Small-Ring Compounds. XXV. Phenylcyclobutadienoquinone and Related Compounds¹

Thermal addition of phenylacetylene to trifluorochloroethylene and tetrafluoroethylene followed by hydrolysis of either product with concentrated sulfuric acid has led to the formation of phenylcyclobutene-1,2-dione(phenylcyclobutadienoquinone). This compound proved to be quite stable and displayed some unusual properties. A number of its substitution products have been prepared and their properties are described. Of these, 4-hydroxy-3-phenylcyclobutene-1,2-dione was of special interest in being a remarkably strong acid with pK_a of 0.37. The stability of phenylcyclobutadienoquinone and the acid strength of the 4-hydroxy derivative are discussed and correlated with delocalization energies calculated by the LCAO method.