Synthesis of mono- and bis-spirooxindole derivatives "on water" using double salt of aluminum sulfate-sulfuric acid as a reusable catalyst.

The preparation of double salt of aluminum sulfate-sulfuric acid (Al(SO)·(HSO)·24HO) by the reaction of aluminum sulfate and sulfuric acid in water is described. Aluminum sulfate-sulfuric acid is characterized via some spectroscopic and microscopic techniques such as infrared spectroscopy (IR), X-ray diffraction spectroscopy (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which corroborated the structure of the double salt. This double salt is soluble in water and insoluble in organic solvents.