View source for Maleic acid

{| class="toccolours" border="1" style="float: right; clear: right; margin: 0 0 1em 1em; border-collapse: collapse;"
! {{chembox header}} | Maleic acid
|-
| align="center" colspan="2" bgcolor="#ffffff" | 
|-
! {{chembox header}} | General
|- 
| [[IUPAC nomenclature|Systematic name]]
| Maleic acid<br/>(''Z'')-Butenedioic acid
|-
| [[Chemical formula|Molecular formula]]
| C<sub>4</sub>H<sub>4</sub>O<sub>4</sub>
|-
| [[Simplified molecular input line entry specification|SMILES]]
| OC(=O)C=CC(=O)O
|-
| [[Molar mass]]
| 116.1 g/mol
|-
| Appearance
| white solid
|-
| [[CAS registry number|CAS number]]
| [110-16-7]
|-
! {{chembox header}} | Properties
|-
| [[Density]] and [[Phase (matter)|phase]]
| 1.59 g/cm³, solid
|-
| [[Soluble|Solubility]] in [[Water (molecule)|water]]
| 78 g/100 ml (25 °C)
|-
<!-- | Other solvents e.g. [[ethanol]], [[acetone]] -->
<!-- | solubility info on other solvents -->
<!-- |- -->
| [[Melting point]]
| 138 °C
|-
| [[Boiling point]]
| 135 °C ''decomp.''
|-
| [[Acid dissociation constant|Acidity]] (p''K''<sub>a</sub>) <!-- omit if not an acid or a base.  If several values, be clear -->
| p''k''<sub>a1</sub> = 1.97, p''k''<sub>a2</sub> = 6.07
|-
| [[Dipole#Molecular dipoles|Dipole moment]]
| ? [[Debye|D]]
|-
! {{chembox header}} | Hazards
|-
| [[Material safety data sheet|MSDS]]
|-
| [[Directive 67/548/EEC|EU classification]]
| Harmful ('''Xn''')
|-
| [[NFPA 704]]
| <!-- {{nfpa|4|4 ox|4}} These are set on "very dangerous" as default- adjust according to actual values -->
|-
| [[List of R-phrases|R-phrases]]
| {{R22}}, {{R36/37/38}}
|-
| [[List of S-phrases|S-phrases]]
| {{S2}}, {{S26}}, {{S28}}, {{S37}}
|-
| [[Flash point]]
| ? °C
|-
| [[RTECS]] number
| OM9625000
|-
! {{chembox header}} | Supplementary data page
|-
| Structure and<br/>properties 
| [[Refractive index|''n'']], [[Dielectric constant|ε<sub>r</sub>]], etc. 
|-
| Thermodynamic<br/>data 
| Phase behaviour<br>Solid, liquid, gas 
|-
| Spectral data
| [[UV/VIS spectroscopy|UV]], [[Infrared spectroscopy|IR]], [[NMR spectroscopy|NMR]], [[Mass spectrometry|MS]]
|-
! {{chembox header}} | Related compounds
|-
| Related [[carboxylic acid|dicarboxylic acids]]
| [[Fumaric acid]]<br/>[[Succinic acid]]
|-
| Related compounds 
| [[Maleic anhydride]]<br/>[[Maleimide]]
|-
| {{chembox header}} | <small>Except where noted otherwise, data are given for<br> materials in their [[standard state|standard state (at 25 °C, 100 kPa)]]<br/>[[wikipedia:Chemical infobox|Infobox disclaimer and references]]</small>
|-
|}
{{SI}}

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==Overview==
'''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 di[[carboxylic 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 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). Both properties of maleic acid can be explained on account of the [[intramolecular]] [[hydrogen bond]]ing 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.

== Synthesis ==
In industry maleic acid is derived from [[maleic anhydride]] by [[hydrolysis]]. Maleic anhydride is produced from [[benzene]] or [[butane]] in an [[organic oxidation|oxidation]] process.


== Reactions ==
Maleic acid and fumaric acid can normally not be interconverted because rotation around a carbon carbon [[double bond]] is not possible. In the laboratory conversion of the cis isomer into the trans isomer is possible by application of light and a small amount of [[bromine]]. Light converts elemental bromine into a bromine [[free radical|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 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. The heated molecule loses a water molecule and becomes an acid anhydride while in the heated 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. This isomer is more stable because the carboxyl groups are no longer on the same side of the molecule, but are now on opposite sides, causing the molecule to become [[non-polar]]. That is why it comes out of the [[Polar molecule|polar]] HCl solution, as a polar solvent will not dissolve a non-polar solute. 

In industry fumaric acid is produced from maleic acid by [[catalysis|catalytic]] [[isomerization]] with [[mineral acid]]s, bromates or [[thiourea]]. 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]].

Maleic acid is converted into [[maleic anhydride]] by [[Dehydration reaction|dehydration]], to [[malic acid]] by [[Hydration reaction|hydration]], and to [[succinic acid]] by [[hydrogenation]]. 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.

==Maleate ion==
The maleate ion is the ionised form of maleic acid. It is of importance to [[biochemistry]].

==External links==
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc11/icsc1186.htm International Chemical Safety Card 1186]
*{{ecb}}
*[http://www.jtbaker.com/msds/englishhtml/m0325.htm MSDS from J. T. Baker]
* [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Mal_Isom-e.htm Light isomerization experiment] (from the University of Regensburg, with video)
* [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/chembox_diprotic_tit-e.htm Titration of Fumaric and Maleic Acid] (from theUniversity of Regensburg, about pH, solubility and melting point)
* [http://www.dsm.com/en_US/html/dfc/rds_oxidation.htm DSM glyoxylic acid production]

[[Category:Dicarboxylic acids]]

[[ja:マレイン酸]]
[[pl:Kwas maleinowy]]
[[pt:Ácido maleico]]
[[zh:馬來酸]]

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