Phenolic Acid: Qualitative Tests, Uses, Natural Occurrence, Examples

In this lesson, we will see about Phenolic acid in detail. Phenolic acid is also called phenol with carboxylic acid.

• Phenolic Acid is found in plant species especially in dry fruits the percentage of Phenolic acid is higher than other plant species.
• Phenolic Acid includes Phenolic ring and carboxylic acid function.
• In all Phenolic acids, the presence of hydroxyl OH on benzene rings i.e., aromatic compounds.
• Acids such as 3, 4-dihydroxybenzoic, p-hydroxybenzoic, vanillic, salicylic acid include one thing in common i.e., phenol.

There are various types of Phenolic acids but to understand Phenolic acids, we first need to understand phenol in detail.

Key Takeaways- Phenol, Phenolic Acid, Carboxylic Acid, Benzene, Carbolic Acid, Chemical, Compound, Electron, Organic, Phenyl

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What is Phenol?

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It is an aromatic organic compound consisting of phenyl group (-C₆H₅) bonded to a hydroxyl group (OH). The IUPAC name is phenol. The chemical formula is C₆H₅-OH. Other names of Phenol are Carboxylic acid, Phenylic acid, Hydroxybenzene, Phenic acid. Phenol is an aromatic organic compound in which one hydroxyl group i.e., OH replaces one hydrogen atom i.e., H in benzene.

To check whether phenol is acidic or not, it is necessary to dissociate the compound. The following reaction clarifies whether phenol is acidic or not.

When any substance dissociates into H+ ions then it shows acidic in nature. When any substance dissociates into OH- ions it shows basic in nature. So, when phenol dissociates it breaks into two in which one is H+, which means phenol is acidic in nature. The substance which replaces H+ more or quickly is more acidic. There are conditions or situations which show the acidity and stability of the chemical compound is:

• Resonance
• S character

In resonance we can identify the stability of the compound, the resonance of phenoxide ions will be checked.

The phenol has resonance, it is stable, so it is acidic in nature. The phenoxide ion is more stable due to resonance. In S character, the more S character in compound it becomes more stable. S character identifies due to hybridization. Phenol has three sigma bonds, so it has sp² hybridization. In which %S character is 33.33% which is acidic more than alcohols.

Effect of substituent on acidity of phenol is given below. There are two types of substituents group which will add on attached on phenol:

1. Electrons donate groups- which have the ability to donate electrons. Ex CH₃, Cl, OH, C₂H₅ etc. After dissociation, when phenoxide ion is attached to the electron donate group it increases the electron density and stability decreases and becomes unstable. Hence acidity of phenoxide ion decreases.
2. Electrons accept a group- which has the ability to accept (gain) electrons. Ex CN, NH₂, NO₂. After dissociation, when phenoxide ion is attached to the electron accept group it decreases the electron density and stability also increases due to that acidity of phenoxide ion also increases. Acidity is directly proportional to stability.

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Qualitative tests for Phenol

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Following are some tests:

1. Litmus test: Phenol turns blue litmus paper into red. This shows that phenol is acidic in nature.
2. Liebermann’s test: When phenol is reacted with NaNO₂ and concentric sulphuric acid, a deep green color is obtained which changes into red when diluted with water. But a small amount of NaOH was added to it. The solution becomes a deep blue color.
3. Ferric chloride test: Aqueous solution of phenols reacts with freshly prepared ferric chloride solution gives colored complex, mostly phenols give dark colored.
4. Bromine water test: Take aqueous solution of phenol and add excess bromine water and it gives yellowish white precipitate.
5. Phthalein dye test: Phenol on heating with phthalic anhydride in the presence of concentric sulphuric acid, it forms colorless condensation compound that is called phenolphthalein. On further reaction with dilute sodium hydroxide (NaOH) it gives a pink color fluorescent compound called fluorescein.

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Uses for Phenol

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Following are the Uses of Phenol:

• It is used in raw materials for drugs such as salol, Aspirin etc.
• It is used in preservatives, antiseptic (Dettol) etc.
• The major uses of phenol in the plastic industry involve its conversion to precursors for plastics.

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Natural Occurrence of Phenol

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Following are some Natural Occurrence of Phenol:

• It occurs in human urine.
• It is found in a castoreum.
• It occurs in Islay scotch whiskey around 30-160 ppm.

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Side Effects of Phenol

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Following are the side effects of Phenol:

• Phenol is cheap because it attracts many small scales. Like in cosmetics but due to safety concerns phenol is banned from use in cosmetics in Europe and Canada.
• As it is known that phenol is acidic in nature, and it can burn our skin.
• Vapors of Phenol are corrosive to the eyes, skin. That means it is toxic in nature.
• Phenol is also a reproductive toxin.

Read More: Reimer and Tiemann

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Examples of Phenol

Following are the Examples of Phenol:

• Vanillic Acid
• Syringic Acid
• Sinapic Acid
• Caffeic Acid
• Ferulic Acid

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Points to Remember

Following are some important points:

• Phenolic ring + carboxylic acid → Phenolic acid
• The phenol has resonance, it is stable, so it is acidic in nature.
• Phenol has three sigma bonds, so it has sp² hybridization.
• In which %S character is 33.33% which is acidic more than alcohols.
• Electrons donate groups- which have the ability to donate electrons. Ex CH₃, Cl, OH, C₂H₅ etc.
• Electron accept group- Which has ability to accept (gain) electrons. Ex CN, NH₂, NO₂.

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