C₆H₅OH — Phenol

Phenol (C₆H₅OH), also known as carbolic acid, is an aromatic compound in which a hydroxyl group (–OH) is directly bonded to a benzene ring. It was first discovered by Friedrich Runge in coal tar and later recognized for its antiseptic properties by Joseph Lister in the 19th century. Phenol is one of the most important industrial chemicals, serving as a precursor to many polymers, drugs, dyes, and synthetic materials.

Phenol exhibits unique chemical behavior due to the resonance interaction between the hydroxyl group and the aromatic ring, making it more acidic than typical alcohols and highly reactive toward electrophilic substitution.

Phenol consists of a benzene ring (C₆H₆) with a hydroxyl group attached to one of the carbon atoms. Its molecular formula is \(C_6H_5OH\). The oxygen atom of the hydroxyl group is sp³ hybridized, forming a sigma bond with hydrogen and another with a carbon atom of the aromatic ring. The unshared pair of electrons on oxygen participates in delocalization with the π-electron system of benzene, enhancing resonance stabilization.

This partial delocalization of lone pair electrons into the ring reduces the polarity of the O–H bond, resulting in moderate acidity (stronger than ethanol but weaker than carboxylic acids).

Phenol can be synthesized through several industrial and laboratory methods:

1. From Chlorobenzene (Dow’s Process)

Chlorobenzene is fused with sodium hydroxide at high temperature and pressure to form sodium phenoxide, which is acidified to yield phenol.

2. From Cumene (Cumene Hydroperoxide Process)

This is the most common industrial method. Cumene (isopropylbenzene) is oxidized in air to form cumene hydroperoxide, which upon acidification yields phenol and acetone.

3. From Benzene Sulfonic Acid

Benzene is sulfonated and then fused with sodium hydroxide to form sodium phenoxide, which upon acidification gives phenol.

4. From Diazonium Salts

Benzene diazonium chloride can be hydrolyzed to phenol using warm water:

Phenol displays properties of both alcohols and aromatic compounds.

Physical Properties:

• Colorless crystalline solid, turning pink on oxidation.
• Melting point: 40.5°C; Boiling point: 181.7°C.
• Soluble in water, alcohol, and ether due to hydrogen bonding.

Chemical Properties:

• 1. Acidity: Phenol is weakly acidic and reacts with alkalis to form phenoxide ions:

\(C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O\)

• 2. Electrophilic Substitution: The hydroxyl group activates the ring and directs substitution to the ortho and para positions.
• Nitration: Reaction with dilute nitric acid forms o-nitrophenol and p-nitrophenol.

\(C_6H_5OH + HNO_3 \rightarrow o ext{-}C_6H_4(OH)(NO_2) + p ext{-}C_6H_4(OH)(NO_2)\)

• Bromination: Reaction with bromine water forms 2,4,6-tribromophenol (white precipitate).

\(C_6H_5OH + 3Br_2 \rightarrow C_6H_2Br_3OH + 3HBr\)

• 3. Oxidation: Phenol oxidizes to quinone upon treatment with oxidizing agents like sodium dichromate.

\(C_6H_5OH + [O] \rightarrow C_6H_4O_2 + H_2O\)

• 4. Reaction with Acetic Anhydride: Forms phenyl acetate (esterification reaction).

\(C_6H_5OH + (CH_3CO)_2O \rightarrow C_6H_5OCOCH_3 + CH_3COOH\)

• 1. Production of Plastics: Phenol is a precursor in making phenol-formaldehyde resins (Bakelite) used in electrical fittings and household goods.
• 2. Antiseptic: Historically used as an antiseptic in surgery (Lister’s antiseptic technique).
• 3. Pharmaceuticals: Used in making aspirin, salicylic acid, and other medicinal compounds.
• 4. Dyes and Explosives: Phenol derivatives are used in dye synthesis and the manufacture of picric acid.
• 5. Solvent and Intermediate: Employed in resin manufacturing, disinfectants, and as a solvent for organic reactions.

Phenol is corrosive and toxic in nature. Direct contact can cause severe burns, while inhalation of vapors affects the respiratory system. Prolonged exposure can damage the liver, kidneys, and central nervous system. In water, phenol can harm aquatic organisms due to its toxicity and persistence.

Proper protective measures such as gloves, masks, and ventilation should be used when handling phenol in laboratories or industries.

• Handle phenol in well-ventilated areas with protective gloves and eyewear.
• Avoid inhaling vapors; use fume hoods in laboratories.
• Store phenol away from heat and oxidizing agents.
• In case of skin contact, wash immediately with polyethylene glycol (PEG) or glycerol, then rinse with water.
• Dispose of phenol waste safely following environmental regulations.