FUMARIC AND MALEIC ACIDS, unsaturated organic acids showing stereoisomerism. Fumaric acid is found in fumitory (Fumaria oficinalis), various fungi (Agaricus piper atus, etc.) and Iceland moss. It is obtained by heating malic acid alone to 150° C, or by heating it with hydrochloric acid or hydro bromic acid. It may also be obtained by boiling monobromo succinic acid with water, and by heating maleic acid to C. It crystallizes in small prisms or needles, and is practically insoluble in cold water. It sublimes to some extent at about 200° C, being partially converted into maleic anhydride and water, the reaction becoming practically quantitative if dehydrating agents be used. Potassium permanganate oxidizes it to racemic acid (see TARTARIC ACID).
Maleic acid is obtained by distilling malic or fumaric acids, or by heating fumaric acid with acetyl chloride to 1 oo° C. It crys tallizes in monoclinic prisms, which are easily soluble in water, melt at 130° C, and boil at 160° C, decomposing into water and maleic anhydride. When heated with concentrated hydrobromic or hydriodic acid, it is converted into fumaric acid. Oxidation converts it into mesotartaric acid. Maleic anhydride is obtained by distilling f umaric acid with phosphoric oxide. It forms tri clinic crystals which melt at 6o° C, and boil at 196° C. On a manufacturing scale maleic anhydride is prepared by aerial oxida tion of benzene vapour with a vanadium pentoxide catalyst. This preparation of the anhydride from benzene, a cyclic compound, affords additional confirmation of the cis-configuration assigned to maleic acid. Both acids yield acetylene by the electrolysis of aqueous solutions of their alkali salts, and on reduction both yield succinic acid, whilst by addition of hydrobromic acid they both yield monobromosuccinic acid. From these results it follows that the two acids are structurally identical, the isomerism being due to differences in the spatial arrangement of their constituent atoms (see STEREOCHEMISTRY), thus: H —C — CO2H Maleic acid H — C — CO2H Fumaric acid H — C — (cis). — C — H (trans) .
The foregoing formulae account for maleic acid readily yielding an anhydride, whereas fumaric acid does not, and for the behaviour of the acids towards bromine, f umaric acid yielding ordinary dibromosuccinic acid, and maleic acid the isomeric isodibromo succinic acid.