Squaric acid

Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula C₄H₂O₄.

The conjugate base of squaric acid is the hydrogensquarate anion C₄HO₄⁻; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C₄O₄²⁻. This is one of the oxocarbon anions, which consist only of carbon and oxygen.

Squaric acid is a reagent for chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases.

Chemical properties

Squaric acid is a white crystalline powder with a thermal decomposition point of 245 °C at ambient pressure. The onset of thermal decomposition depends on the different thermodynamic conditions such as heating rates.

The structure of squaric acid is not a perfect square, as the carbon–carbon bond lengths are not quite equal. The high acidity with pK_a = 1.5 for the first proton and pK_a = 3.4 for the second is attributable to resonance stabilization of the anion. Because the negative charges are equally distributed between each oxygen atom, the dianion of squaric acid is completely symmetrical (unlike squaric acid itself) with all C-C and C-O bond lengths identical.