Maleic acid or (Z)-butenedioic acid or cis-butenedioic acid or malenic acid or maleinic acid or toxilic acid is an organic compound that is a dicarboxylic acid (molecule with two carboxyl groups). The molecule consists of an ethylene group flanked by two carboxylic acid groups. Maleic acid is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer. The cis isomer is the less stable one of the two; the difference in heat of combustion is 22.7 kJ/mol. The physical properties of maleic acid are very different from those of fumaric acid. Maleic acid is soluble in water, whereas fumaric acid is not; and the melting point of maleic acid (130 - 139 °C) is also much lower than that of fumaric acid (287 °C). Both properties of maleic acid can be explained on account of the intramolecularhydrogen bonding that takes place at the expense of intermolecular interactions.
Maleic acid should not be confused with malic acid or malonic acid, both of which are different types of dicarboxylic acids. In biology the compound goes by the name ionised maleate.
Isomerization. Maleic acid and fumaric acid can normally not be interconverted because rotation around a carbon carbon double bond is not possible. In the laboratory, conversion of the cis isomer into the trans isomer is possible by application of light and a small amount of bromine. Light converts elemental bromine into a bromine radical, which attacks the alkene in a radical addition reaction to a bromo-alkane radical; and now single bond rotation is possible. The bromine radicals recombine and fumaric acid is formed. In another method (used as a classroom demonstration), maleic acid is transformed into fumaric acid through the process of heating the maleic acid to a high temperature in a 12 molar hydrochloric acid solution. When heated the molecule loses a water molecule and becomes an acid anhydride. Once the heat is removed, the acid anhydride takes back the water molecule, but reforms as apolar fumaric acid. That is why it comes out of the polar HCl solution, as a polar solvent will not dissolve a non-polar solute. In industry, fumaric acid is also produced from maleic acid by catalyticisomerization with mineral acids, bromates, or thiourea. Again the large difference in water solubility makes fumaric acid purification easy.