C4H4O4, IUPAC cis-butenedioic acid, melting point 139°C. A geometrical isomer of fumaric acid, but as the carboxyl groups are on the same side of the double bond, it forms an internal hydrogen bond, indicated in the formula, and thus has a much lower melting point than fumaric acid. It easily forms an anhydride by the loss of a water molecule with the formation of a ring.
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). Maleic acid is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer. 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).
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.
Reactions
Maleic acid and fumaric acid can normally not be interconverted because rotation around a carbon carbon double bond is not possible.
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. 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.
In industry fumaric acid is produced from maleic acid by catalytic isomerization with mineral acids, bromates or thiourea. Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.
Maleic acid is converted into maleic anhydride by dehydration and to succinic acid by dehydrogenation. It reacts with thionyl chloride or phosphorus pentachloride to give the maleic acid chloride (it is not possible to isolate the mono acid chloride).
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