How Is Phthalic Acid Produced and Why Is It Important in Chemistry?
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
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.
Structures
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.
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: Structure, Reactions & Applications
1. What is phthalic acid and what is its chemical structure?
Phthalic acid is an aromatic dicarboxylic acid with the chemical formula C₆H₄(COOH)₂. Its structure consists of a benzene ring with two carboxyl (-COOH) groups attached to adjacent carbon atoms, specifically at positions 1 and 2. This 'ortho' arrangement is crucial to its chemical properties. Its IUPAC name is benzene-1,2-dicarboxylic acid.
2. How is phthalic acid commercially prepared?
Phthalic acid is primarily produced through a two-step industrial process. First, o-xylene or naphthalene is catalytically oxidised in the vapour phase to form phthalic anhydride. In the second step, this anhydride undergoes hydrolysis (reaction with hot water) to yield phthalic acid. This method is highly efficient for large-scale production.
3. What are the major applications and uses of phthalic acid?
While phthalic acid itself has limited direct use, it is a vital intermediate, mainly in the form of its anhydride. Its key applications include:
- Production of Plasticizers: Its esters, known as phthalates, are widely used to make plastics like PVC more flexible and durable.
- Synthesis of Dyes: It is a precursor for producing phenolphthalein and other phthalein dyes used as indicators.
- Manufacturing Resins: It is used to create alkyd and polyester resins, which are components of paints and varnishes.
- Other Chemicals: It is also used to synthesise perfumes, saccharin, and various other organic compounds.
4. Is phthalic acid considered a strong or a weak acid? Explain why.
Phthalic acid is a relatively weak dibasic acid. Although it is stronger than acetic acid, it is much weaker than mineral acids like HCl. The first dissociation (pKa1 ≈ 2.9) is more acidic than the second (pKa2 ≈ 5.5). The electron-withdrawing nature of the benzene ring enhances its acidity compared to aliphatic dicarboxylic acids, but it does not fully dissociate in water, which is characteristic of weak acids.
5. Why does phthalic acid easily form an anhydride on heating, while its isomers do not?
Phthalic acid readily forms phthalic anhydride upon gentle heating because its two carboxylic acid groups are located in the ortho (1,2) positions. This close proximity allows for an easy intramolecular dehydration reaction, where a water molecule is eliminated to form a stable five-membered anhydride ring. In contrast, its isomers, isophthalic acid (1,3-position) and terephthalic acid (1,4-position), have their carboxyl groups too far apart on the benzene ring to undergo this intramolecular ring formation.
6. What is the difference between converting phthalic acid to phthalic anhydride and the reverse reaction?
The conversion is a reversible reaction based on the presence or absence of water.
- Acid to Anhydride (Dehydration): Heating phthalic acid to around 200°C causes it to lose a molecule of water (dehydration) to form phthalic anhydride.
- Anhydride to Acid (Hydrolysis): Reacting phthalic anhydride with hot water adds a water molecule back across the anhydride bond (hydrolysis), reforming the two carboxylic acid groups of phthalic acid.
7. How does the structure of phthalic acid contribute to its role in making plasticizers?
The structure of phthalic acid is ideal for creating plasticizers through a process called esterification. The two carboxylic acid groups can react with long-chain alcohols (like 2-ethylhexanol) to form large, flexible ester molecules called phthalates. When these bulky phthalate molecules are mixed into a polymer like PVC, they get in between the rigid polymer chains, preventing them from packing tightly. This increases the space between chains, allowing them to slide past each other, which results in a much more flexible and softer plastic.
8. What are some key physical properties of phthalic acid?
Phthalic acid is a white crystalline solid at room temperature. It is sparingly soluble in cold water but its solubility increases significantly in hot water. It is readily soluble in organic solvents like ethanol. When heated, it does not have a sharp boiling point; instead, it melts around 210°C with decomposition, losing water to form phthalic anhydride.
