{{short description|Dicarboxylic acid}} {{Distinguish|malic acid|malonic acid}} {{chembox | Watchedfields = changed | verifiedrevid = 477313504 | OtherNames = (''Z'')-Butenedioic acid; ''cis''-Butenedioic acid; Malenic acid; Maleinic acid; Toxilic acid | ImageFile = Maleinsäure.svg | ImageClass = skin-invert | ImageSize = | ImageName = Skeletal formula of maleic acid | ImageFile1 = Maleic-acid-3D-balls-A.png | ImageClass1 = bg-transparent | ImageSize1 = | ImageName1 = Ball-and-stick model of the maleic acid molecule | ImageFile2 = Maleic-acid-3D-vdW-A.png | ImageClass2 = bg-transparent | ImageSize2 = | ImageName2 = Space-filling model of the maleic acid molecule | ImageFile3 = Sample of maleic acid.jpg | ImageSize3 = 175px | PIN = (2''Z'')-But-2-enedioic acid <!-- Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) -->

|Section1={{Chembox Identifiers | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 18300 | SMILES = O=C(O)\C=C/C(=O)O | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 392248 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 91XW058U2C | KEGG_Ref = {{keggcite|correct|kegg}} | KEGG = C01384 | InChI = 1/C4H4O4/c5-3(6)1-2-4(7)8/h1-2H,(H,5,6)(H,7,8)/b2-1- | InChIKey = VZCYOOQTPOCHFL-UPHRSURJBG | ChEMBL_Ref = {{ebicite|correct|EBI}} | ChEMBL = 539648 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/C4H4O4/c5-3(6)1-2-4(7)8/h1-2H,(H,5,6)(H,7,8)/b2-1- | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = VZCYOOQTPOCHFL-UPHRSURJSA-N | CASNo_Ref = {{cascite|correct|CAS}} | CASNo = 110-16-7 | EINECS = 203-742-5 | RTECS = OM9625000 | PubChem = 444266 | 3DMet = B00280 | Beilstein = 605762 | Gmelin = 49854 | DrugBank = DB04299 }} |Section2={{Chembox Properties | C=4 | H=4 | O=4 | Appearance = White solid | Density = 1.59 g/cm<sup>3</sup> <ref name=Merck>{{Merck12th}}</ref> | Solubility = 478.8 g/L at 20 C<ref name=GESTIS/> | MeltingPtC = 135 | MeltingPt_notes = (decomposes)<ref name=GESTIS>{{GESTIS|ZVG=14640}}</ref> | pKa = p''K''<sub>a1</sub> = 1.90<br />p''K''<sub>a2</sub> = 6.07 <ref>{{cite book|title=CRC Handbook of Chemistry and Physics |edition=73rd |publisher=CRC Press |location=Boca Raton, Florida |date=1993}}</ref> | MagSus = {{val|-49.71e-6|u=cm<sup>3</sup>/mol}} }} |Section3={{Chembox Structure | Dipole = }} |Section7={{Chembox Hazards | ExternalSDS = [http://hazard.com/msds/mf/baker/baker/files/m0325.htm MSDS from J. T. Baker] | GHSPictograms = {{GHS07}} | GHSSignalWord = Warning | HPhrases = {{H-phrases|302|315|317|319|335}} | PPhrases = {{P-phrases|261|264|270|271|272|280|301+312|302+352|304+340|305+351+338|312|321|330|332+313|333+313|337+313|362|363|403+233|405|501}} | NFPA-H = 3 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = | FlashPt = }} |Section8={{Chembox Related | OtherFunction_label = carboxylic acids | OtherFunction = fumaric acid<br />succinic acid<br />crotonic acid | OtherCompounds = maleic anhydride<br />maleimide }} }}

'''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 HO<sub>2</sub>CCH=CHCO<sub>2</sub>H. Maleic acid is the ''cis'' isomer of butenedioic acid, whereas fumaric acid is the ''trans'' isomer. Maleic acid is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride, which has many applications.<ref name=Ullmann/>

==Etymology== 'Maleic acid' from the French {{lang|fr|acide maléique}} altered from {{lang|fr|acide malique}}, malic acid, from which it was synthesised.<ref>[https://www.researchgate.net/publication/231267880_The_Origin_of_the_Names_Malic_Maleic_and_Malonic_Acid Origin of the Names Malic, Maleic, and Malonic Acid] ResearchGate.net</ref> The word 'malic' is derived from Latin {{lang|la|mālum}}, meaning 'apple'.

== Physical properties == Maleic acid has a heat of combustion of −1,355.2{{nbsp}}kJ/mol, −20.5{{nbsp}}kJ/mol higher than that of fumaric acid.<ref>[http://www.southalabama.edu/chemistry/barletta/felthouse.pdf Maleic Anhydride, Maleic Acid, and Fumaric Acid] {{Webarchive|url=https://web.archive.org/web/20131102113133/http://www.southalabama.edu/chemistry/barletta/felthouse.pdf |date=2013-11-02 }}, Huntsman Petrochemical Corporation</ref> Maleic acid is more soluble in water than fumaric acid. The melting point of maleic acid (135&nbsp;°C) is also much lower than that of fumaric acid (287&nbsp;°C). As confirmed by X-ray crystallography, maleic acid is planar. Two normal hydrogen bonds are observed, one intramolecular and one intermolecular. Crystallographic analysis shows that the intramolecular hydrogen bond is little affected in the monopotassium salt.<ref>{{cite journal | first= M. N. G. |last=James |first2=G. J. B. |last2=Williams| title= A Refinement of the Crystal Structure of Maleic Acid | journal=Acta Crystallographica B| year= 1974 | volume=30 |issue=5 | pages=1249–1275 | doi= 10.1107/S0567740874004626 | bibcode= 1974AcCrB..30.1249J | doi-access= free }}</ref>

==Production and industrial applications== In industry, maleic acid is derived by hydrolysis of maleic anhydride, the latter being produced by oxidation of benzene or butane.<ref name=Ullmann>{{cite book|doi=10.1002/14356007.a16_053 | chapter=Maleic and Fumaric Acids | title=Ullmann's Encyclopedia of Industrial Chemistry | date=2000 | last1=Lohbeck | first1=Kurt | last2=Haferkorn | first2=Herbert | last3=Fuhrmann | first3=Werner | last4=Fedtke | first4=Norbert | isbn=9783527303854 }}</ref>

Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.<ref>[http://www.dsm.com/en_US/html/dfc/rds_oxidation.htm DSM glyoxylic acid production] {{webarchive|url=https://web.archive.org/web/20051130094344/http://www.dsm.com/en_US/html/dfc/rds_oxidation.htm |date=2005-11-30 }}</ref>

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.

===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.<ref>[http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Mal_Isom-e.htm Light isomerization experiment] {{webarchive|url=https://web.archive.org/web/20051127044811/http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Mal_Isom-e.htm |date=2005-11-27 }} (from the University of Regensburg, with video)</ref> 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. The use of catalytic amounts of aqueous thiyl radicals also accelerates the isomerization reaction.<ref>{{cite journal|author1=Kosaka, S. |author2=Kurebayashi, K. |author3=Yamato, N. |author4=Tanaka, H. |author5=Haruta, N. |author6=Yamamoto, M.|title= Thiyl Chemistry: Cysteine-Catalyzed Maleate Isomerization via Aqueous Thiyl Radical Processes|journal=Green Chemistry|year=2025|volume=27|issue=10 |pages=2743–2750|doi=10.1039/d4gc06310d }}</ref> In another method (used as a classroom demonstration), maleic acid is transformed into fumaric acid through the process of heating the maleic acid in hydrochloric acid solution. Reversible addition (of H<sup>+</sup>) leads to free rotation about the central C–C bond and formation of the more stable and less soluble fumaric acid.

Some bacteria produce the enzyme maleate isomerase, which is used by bacteria in nicotinate metabolism. This enzyme catalyses isomerization between fumarate and maleate.

==Other reactions== 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).<ref>{{cite journal | title = Catalytic Hydrogenation of Maleic Acid at Moderate Pressures A Laboratory Demonstration | first = Kwesi | last = Amoa | pages = 1948 | journal = Journal of Chemical Education | volume = 84 | issue = 12 | year = 2007 | doi = 10.1021/ed084p1948| bibcode = 2007JChEd..84.1948A }}</ref> It reacts with thionyl chloride or phosphorus pentachloride to give the maleic acid chloride (it is not possible to isolate the monoacid chloride). Maleic acid, being electrophilic, participates as a dienophile in many Diels–Alder reactions.

== {{anchor|Maleate ion}} Maleates== 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.

===Use in pharmaceutical drugs=== Many drugs that contain amines are provided as the maleate acid salt, for example, carfenazine, chlorpheniramine, pyrilamine, methylergonovine, and thiethylperazine.<ref>{{Cite web |title=NCATS Inxight Drugs — Fumaric acid |url=https://drugs.ncats.io/drug/88XHZ13131 |access-date=2023-11-10 |website=drugs.ncats.io |language=en}}</ref>

==See also== * Fumaric acid * Malic acid * Malonic acid * Succinic acid

==References== {{Reflist}}

==External links== {{commons}} * [https://web.archive.org/web/20160304193119/http://www.inchem.org/documents/icsc/icsc/eics1186.htm International Chemical Safety Card 1186] * [http://www.aim.env.uea.ac.uk/aim/accent2/inputpage.php Calculator: Water and solute activities in aqueous maleic acid] {{Webarchive|url=https://web.archive.org/web/20090511021547/http://www.aim.env.uea.ac.uk/aim/accent2/inputpage.php |date=2009-05-11 }}

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Category:Dicarboxylic acids Category:Enoic acids