Squaric acid

Squaric acid, also called quadratic acid, because its four carbon atoms form a square, is an acid with chemical formula 424 and CAS number 2892-51-5. It is one of the oxocarbonic acids.

It appears a white crystalline powder, and the melting point is 293 °C.

Medically, it can be used for the successful treatment of warts. It is also used treating alopecia areata or alopecia totalis/universalis (autoimmune hair loss) through topical immunotherapy involving the production of an allergic rash.

Squaric acid is also used to synthesize photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases.

The high acidity with pKa = 1.5 for the first proton and pKa = 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.



The original synthesis started from reaction of 1-chloro-1,2,2-trifluoroethylene with zinc to perfluorocyclobutene. This compound was converted to 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene with ethanol. Hydrolysis gives the squaric acid.