maleic acid

Maleic acid
IUPAC name	Maleic acid (Z)-Butenedioic acid
Identifiers
CAS number	110-16-7 Y
EC number	203-742-5
RTECS number	OM9625000
SMILES	OC(=O)C=CC(=O)O
Properties
Molecular formula	C4H4O4
Molar mass	116.1 g/mol
Appearance	white solid
Density	1.59 g/cm³, solid
Melting point	131–139°C decomposes
Boiling point	135 °C decomp.
Solubility in water	78 g/100 ml (25 °C)
Solubility in ethanol, methanol	ethanol 3.41 M, methanol 3.82 M [1]
Acidity (pKa)	pka1 = 1.92 pka2 = 6.27
Hazards
MSDS	MSDS from J. T. Baker
EU classification	Harmful (Xn)
R-phrases	R22, R36/37/38
S-phrases	(S2), S26, S28, S37
NFPA 704	0 3 0
Related compounds
Related carboxylic acids	fumaric acid succinic acid crotonic acid
Related compounds	maleic anhydride maleimide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Maleic acid or (Z)-butenedioic acid or cis-butenedioic acid. is an organic compound that is a dicarboxylic acid (molecule with two carboxyl groups). Other names are malenic acid, maleinic acid and toxilic acid.
Maleic acid is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer.

Physical properties

The physical properties of maleic acid are very different from those of fumaric acid. Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion is 22.7 kJ/mol.

Maleic acid is soluble in water, whereas fumaric acid is not. The melting point of maleic acid (131 - 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^[2] that takes place at the expense of intermolecular interactions.

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

• Isomerization. Maleic acid and fumaric acid can normally not be interconverted because rotation around a carbon carbon double bond is not energetically favourable. In the laboratory, conversion of the cis isomer into the trans isomer is possible by application of light and a small amount of bromine ^[3]. 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^{[citation needed]} 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. In industry, fumaric acid is also produced from maleic acid by catalytic isomerization with mineral acids, bromates, or thiourea^{[citation needed]}. Again 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 ^[4].
• Maleic acid is converted into maleic anhydride by dehydration, to malic acid by hydration, and to succinic acid by hydrogenation (ethanol / Palladium on carbon) ^[5]. 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.

Maleates

The maleate ion is the ionised 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.

External links

Wikimedia Commons has media related to: Maleic acid

• International Chemical Safety Card 1186
• European Chemicals Bureau
• 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

References