Maleic acid


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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 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.


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



The maleate ion is the ionised form of maleic acid. It is of importance to biochemistry. The maleate ion is useful in biochemistry as an inhibitor of transaminase reactions. Maleic acid esters are also called maleates, for instance dimethyl maleate.


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