Maleic acid is an
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
that is a
dicarboxylic acidDicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...
, a molecule with two carboxyl groups. Maleic acid is the
cis-isomer of butenedioic acid, whereas
fumaric acidFumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis...
is the
trans-isomer. It is mainly used as a precursor to fumaric acid, and relative to its parent
maleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
, maleic acid has few applications.
Physical properties
Maleic acid is a less stable molecule than fumaric acid. The difference in
heat of combustionThe heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...
is 22.7 kJ·mol
−1. The heat of combustion is -1355 kJ/mole. Maleic acid is more soluble in water, than
fumaric acidFumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis...
. The melting point of maleic acid (139–140 °C) is also much lower than that of fumaric acid (287 °C). Both properties of maleic acid can be explained on account of the
intramolecularIntramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...
hydrogen bondA hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
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
hydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of
maleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
, the latter being produced by oxidation of
benzeneBenzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
or
butaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
.
Isomerization to fumaric acid
The major industrial use of maleic acid is its conversion to fumaric acid. This conversion, an isomerization, is
catalysedCatalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by a variety of reagents, such as
mineral acidA mineral acid is an acid derived from one or more inorganic compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.-Characteristics:...
s and
thioureaThiourea is an organosulfur compound of with the formula SC2 . It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad...
. 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 bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
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
bromineBromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
. 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 acidFumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis...
through the process of heating the maleic acid in 12 M
hydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric 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.
Maleic acid is an industrial raw material for the production of
glyoxylic acidGlyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, and oxalic acid, glyoxylic acid is one of the C2 carboxylic acids. It is a colourless solid that occurs naturally and is useful industrially....
by
ozonolysisOzonolysis is the cleavage of an alkene or alkyne with ozone to form organic compounds in which the multiple carbon–carbon bond has been replaced by a double bond to oxygen...
.
Other reactions
Although not practised commercially, maleic acid can be converted into
maleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
by
dehydrationIn chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions...
, to
malic acidMalic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...
by
hydrationIn organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
, and to
succinic acidSuccinic acid is a dicarboxylic acid. Succinate plays a biochemical role in the citric acid cycle. The name derives from Latin succinum, meaning amber, from which the acid may be obtained....
by
hydrogenationHydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
(
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
/
palladium on carbonPalladium on carbon, often referred to as Pd/C, is a form of palladium used for catalysis. It is usually used for catalytic hydrogenations in organic chemistry...
). It reacts with
thionyl chlorideThionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...
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
biochemistryBiochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
as an inhibitor of transaminase reactions. Maleic acid
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s are also called maleates, for instance
dimethyl maleateDimethyl maleate is an organic compound with the formula C6H8O4. It is the -isomer of the dimethyl ester of fumaric acid.-Synthesis:Dimethyl maleate can be synthesized from maleic anhydride and methanol, with sulfuric acid acting as acid catalyst, via a nucleophilic acyl substitution for the...
.
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