- Maleic acid
Maleic acid Identifiers CAS number ChemSpider UNII EC number KEGG ChEBI ChEMBL RTECS number OM9625000 Jmol-3D images Image 1 Properties Molecular formula C4H4O4 Molar mass 116.07 g mol−1 Appearance White solid Density 1.59 g/cm³  Melting point
135 °C, 408 K, 275 °F (decomposes)
Solubility in water 788 g/L Acidity (pKa) pka1 = 1.9
pka2 = 6.07 
Hazards MSDS MSDS from J. T. Baker EU classification Harmful (Xn) R-phrases S-phrases NFPA 704 Related compounds Related carboxylic acids fumaric acid
Related compounds maleic anhydride
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Maleic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Maleic acid is the cis-isomer of butenedioic acid, whereas fumaric acid is the trans-isomer. It is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride, maleic acid has few applications.
Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion is 22.7 kJ·mol−1. The heat of combustion is -1355 kJ/mole. Maleic acid is more soluble in water, than fumaric acid. The melting point of maleic acid (139–140 °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 in maleic acid at the expense of intermolecular interactions, and that are not possible in fumaric acid for geometric reasons.
Production and industrial applications
Isomerization to fumaric acid
The major industrial use of maleic acid is its conversion to fumaric acid. This conversion, an isomerization, is catalysed by a variety of reagents, such as mineral acids and thiourea. Again, the large difference in water solubility makes fumaric acid purification easy.
The isomerization is a popular topic in schools. Maleic acid and fumaric acid do not spontaneously interconvert because rotation around a carbon carbon double bond is not energetically favourable. However, conversion of the cis isomer into the trans isomer is possible by photolysis in the presence of 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 in 12 M hydrochloric acid solution. Reversible addition (of H+) leads to free rotation about the central C-C bond and formation of the more stable and less soluble fumaric acid.
Although not practised commercially, maleic acid can be converted into maleic anhydride by dehydration, to malic acid by hydration, and to succinic acid by hydrogenation (ethanol / palladium on carbon). 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, being electrophilic, participates as a dienophile in many Diels-Alder reactions.
The maleate ion is the ionized form of maleic acid. The maleate ion is useful in biochemistry as an inhibitor of transaminase reactions. Maleic acid esters are also called maleates, for instance dimethyl maleate.
- ^ Budavari, Susan, ed. (1996), The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals (12th ed.), Merck, ISBN 0911910123
- ^ a b Record in the GESTIS Substance Database from the IFA
- ^ CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL., 1993
- ^ Maleic Anhydride, Maleic Acid, and Fumaric Acid, Huntsman Petrochemical Corporation
- ^ M. N. G James, G. J. B Williams (1974). "A Refinement of the Crystal Structure of Maleic Acid". Acta Crystallographica B30 (5) (5): 1249–1275. doi:10.1107/S0567740874004626.
- ^ Kurt Lohbeck, Herbert Haferkorn, Werner Fuhrmann and Norbert Fedtke "Maleic and Fumaric Acids" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000. doi:10.1002/14356007.a16_053
- ^ Light isomerization experiment (from the University of Regensburg, with video)
- ^ DSM glyoxylic acid production
- ^ Kwesi Amoa (2007). "Catalytic Hydrogenation of Maleic Acid at Moderate Pressures A Laboratory Demonstration". Journal of Chemical Education 84 (12): 1948. doi:10.1021/ed084p1948.
- International Chemical Safety Card 1186
- Titration of Fumaric and Maleic Acid (from the University of Regensburg, about pH, solubility and melting point)
- Calculator: Water and solute activities in aqueous maleic acid
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