Maleic acid or ''cis''-butenedioic acid is an organic compound that is a

dicarboxylic acid In organic chemistry, a dicarboxylic acid is an organic compound containing two carboxyl groups (). The general molecular formula for dicarboxylic acids can be written as , where R can be aliphatic or aromatic. In general, dicarboxylic acids show ...

, a molecule with two carboxyl groups. Its chemical formula is HO₂CCH=CHCO₂H. 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.

Physical properties

Maleic acid has a ''

heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...

'' of -1,355 kJ/mol., 22.7 kJ/mol higher than that of fumaric acid. 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 bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

ing 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

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

butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name but ...

. 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.

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 Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

in the presence of a small amount of bromine. Light converts elemental bromine into a bromine

radical Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...

, 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 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. Some bacteria produce the enzyme maleate isomerase, which is used by bacteria in

nicotinate Niacin, also known as nicotinic acid, is an organic compound and a form of vitamin B3, an essential human nutrient. It can be manufactured by plants and animals from the amino acid tryptophan. Niacin is obtained in the diet from a variet ...

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 Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...

, and to succinic acid by hydrogenation ( ethanol /

palladium on carbon Palladium on carbon, often referred to as Pd/C, is a form of palladium used as a catalyst. The metal is supported on activated carbon to maximize its surface area and activity. Uses Hydrogenation Palladium on carbon is used for catalytic hydrog ...

). 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.

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 Dimethyl maleate is an organic compound with the formula C6H8O4. It is the dimethyl ester of maleic acid. Synthesis Dimethyl maleate can be synthesized from maleic anhydride and methanol, with sulfuric acid acting as acid catalyst, via a nucleop ...

Use in pharmaceutical drugs

Many drugs that contain amines are provided as the maleate acid salt, e.g.

carfenazine Carfenazine (INN) (former developmental code name WY-2445), or carphenazine ( BAN), also known as carphenazine maleate (USAN) (brand name Proketazine; former developmental code name NSC-71755), is an antipsychotic and tranquilizer of the phenothia ...

, chlorpheniramine,

pyrilamine Mepyramine, also known as pyrilamine, is a Antihistamine#H1-receptor antagonists, first generation antihistamine, targeting the Histamine H1 receptor, H1 receptor as an inverse agonist. Mepyramine rapidly permeates the brain, often causing drowsi ...

methylergonovine Methylergometrine, also known as methylergonovine and sold under the brand name Methergine, is a medication of the ergoline and lysergamide groups which is used as an oxytocic in obstetrics and in the treatment of migraine. It reportedly produces ...

, and

thiethylperazine Thiethylperazine (Torecan, Norzine) is an antiemetic of the phenothiazine class. It is an antagonist of dopamine receptors (DRD1, DRD2, DRD4) as well as of 5-HT2A, 5-HT2C receptors, mAChRs (1 through 5), α1 adrenergic receptor and H1 recepto ...

References

External links

International Chemical Safety Card 1186

Calculator: Water and solute activities in aqueous maleic acid
{{Authority control Dicarboxylic acids Enoic acids