Maleic Acid

Maleic acid is an organic compound and the cis-isomer form of the butenedioic acid. Hence, maleic acid is a dicarboxylic acid which means that the maleic acid structure consists of two carboxyl groups. The maleic acid molecular formula is HOOCCH=CHCOOH, which clearly shows the presence of the double bond of carbon as depicted by the name butenedioic acid. This can also be seen in maleic acid IUPAC name or nomenclature which identifies maleic acid as (2Z) But-2-enedioic acid. 

Maleic Acid - The Cis Form of Butenedioic Acid

From the given maleic acid formula it is clear that it is a four-carbon, double bond containing organic molecule with two carboxylic groups attached at both ends. Maleic acid is actually the cis isomer form of butenedioic acid. Hence, it is scientifically known as cis-butenedioic acid. On the other hand, the trans isomer form of butenedioic acid is known as fumaric acid. Thus, maleic acid and fumaric acid are isomers of the same organic compound. But this maleic acid is not to be confused with malic acid. Maleic acid is cis-but-2-enedionic acid whereas malic acid is 2-hydroxybutanedioic acid. Thus, what makes malic acid and fumaric acid/maleic acid essentially different is that in malic acid there is a hydroxyl group attached to the second carbon while in the structure of maleic acid and fumaric acid double bond is present on the second carbon. Both, the maleic acid structure and fumaric acid structure, which are the cis and trans isomers of butenedioic acid are shown below:

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Maleic acid, like its parent maleic anhydride is a white solid in appearance. Another relation between maleic and fumaric acid is that maleic acid is a precursor to fumaric acid and the wide-scale industrial applications of maleic acid involve the production of fumaric acid.  

Maleic Acid Properties

Some of the properties of maleic acid are given below:

• The heat of combustion of maleic acid is -1335 kJ/mol and is higher than the fumaric acid. 

• Also, maleic acid and fumaric acid have different solubility properties as well. Maleic acid is more soluble in water than the trans counterpart fumaric acid. 

• The melting point of maleic acid is 135℃  and it is lower than fumaric acid which has a melting point of 187℃.

• The properties of maleic acid are mostly based on the intramolecular maleic acid structure which is defined by intramolecular hydrogen bonding. This is another difference between maleic and fumaric acid, because of structural constraints fumaric acid does not have intramolecular hydrogen bonding. 

• The molar mass of maleic acid is 116.072 g/mol. It is a white solid with a density of 1.59 g/cm3. 

Industrial Production and Applications of Maleic Acid 

Industrially, maleic acid production is driven by maleic anhydride as the source material. The hydrolysis of maleic anhydride and maleic anhydride itself is produced by the oxidation of benzene or butane. 

Although the primary industrial use of maleic acid is the production of fumaric acid, there are other uses as well which are listed below:

• It is used as an industrial raw material for the production of glyoxylic acid by ozonolysis. 

• It can also be used as acid addition salts along with drugs to make them more stable. An example of this is the indacaterol maleate. 

• It is also used as an adhesion promoter for many different substrates such as metals coated with zinc and nylon. An important example of this is the galvanized steel that is used in methyl methacrylate-based adhesives. 

• It can also be used for the conversion of maleic anhydride by dehydration. Although, this is not a very prominent commercial technique. 

• Maleic acid can also be used to convert into malic acid by hydration and also for the conversion into succinic acid through the process of hydrogenation in presence of ethanol or palladium on carbon. 

• The reaction of maleic acid with thionyl chloride or phosphorus pentachloride gives maleic acid chloride. 

• It is also used in Diels-Alder reactions because maleic acid being electrophilic in nature can act as a dienophile.  

• Maleate ion, which is an ionized form of maleic acid, is useful in biochemistry applications of the inhibitor of the transaminase reactions. Maleic acid esters are known as maleates for example dimethyl maleate. 

• Many of the drugs that consist of amines are usually provided as maleic acid esters such as carefenazine, methylergonovine, etc. 

Conversion of Maleic Acid to Fumaric Acid

There are wide to lesser applications of maleic acid. The major industrial utilization of maleic acid is in the production of fumaric acid. The conversion of maleic acid into fumaric acid is carried out by the isomerization process. The isomerization is catalyzed by a variety of reagents like mineral acids and thiourea. One of the easiest ways to separate maleic acid from fumaric acid is the difference in the solubility in water. Maleic is highly soluble in water whereas fumaric acid is very less and thus, it becomes easier to purify fumaric acid.

A notable point is that maleic acid cannot be readily converted into fumaric acid on its own. The structural constraints of the presence of double bonds of maleic acid prevent it from spontaneously converting into fumaric acid. This is because the rotation around the central carbon-carbon double bond is not energetically favourable. Hence, different methods such as photolysis is required to convert the cis isomer to trans isomer. The photolysis reaction i.e. breaking of the bonds using light, for conversion of maleic acid to formic acid requires small amounts of bromine as well. In the reaction process, light converts the bromine atoms into bromine radicals. This bromine radical then goes and attacks the alkene that is present through the radical reaction to bromo-alkene radical. Hence, this converts the carbon-carbon double bond into a single bond. This single bond can be rotated and the cis isomer of this intermediate product gets converted to trans isomer. After this, the bromine radicals recombine and the double bond in-between the two carbon atoms is restored. 

There is another process or reaction via which maleic acid gets converted to fumaric acid. This method is easy and safe and is also used as a part of the classroom teaching process. In this method, the conversion of maleic acid takes place by heating it in a hydrochloric acid solution. Due to the heating, the dissociated H+ ions as free radicals perform the same function as the bromine radicals and hence, are reversibly added to the double-bonded carbons. Once the double bond is converted to a single bond it is then rotated and as the H+ ions leave the single bond gets converted to a double bond again. Later on, because the fumaric acid is less soluble in water it is easily purified from the reaction. 

There are some examples of living organisms being able to convert maleic acid into fumaric acid as well. Some bacteria produce certain enzymes, known as maleate isomerase, which are used by the bacteria during the process of nicotinate metabolism. This enzyme is responsible for the isomerization of maleic acid to fumaric acid. These acids are also known as maleate and fumarate.