What Is Maleic Acid? Key Facts, Industrial Uses, and FAQs
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.
FAQs on Maleic Acid: Properties, Structure, and Applications
1. What is maleic acid and what is its chemical formula?
Maleic acid is an organic compound classified as a dicarboxylic acid, meaning it has two carboxyl functional groups (-COOH). Its chemical formula is C₄H₄O₄. It is specifically the cis-isomer of butenedioic acid, where the two carboxyl groups are located on the same side of the carbon-carbon double bond. This structure gives it distinct properties compared to its trans-isomer, fumaric acid.
2. What does the structure of maleic acid look like?
The structure of maleic acid is defined by a four-carbon chain containing a double bond between the second and third carbon atoms. The key feature is its cis-configuration, where the two carboxylic acid (-COOH) groups attached to the double-bonded carbons are on the same side. This proximity of the functional groups is crucial for its chemical behaviour, such as its ability to form a cyclic anhydride upon heating.
3. What are the key properties of maleic acid?
Maleic acid exhibits several distinct physical and chemical properties.
- Appearance: It is a white crystalline solid at room temperature with a sour taste.
- Solubility: It is highly soluble in water, which is a key difference from its trans-isomer, fumaric acid.
- Acidity: It is a diprotic acid, meaning it can donate two protons. Its first dissociation constant (pKa1) is lower (more acidic) than that of fumaric acid.
- Thermal Reaction: When heated to about 135°C, it readily loses a molecule of water to form maleic anhydride, a cyclic compound. This reaction is unique to the cis-isomer.
4. What are the major industrial applications of maleic acid?
Maleic acid is an important industrial chemical with several key applications. Its primary uses include:
- The manufacture of unsaturated polyester resins, which are used in fiberglass-reinforced plastics.
- Production of its more stable isomer, fumaric acid, which is used as a food acidulant.
- As a raw material for synthesising other important chemicals like malic acid and succinic acid.
- Used in the production of lubricant additives, copolymers, surface coatings, and agricultural chemicals.
5. How does maleic acid differ from its isomer, fumaric acid?
Maleic acid and fumaric acid are geometric isomers of butenedioic acid, leading to significant differences in their properties. The main distinctions are:
- Structure: Maleic acid is the cis-isomer (functional groups on the same side of the double bond), while fumaric acid is the trans-isomer (groups on opposite sides).
- Stability: Fumaric acid is more stable due to less steric repulsion between the large carboxyl groups.
- Solubility: Maleic acid is much more soluble in water because its structure allows for intramolecular hydrogen bonding, which interferes less with intermolecular bonding with water.
- Melting Point: Fumaric acid has a significantly higher melting point (287°C) compared to maleic acid (135°C) due to its more symmetrical structure, which allows for more efficient crystal packing.
- Reactivity: Only maleic acid can form a cyclic anhydride upon heating due to the proximity of its carboxyl groups.
6. Why is maleic acid considered a stronger acid than fumaric acid for the first proton donation?
Maleic acid is more acidic in its first dissociation (has a lower pKa1) because its conjugate base is uniquely stabilised. After losing the first proton, the resulting maleate ion can form a strong intramolecular hydrogen bond between the hydrogen of the remaining carboxyl group and the oxygen of the newly formed carboxylate group. This internal stabilisation makes the removal of the first proton more energetically favourable. Fumaric acid, being a trans-isomer, has its carboxyl groups too far apart to form such a bond, so its conjugate base is less stable.
7. Why is the ability to form maleic anhydride an important and unique feature of maleic acid?
The ability to form maleic anhydride upon heating is a direct result of maleic acid's cis-geometry. The two carboxyl groups are held close together, allowing them to easily react and eliminate a water molecule to form a stable five-membered ring. Its isomer, fumaric acid, cannot undergo this reaction because its trans-geometry places the carboxyl groups on opposite sides of the molecule, making it sterically impossible for them to interact. This unique reactivity makes maleic acid a crucial precursor in the synthesis of polymers and resins where maleic anhydride is the key building block.
8. How is maleic acid typically prepared on an industrial scale?
The primary industrial method for producing maleic acid is through the hydrolysis of maleic anhydride. This is a two-step process. First, maleic anhydride is produced by the catalytic vapour-phase oxidation of an organic compound, most commonly n-butane or benzene, using a catalyst like vanadium pentoxide. The resulting maleic anhydride is then dissolved in water and hydrolysed to yield maleic acid.
