Maleic acid

Maleic acid, maleinic acid

Names
Preferred IUPAC name (2Z)-But-2-enedioic acid
Other names (Z)-butenedioic acid, cis-butenedioic acid, malenic acid, maleinic acid, toxilic acid
Identifiers
CAS Number	110-16-7 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:18300 Y
ChEMBL	ChEMBL539648 Y
ChemSpider	392248 Y
ECHA InfoCard	100.003.403
EC Number	203-742-5
KEGG	C01384 Y
RTECS number	OM9625000
UNII	91XW058U2C Y
InChI InChI=1S/C4H4O4/c5-3(6)1-2-4(7)8/h1-2H,(H,5,6)(H,7,8)/b2-1- Y Key: VZCYOOQTPOCHFL-UPHRSURJSA-N Y InChI=1/C4H4O4/c5-3(6)1-2-4(7)8/h1-2H,(H,5,6)(H,7,8)/b2-1- Key: VZCYOOQTPOCHFL-UPHRSURJBG
SMILES O=C(O)\C=C/C(=O)O
Properties
Chemical formula	C4H4O4
Molar mass	116.07 g·mol−1
Appearance	White solid
Density	1.59 g/cm³
Melting point	135 °C (275 °F; 408 K) (decomposes)
Solubility in water	478.8 g/L at 20 C
Acidity (pKa)	pka1 = 1.9 pka2 = 6.07
Magnetic susceptibility (χ)	-49.71·10−6 cm3/mol
Hazards
Safety data sheet	MSDS from J. T. Baker
EU classification (DSD)	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
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Maleic acid or cis-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO₂CCHCHCO₂H. 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 (135 °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.

In industry, maleic acid is derived by hydrolysis of maleic anhydride, the latter being produced by oxidation of benzene or butane.

Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.

Maleic acid may be used to form acid addition salts with drugs to make them more stable, such as indacaterol maleate.

Maleic acid is also used as an adhesion promoter for different substrates, such as nylon and zinc coated metals e.g galvanized steel, in methyl methacrylate based adhesives.

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.