Maleic acid

Overview
Maleic acid (ionised maleate in biology) or (Z)-butenedioic acid or cis-butenedioic acid or malenic acid or maleinic acid or toxilic acid is an organic compound which 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 that 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 intramolecular hydrogen 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.

Synthesis
In industry maleic acid is derived from maleic anhydride by hydrolysis. Maleic anhydride is produced from benzene or butane in an oxidation process.

Reactions
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 the classroom, maleic acid is transformed into Fumaric acid through the process of heating the maleic acid to a high temperature in a 12mol HCl solution. The heated molecule loses a water molecule and becomes an acid anhydride while in the heated solution. Once the heat is removed, the acid anhydride takes back the water molecule, but reforms as fumaric acid, the more stable isomer of butenedioic acid. This isomer is more stable because the carboxyl groups are no longer on the same side of the molecule, but are now on opposite sides, causing the molecule to become non-polar. 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 produced from maleic acid by catalytic isomerization with mineral acids, bromates or thiourea. The large difference in water solubility makes fumaric acid purification easy. Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.

Maleic acid is converted into maleic anhydride by dehydration, to malic acid by hydration, and to succinic acid by hydrogenation. It reacts with thionyl chloride or phosphorus pentachloride to give the maleic acid chloride (it is not possible to isolate the mono acid chloride). Maleic acid is a reactant in many Diels-Alder reactions.

Maleate ion
The maleate ion is the ionised form of maleic acid. It is of importance to biochemistry.