2-Hydroxyisophthalic acid: Hydrogen-bonding patterns in the monohydrate and the tetraphenyl-phosphonium salt. An instance of dramatic acidity enhancement by symmetric, internally hydrogen-bonded anion stabilization

Shaleen Bawa, Marie L. Coté, Patrick Dubois, Roger A. Lalancette, Hugh W. Thompson

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The monohydrate of the title phenolic diacid (C8H 6O5-H2O, 2-hydroxybenzene-1,3-dioic acid or 3-carboxysalicylic acid) adopts a planar conformation, with the phenol hydrogen internally hydrogen-bonded to the carbonyl of one highly ordered carboxyl, which, in turn, donates a hydrogen bond to the oxygen of water. The second carboxyl is disordered and hydrogen-bonded both to water and to the disordered carboxyl of a centrosymmetrically related neighbor in a static disorder arrangement extending over two full asymmetric units. The water accepts either one or two hydrogen bonds and donates a long bifurcated hydrogen bond shared equally by O atoms of the phenol and the disordered carboxyl. The hydrogen bonding includes no standard carboxyl pairing and is entirely two-dimensional. The resulting planar ribbons stack translationally at a distance of 3.413 (8) Å, in an offset arrangement having non-translational interplanar distances of 0.821 (5) and 2.592 (6) Å. This structure is compared with two previously reported for this compound. The title compound forms a monoanion, whose tetraphenylphosphonium salt is described (C32H 25O5P, tetraphenylphosphonium 2,6-dicarboxyphenolate, tetraphenylphosphonium 2-oxidoisophthalic acid or tetraphenylphosphonium 3-carboxy-salicylate). The, phenol oxygen is the site of formal negative charge on the anion, which is stabilized in a planar arrangement by symmetrical hydrogen bonds from both ortho-carboxyl groups. The energetics of this arrangement, the phenol and carboxyl acidities, and factors affecting those acidities and providing anion stabilization are discussed.

Original languageEnglish (US)
Pages (from-to)438-446
Number of pages9
JournalActa Crystallographica Section B: Structural Science
Issue number4
StatePublished - Aug 1 2004


All Science Journal Classification (ASJC) codes

  • Biochemistry, Genetics and Molecular Biology(all)

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