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Phthalic Acid

Last updated date: 16th May 2024
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Phthalic Acid Meaning

Phthalic Acid IUPAC name is 1, 2-, benzenedicarboxylic acid. It is a colorless, crystalline organic compound that is usually manufactured and marketed as anhydride. In the late twentieth century, annual phthalic anhydride production surpassed 1,000,000 metric tons; the majority of it was used as a component of polyesters, such as alkyd resins. 


In the production of anthraquinone (a dye intermediate), phenolphthalein (a laxative and acid-base indicator), and phthalocyanine pigments, smaller amounts were used. Phthalic Acid is the main chemical compound that is applied as an anhydride to create chemicals like dyes, phthalates, saccharin, perfumes, and many other useful products. The Phthalic Acid boiling point is 289 °C (1013 hPa) (decomposition).


Phthalic Acid Structure

The Phthalic Acid structure is very similar to that of the aromatic carboxylic acid and thus is known as one of the simplest acids of this family. As Phthalic Acid is chemically known as benzene-1,2-dicarboxylic acid, thus it is clear from its naming that Phthalic Acid structure consists of a benzene ring that is attached with two carboxyl groups at 1 and 2 positions or in other words an additional carboxyl group attached to the benzoic acid at the ortho position. Thus the chemical formula of Phthalic Acid or Phthalic Acid formula is  C6H4(CO2H)2 which means there is a six carbon ring with alternating double bond that makes it aromatic in nature with a carboxyl group ( -COOH) to one of the carbon atom of the aromatic benzene ring and the other carboxyl group ( -COOH) at the ortho position. This acid is very stable in nature and is considered as weak acid but it reacts vigorously with the strong bases. 


Production of Phthalic Acid

Phthalic Acid is generated by the catalytic oxidation of naphthalene or ortho-xylene straight to phthalic anhydride and following hydrolysis of the anhydride.


Auguste Laurent, a French chemist, developed Phthalic Acid in 1836 by oxidizing naphthalene tetrachloride. 


He called the resulting product "naPhthalic Acid" because he thought it was a naphthalene derivative. 


Laurent gave it its current name after Swiss chemist Jean Charles Galissard de Marignac determined its right formula. The oxidation of naphthalene tetrachloride with nitric acid, or, better still, the oxidation of the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate as a catalyst, was a popular manufacturing process in the nineteenth century.


Reactions of Phthalic Acid 

With pKas of 2.89 and 5.51, it is a dibasic acid. In analytical chemistry, potassium hydrogen phthalate, a monopotassium salt, is a regular acid. Typically, phthalate esters are made from phthalic anhydride, which is readily available. The 1,3-cyclohexadiene derivative is generated by reducing Phthalic Acid with sodium amalgam in the presence of water.


Phthalic Acid Properties 

Phthalic Acid Formula

C8H6O4 or C6H4(COOH)2 or H2C8H4O4

Molecular Weight

166.132 g/mol


1.593 g/cm3

Chemical Names

ortho-Phthalic Acid, 1,2-Benzene Dioic acid,

Benzene-1,2-dioic acid

Solubility in Water

0.6 g / 100 mL

Melting Point

207 °C


Phthalic Acid Formula

Phthalic Acid is a dicarboxylic acid of benzene that has two carboxyl groups in ortho places. It acts as a xenobiotic metabolite in humans. It's the phthalate(1-) and phthalate conjugate acid. 


Phthalic Acid Uses 

There are various Phthalic Acid uses :

  • Phthalic Acid is used mainly in the form of anhydride to produce other chemicals such as dyes, perfumes, saccharin, phthalates, and many other useful products.

  • Plasticizers such as Phthalic Acid esters (phthalates) are used in a wide range of consumer goods, commodities, and construction materials. 

  • As a result, phthalates can be present in high concentrations in both the air and dust of human homes and workplaces. 

  • Phthalates are also common pollutants in food and the environment.


Can Phthalic Acid be Dangerous?

Irritation of the skin, eyes, mucous membranes, and respiratory passages may occur after exposure to this compound. It can cause narcosis at high concentrations. 

Phthalic Acid is a carboxylic acid. Extreme heat has a negative impact on this chemical.


Anhydride Phthalic: Meaning, Uses, and Formula

The organic compound is phthalic anhydride, formula - C8H4O3. It is Phthalic Acid's anhydride. 


The most common commercial form of Phthalic Acid is phthalic anhydride. It was the first commercially available dicarboxylic acid anhydride.


Phthalic anhydride is a colorless to white lustrous solid that comes in the form of needles and has a slight odor. Skin irritant and moderately harmful by inhalation or ingestion.


The oxidation of naphthalene in concentrated sulphuric acid in the presence of mercury sulfate was the first step in the production of phthalic anhydride. The effluent gasses are cooled before passing into switch condensers, where the phthalic anhydride solidifies on the walls and is retrieved by sublimation.


Anhydride phthalic is used in a wide variety of applications around the world, from the plastics industry to resin synthesis, agricultural fungicides, and amines. It is currently made by oxidizing o-xylene and naphthalene in the vapor process. 


Reactions of Phthalic Anhydride

The reaction of phthalic anhydride or the acid with alcohol produces Phthalic Acid esters, which are used in diffusion pumps and to replace mercury in manometers. 


Diethyl and dihexyl esters are commonly used in this application. Insect repellents such as dimethyl phthalate are effective. In the production of so-called alkyd resins, phthalic anhydride is widely used. Polyesters of acids with two carboxyl groups and polyhydric alcohols make up these resins.


The acidic amino hydrogen atom between the two carbonyl groups causes phthalimide to form metallic salts. The potassium salt of phthalimide is used in Gabriel's synthesis of amines and amino acids, as you might remember. As previously mentioned, the Hofmann degradation of phthalimide provides a convenient method of obtaining anthranilic acid. When phthalic anhydride is treated in the cold with alkaline hydrogen peroxide, acidified monoperPhthalic Acid is created.


Below given are the 2D and 3D conformer structures of Phthalic Acid. Structures are representations of the arrangement of atoms and their ordering. 3D structures are compounds represented with the help of dashes, wedges and straight lines. Through 3D structure, one can easily identify and learn the positions of atoms with respect to each other. While 2D structures are just atoms drawn on the surface with 2 axes involved. 2D structure helps to understand functional performance and for detailed study analysis. 

2D structure

This 2D structure helps to give the chemical composition of compounds along with the presence of single or double bonds. 

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3D Conformer Structure

This is a ball and sticks 3D model of Phthalic Acid. Where the white terminates represent hydrogen atoms. And rests are carbon and compounds of carbon. 

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Quick Summary

Here below is a quick summary of all the properties of Phthalic Acid in tabular form. 

Name of Compound

Phthalic Acid


1, 2-, benzene dicarboxylic acid




Aromatic Crystalline Organic Compound

Chemical Formula



Catalytic oxidation of Naphthalene 


Dibasic acid

Molecular weight

166.132 g/mol


1.593 g/cm3


Is soluble in water, 0.6g/100ml

Melting point

207o C


Dyes, Construction materials, Pollutants, etc. 


Is sensitive to heat. May cause irritations if exposed to the human body

This was all about Phthalic Acid. For more such information, access free resources available on the Vedantu website useful for the state boards, CBSE, ICSE, and competitive examinations.

FAQs on Phthalic Acid

1. How Do You Make Phthalic Anhydride?

Phthalic anhydride is used to catalyze the oxidation of ortho-xylene or naphthalene. To keep phthalic anhydride separate from by-products like o – xylene in water or maleic anhydride, a collection of "switch condensers" is required. Phthalic anhydride can also be made with Phthalic Acid. Phthalic anhydride is being used in a wide variety of applications around the world, from the plastics industry to resin synthesis, agricultural fungicides, and amines. It is currently made by oxidizing o-xylene and naphthalene in the vapor process. To reduce the strain on fossil usage and the consequences of oil depletion issues, the construction of an alternative production route to the petrochemical one is highly desirable. 

2. How to Convert Phthalic Anhydride to Phthalic Acid?

We can transform Phthalic Acid into phthalic anhydride by dehydrating it. Since Phthalic Acid is a dicarboxylic acid with two or three carboxylic groups divided by two or three carbon atoms, it loses water molecules when heated, forming acid anhydrides. That’s how the conversion goes on. Phthalic anhydride is a colorless to white lustrous solid that comes in the form of needles and has a slight odor. Skin irritant and moderately harmful by inhalation or ingestion. The melting point is 64°F, and the flashpoint is 305°F. When combined with water, it produces a corrosive solution. 

3. Is Phthalic Acid Strong?

Phthalic Acid is a weak dibasic acid that has a pKa values 2.89 and 5.51 but it reacts very vigorously when it comes in contact with any strong bases.

4. Is Phthalic Acid safe?

Phthalic Acid is an aromatic dicarboxylic acid. The molecular formula of Phthalic Acid is C6H4(CO2H)2. Phthalic Acid is an isomer of isoPhthalic Acid and terePhthalic Acid. Any acid irrespective of its composure is harmful to the human body. This chemical is very sensitive to extreme heat. If Phthalic Acid is exposed to the body, irritations of skin, eyes, membranes or respiratory organs may occur. If a high amount of acid has been in contact, it can cause narcosis. 

5. What is the other name of Phthalic Acid?

Phthalic Acid which is closely related to the derivative of phthalic anhydride is a commodity produced on a large scale. Phthalic Acid is obtained from agricultural soils. The molecular formula of Phthalic Acid is C6H4(CO2H)2. The alternative name or the IUPAC name is 1,2-benzene dicarboxylic acid benzene-1,2-dicarboxylic acid o-Phthalic Acid o-dicarboxybenzene o-benzene dicarboxylic acid ortho-Phthalic Acid.