MELLITIC ACID (benzene hexacarboxylic acid), C6(COOH)6, was first discovered in 1799 by M. H. Klaproth in the mineral honey stone, which is the aluminium salt of the acid., The acid may be prepared by warming honey stone with ammonium carbonate, boiling off the excess of the ammonium salt and adding ammonia to the solution. The precipitated alumina is nltered off, the filtrate evaporated and the ammonium salt of the acid purified by recrystallization. The ammonium salt is then converted into the lead salt by precipitation with lea acetate and the lead salt decomposed by sulphuretted hydrogen.
The acid may also be prepared by the oxidation of pure carbon,
or of hexamethyl benzene, in the cold, by alkaline potassium
permanganate (F. Schulze, Ber., 1871, 4, p. 802; C. Friedel and J. M;
Crafts, Ann. chim. phys., 1884 [6], 1, p. 470). It crystallizes in line
silky needles and is soluble in water and alcohol. It is a very stable
compound, chlorine, concentrated nitric acid and hydriodic acid
having no action upon it. It is decomposed, on dry distillation,
into carbon dioxide and pyromellitic acid, C10H6O8; when distilled
with lime it gives carbon dioxide and benzene. Long digestion of
the acid with excess of phosphorus pentachloride results in the
formation of the acid chloride, C6(COCl)6, which crystallizes in
needles, melting at 190° C. By heating the ammonium salt of the
acid to 150–160° C. as long as ammonia is evolved, a mixture of
paramide (mellimide), C6(CO
CO 
NH)3, and ammonium euchroate is
obtained. The mixture may be separated by dissolving out the
ammonium euchroate with water. Paramide is a white amorphous
powder, insoluble in water and alcohol.