Maleic anhydride is an organic compound with the formula C2H2(CO)2O. It is the acid anhydride of maleic acid. It is a colorless or white solid with an acrid odor. It is produced industrially on a large scale for applications in coatings and polymers. Production Maleic anhydride is produced by vapor-phase oxidation of n-butane. The overall process converts the methyl groups to carboxylate and dehydrogenates the backbone. The selectivity of the process reflects the robustness of maleic anhydride, with its conjugated double-bond system. Traditionally maleic anhydride was produced by the oxidation of benzene or other aromatic compounds. As of 2006, only a few smaller plants continue to use benzene. In both cases, benzene and butane are fed into a stream of hot air, and the mixture is passed through a catalyst bed at high temperature. The ratio of air to hydrocarbon is controlled to prevent the mixture from igniting. Vanadium pentoxide and molybdenum trioxide are the catalysts used for the benzene route, whereas vanadium phosphate is used for the butane route: C4H10 + 3.5 O2 → C4H2O3 + 4 H2O ∆H = −1236 kJ/mol The main competing process entails full combustion of the butane, a conversion that is twice as exothermic as the partial oxidation. The traditional method using benzene became uneconomical due to the high and still rising benzene prices and by complying with the regulations of benzene emissions. In addition, in the production of maleic anhydride (4 C-atoms) a third of the original carbon atoms is lost as carbon dioxide when using benzene (6 carbon atoms). The modern catalytic processes start from a 4-carbon molecule and only attaches oxygen and removes water; the 4-C-base body of the molecule remains intact. Overall, the newer method is therefore more material efficient. Parallels exist with the production of phthalic anhydride: While older methods use naphthalene, modern methods use o-xylene as feedstock. Reactions The chemistry of maleic anhydride is very rich, reflecting its ready availability and bifunctional reactivity. It hydrolyzes, producing maleic acid, cis-HOOC–CH=CH–COOH. With alcohols, the half-ester is generated, e.g., cis-HOOC–CH=CH–COOCH3. Maleic anhydride is a classic substrate for Diels-Alder reactions. It was used for work in 1928, on the reaction between maleic anhydride and 1,3-butadiene, for which Otto Paul Hermann Diels and Kurt Alder were awarded the Nobel Prize in 1950. It is through this reaction that maleic anhydride converted to many pesticides and pharmaceuticals. Their 1928 patent also provided many other