Br.C([O:4][P:5]([C:10]1[CH:15]=[CH:14][C:13]([P:16]([O:21]CC)(=[O:20])[O:17]CC)=[CH:12][CH:11]=1)(=[O:9])[O:6]CC)C.C(OCC)C.C(=O)=O>C(O)(=O)C.C(O)C>[C:13]1([P:16]([OH:21])(=[O:17])[OH:20])[CH:12]=[CH:11][C:10]([P:5]([OH:6])(=[O:4])[OH:9])=[CH:15][CH:14]=1
93 ml of HBr at 33% by weight in acetic acid are added to 6.68 g of 1.4-benzene-diphosphonic acid tetra-ethyl ester and the mixture is heated to 80° C. for 4 hours. After cooling to ambient temperature 200 ml of diethyl-ether are added and the mixture, after brief agitation, is cooled to -78° C. in a bath of ethanol and dry ice. The oily precipitate obtained is recovered by decantation and subjected twice more to the dispersion process with ether and low-temperature precipitation. The oily residue obtained is vacuum dried (1 mm Hg) at 50° C. for 8 hours, obtaining 4.0 g of 1,4-benzene-diphosphonic acid in the form of a white solid. Yield in moles 89%.
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