Phenol is an aromatic organic compound. Its molecular formula is C₆H₆O. Phenol is characterized by a hydroxyl group attached to a carbon atom that is part of an aromatic ring. It is also known as carbolic acid and consists of hydroxyl and phenyl groups attached to each other. In this article, we look into what phenol is, its structure, its classification, general properties, preparation and reactions, etc.

What is Phenol?

Phenol is an organic compound with an aromatic ring. The molecular formula of phenol is C₆H₅OH. It can be obtained from the distillation of coal tar or crude. It is a white crystalline solid that is volatile. It can be manufactured as a chemical or obtained as a natural substance. Its appearance is colourless-to-white solid when pure, while the commercial product is a liquid.

It is used in various applications, including surgical treatments, chemical peels, food and cosmetic preservatives, and cancer prevention. Being an antioxidant it have some health benefits. It is more acidic than typical alcohol.

Structure of Phenol

Phenol, also known as carbolic acid, has a simple chemical structure. It consists of a benzene ring with a hydroxyl (-OH) group attached to it. The chemical formula for phenol is C₆H₅OH. The benzene ring is a hexagonal ring of six carbon atoms with alternating single and double bonds between them. The hydroxyl group is attached to one of the carbon atoms in the benzene ring.

Phenol

Nomenclature of Phenol

The IUPAC name of phenol is Benzenol. However, the common name “phenol” is widely accepted and more commonly used than the IUPAC name. The IUPAC name Benzenol is derived from the systematic naming of the compound, where “benzene” refers to the parent hydrocarbon structure, and “ol” indicates the presence of a hydroxyl group (-OH).

Classification of phenol

Phenols can be classified based on the number of hydroxyl groups attached. They can be classified as follows:

• Monohydric Phenol
• Dihydric Phenol
• Trihydric Phenol

Monohydric Phenol (Containing One -OH group)

Monohydric Phenols are aromatic compounds with a single hydroxyl (-OH) group attached to a benzene ring. They can be represented by the general formula C₆H₅O. An example of a monohydric phenol is phenol itself, also known as carbolic acid.

Dihydric Phenol (Containing Two -OH groups)

Dihydric phenol is a type of phenol with two hydroxyl (-OH) groups directly attached to an aromatic ring. It may be ortho-, meta-, or para-derivative. Examples of dihydric phenols include benzene-1,2-diol, also known as catechol, and benzene-1,3-diol, also known as resorcinol.

Trihydric Phenol (Containing Three -OH groups)

Trihydric phenols are a phenolic compound containing three hydroxyl (OH) groups. Examples of trihydric phenols include 1,2,3-Benzenetriol (Pyrogallol), 1,2,4-Benzenetriol (Hydroxyquinol), and phloroglucinol (1,3,5). These compounds are solids slightly soluble in water but miscible with organic solvents.

Properties of phenol

Phenol is also called carbolic acid. It exhibits several distinctive physical and chemical properties:

State and Smell of Phenol

• Phenol is a colourless-to-white solid when pure, but the commercial product is a liquid with a distinct, sickeningly sweet, and tarry odor.

Solubility

• Phenol is moderately soluble in water, with about 8 g of phenol dissolving in 100 g of water. An increase in the size of the aryl group decreases its solubility.

Boiling Point

• Phenol has a higher boiling point than other hydrocarbons, increasing with more carbon atoms. Boiling Point of Phenol is 181.7℃

Molecular Mass

• The molecular mass of phenol is 94.11 g/mol

Melting Point

• The melting point of phenol is approximately 40.5°C to 41°C (104.9 °F)

Density

• The density of phenol is about 1.07 g/cm³ or 9.9lb/gal

Chemical Properties of Phenol

• Phenols are more acidic than aliphatic alcohols and water, but not as acidic as carboxylic acids.
• They react with active metals such as sodium and potassium to form the corresponding phenoxide, which indicate their acidic nature.
• Phenols exhibit chirality within their molecules due to the absence of planar and axial symmetry in the phenol molecule.

Acidity of Phenol

Phenol is more acidic than aliphatic alcohols due to the resonance stabilization of the phenolate anion. The sp² hybridized carbon atom of the benzene ring attached directly to the hydroxyl group acts as an electron sink, leading to the delocalization of the negative charge on the oxygen atom and an increase in the polarity of the O-H bond.

It allows Phenol to lose a hydrogen ion and form a phenoxide ion, making it a weak acid with a pH of around 6.6. The enhanced acidity of Phenol is attributed to the resonance stabilization of the phenolate anion, which is not present in the conjugate bases of simple alcohols.

Tautomerism of Phenol

Phenol can exist in two tautomeric forms, keto and enol. In the keto form, the oxygen of the hydroxyl group is double-bonded to the carbon of the aromatic ring, while in the enol form, the hydroxyl group is double-bonded to the adjacent carbon of the ring, forming a double bond between the carbon and the oxygen.

The enol form is less stable than the keto form due to the unfavourable charge separation between the oxygen and the carbon atoms. However, resonance, hydrogen bonding, or aromaticity can stabilize the enol form.

Natural Occurrence of Phenol

Phenol is a naturally occurring compound that can be found in various sources. Plants and microorganisms produce phenolic compounds in response to ecological pressures such as UV radiation and wounding. Natural phenols can be found in food consumed in human diets, such as fruits, vegetables, herbs, and spices, as well as in fatty matrices like olive oil.

Phenol is also a normal metabolic product excreted in human urine. Examples of natural sources of phenols include wine, vinegar, vegetables, herbs, spices, nuts, and algae.

Preparation of Phenol

Phenol can be prepared from various compounds such as haloarenes, benzene sulfonic acid, cumene, diazonium salts, Grignard reagents, etc. It can be prepared by laboratory and commercial methods. Let’s discuss these two methods below:

Laboratory Preparation of Phenol

In the laboratory, Phenol is prepared from benzene derivatives using the following method.

From benzene sulphonic acid: When benzene is treated with the mixture of( H₂SO₄ and SO₃) it forms benzene sulphonic acid. It reacts with NaOH and forms sodium benzene sulphonate, which is converted into phenoxide by heating with molten sodium hydroxide. This acidification gives Phenol.

C₆H₆ (Benzene) + H₂S₂O₇ (oleum) →C₆H₅SO₃H (Benzene sulphonic acid) + NaOH → C₆H₅SO₃^–Na⁺

C₆H₅SO₃^–Na⁺ + NaOH –(573/623) → C₆H₅NaO (Sodiumphenoxide) + HCl →C₆H₅OH

Grignard Reagent: When oxygen gas is passed from the ether solution of the Grignard reagent, it forms an oxy compound, which, on further hydrolysis with dilute HCl, gives Phenol.

C₆H₅MgBr + O₂ →C₆H₅OMgBr

C₆H₅OMgBr + water →C₆H₅OH

From diazonium salts: An aqueous solution of benzene diazonium salt on warming gives Phenol.

R−N₂⁺X⁻ + Water →C₆H₅OH

Commercial Preparation of Phenol

From Haloarenes(Dow process): Phenol can be prepared by heating chlorobenzene with 10% aqueous NaOH at about 623K under 300atm pressure in the presence of copper salt, which acts as a catalyst to form sodium phenoxide. The salt, when treated with dilute HCl, gives Phenol. This method is called the Dow process.

C₆H₅Cl + NaOH →C₆H₅O^–Na⁺ + HCl –› Phenol

From cumene(Hock process): Cumene is oxidized in the presence of air to cumene hydroperoxide, which further hydrolysis with dilute H₂SO₄ gives Phenol.

Cumene + oxygen →cumenehydroperoxide + water →Phenol

Reaction of Phenol

The reaction of phenols are as follows:

Action of zinc dust: When Phenol is heated with Zn dust it reduces to benzene.

C₆H₅OH + Zn(dust) →C₆H₆ + ZnO

Action of Ammonia: Phenol reacts with ammonia in the presence of anhydrous ZnCl₂ to give aniline.

Phenol + NH₃ –––573 K Anhydrous––› ZnCl₂ + C₆H₅NH₃ + H₂O

Action of Acid chlorides: When Phenol is treated with an acid chloride in the presence of Pyridine, it forms an ester.

C₆H₅OH + R-COCl ––Lewis acid––› Aryl ketone + HCl

Action of Acid anhydride: When phenol is treated with an acid anhydride in the presence of a small amount of concentration H₂SO₄,it forms an ester.

Phenol + Acid anhydride –––Warm––› Ar-COOR’ + Acid

Special Reaction of Phenol

Phenol undergoes various chemical reactions to a number of compounds. Some of these reactions are mentioned below:

Electrophilic Aromatic Substitution Reactions

Phenol undergoes various electrophilic aromatic substitution reactions due to the presence of the hydroxyl group, which is strongly activating and ortho/para directing. These reactions include halogenation, sulfonation, nitration, and Friedel–Crafts.

• Halogenation: Phenol undergoes halogenation in the presence of a halogen carrier such as iron(III) halide. The general reaction is:

Phenol + X₂ –––FeX₃––› C₆H₄OHX+ HX

• Sulfonation: Phenol undergoes sulfonation with fuming sulfuric acid to produce a mixture of ortho- and para-sulfonated products. The general reaction is:

Phenol + SO₃ ––H₂SO₄––› C₆H₄OHSO₃H+ H₂O

• Nitration: Phenol undergoes nitration with dilute nitric acid at low temperature to give ortho and para nitrophenols. The general reaction is:

Phenol + HNO₃ –––298K––› C₆H₄OHNO₂ + H₂O

• Friedel-Crafts Alkylation: Phenol can undergo Friedel-Crafts alkylation to introduce an alkyl group onto the benzene ring. The general reaction is:

Phenol + R-Cl ––– AlCl₃––› C₆H₄OHR+ HCl

Oxidation to Quinones: Phenol can be oxidized to form quinones, such as benzoquinone, through reactions with oxidizing agents like potassium dichromate.

Quinone + Oxidizing agent –––––› Oxidized quinone

Kolbe’s Reaction: When Phenol is treated with sodium hydroxide, it forms the phenoxide ion, which undergoes electrophilic substitution to form ortho-hydroxybenzoic acid. This reaction is called Kolbe’s reaction.

C₆H₅OH + CO₂ + NaOH –––––› Aromatic hydroxy acid

Reimer-Tiemann Reaction: Treatment of Phenol with chloroform in the presence of sodium hydroxide leads to the forming an aldehyde group at the ortho position of the benzene ring, called the Reimer-Tiemann reaction.

Phenol + CHCl₃ + NaOH ––––› C₆H₄OHCHO + NaCl + H₂O

Esterification: Phenol can react with acetyl chloride and acetic anhydride to form esters, where an alkoxy group replaces the hydroxyl group.

Phenol + Carboxylic acid derivative → C₆H₅COOR’ + By product

Derivatives of Phenol

Phenol can be used to make various other organic compounds. The reactions make other organic compounds is discussed below:

Phenol to Benzene

Phenol can be converted to benzene by removing -OH group from phenol. This can be done by dehydration. In the process of dehydration, phenol is heated in the presence of sulphuric acid catalyst. The reaction for the same is given below:

C₆​H₅​OH —Heat, H₂SO₄ —-> C₆​H₆​+H₂​O

Phenol to Chlorobenzene

To convert phenol to chlorobenzene, -OH group from Phenol is replaced with Cl atom. This is done by treating phenol with a chlorinating agent such as SOCl₂ or PCl₅. The reaction for the same is given as follows:

C₆​H_5​OH + SOCl₂​→C₆​H_5​Cl + SO₂ ​+ HCl

or

C₆H₅OH + PCl₅→C₆H₅Cl + POCl₃ +HCl

Phenol to Benzoquinone

To convert phenol to benzoquinone, phenol is oxidized into quinone compound with the help of oxidizing agent such as potassium permangante or chromic acid. The reaction for the same is given below:

2C₆​H₅​OH + O₂​→C₆​H₄​O₂​ + 2H₂​O

Uses of Phenol

Due to its unique properties, Phenol has a wide range of applications across various industries. Some of the critical applications of Phenol include:

• Medical Uses: Phenol finds application in the medical field as a local anaesthetic for otology procedures, a sore throat treatment, and an active ingredient in oral analgesics. It also possesses hemostatic and antiseptic properties, making it suitable for medical applications.
• Pharmaceuticals: Phenol is an essential ingredient in the production of various pharmaceutical drugs, lotions, ointments, and ear drops used to prevent bacterial or fungal infections
• Industrial Synthesis: Phenol is a crucial precursor for producing various materials and compounds. It synthesizes plastics, polycarbonates, epoxies, Bakelite, and nylon.
• Polymer Industry: It is used in the polymer industry for the preparation of phenolic resins, such as Bakelite, through its polymerization reaction with formaldehyde
• Other Uses: Phenol is also utilized in the extraction of nucleic acids, the production of explosive materials, and the manufacturing of azo dyes, and it is used as a preservative in the wood industry.

Toxicity of Phenol

Phenol can cause severe burns upon skin contact, and even dilute solutions may result in systemic toxicity. It is readily absorbed through multiple routes of exposure (ingestion, dermal, inhalation) and distributes widely in the body, leading to multi organ failure in severe cases.

It is a toxic substance that can cause a range of harmful effects on the body. It is a protoplasmic poison with dual hydrophilic and lipophilic properties. It can penetrate cellular membranes easily, denature proteins, and lead to cell death and necrosis. Following safety guidelines and precautions when working with Phenol is essential to prevent exposure and minimize the associated risks.