Benzophenone-3,3',4,4'-tetracarboxylic acid dihydrate.

The title compound, C17H10O9,2H2O, crystallizes in the centrosymmetric space group P2(1)/a but does not exhibit strong eight-membered cyclic dimer hydrogen bonds about centers of symmetry or otherwise. A richly three-dimensional hydrogen-bonding network is observed, however, which arises from 19 hydrogen bonds per asymmetric unit; it includes a 14-membered cyclic dimer about about a center of symmetry and hydrogen-bonding rings of 18 and 30 members involving the acid molecule and a water molecule, both also about centers of symmetry. The carboxyl H and O atoms are ordered in each of the carboxyl groups. The configuration of the benzophenone core is quite similar to that in benzophenone itself and the configuration of the adjacent carboxyl groups is quite similar to that in phthalic acid and naphthalene-2,3-dicarboxylic acid.