Mellitic acid

Mellitic acid, also called graphitic acid, benzene hexacarboxylic acid, and benzene-1,2,3,4,5,6-hexacarboxylic acid (IUPAC name) is an acid first discovered in 1799 by M. H. Klaproth in the mineral mellite (honeystone), which is the aluminium salt of the acid.

Preparation
Mellitic acid may be prepared by warming mellite with ammonium carbonate, boiling off the excess of the ammonium salt and adding ammonia to the solution. The precipitated alumina is filtered 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 lead acetate and the lead salt decomposed by hydrogen sulfide. The acid may also be prepared by the oxidation of pure carbon, or of hexamethyl benzene, in the cold, by alkaline potassium permanganate, or by hot concentrated nitric acid.

Reactions
It crystallizes in fine 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, C10H6O3; 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, 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(CONH)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.

The high stability of mellitic acid salts and that they are the endproduct of the oxidation of polycyclic aromatic hydrocarbons, which are present in the solar system, made them a possible organic substance in Martian soil.

Mellitic anhydride
The anhydride of mellitic acid, C12O9, is one of the exotic oxides of carbon as it only consists of carbon and oxygen like the other oxides CO2, CO, C3O2 and C2O.