtered, washed with water and vacuum dried (20 mm Hg) at 45° C. for one night. 2.29 g of 4,4'-biphenyl-diphosphonic acid, with a molar yield of 66% are obtained.
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