Phenols - Detailed Notes for JEE Exam Preparation

1. Nomenclature and Structure of Phenols

• Phenols are a class of organic compounds characterized by the presence of a hydroxyl group (-OH) directly attached to a benzene ring.
• They can be classified as monohydric phenols (one -OH group), polyhydric phenols (multiple -OH groups), or substituted phenols (other functional groups attached to the benzene ring).
• Some important phenols include phenol, cresols, xylenols, resorcinol, catechol, and hydroquinone.

2. Reaction with Hydrogen Halides (Electrophilic Substitution)

• Phenols undergo electrophilic substitution reactions with hydrogen halides (HX) to form alkyl halides.
• The reaction proceeds via the formation of a phenoxide ion, which acts as a nucleophile and attacks the electrophilic hydrogen halide.
• The reactivity of phenols towards electrophilic substitution is influenced by the presence of substituents on the benzene ring, such as alkyl groups (activating) and electron-withdrawing groups (deactivating).

3. Friedel-Crafts Reaction

• Phenols undergo Friedel-Crafts acylation and alkylation reactions, similar to benzene.
• In Friedel-Crafts acylation, phenols react with acyl chlorides (R-COCl) in the presence of a Lewis acid catalyst (e.g., AlCl₃) to form ketones (R-CO-Ar).
• In Friedel-Crafts alkylation, phenols react with alkyl halides (R-X) in the presence of a Lewis acid catalyst to form alkylated phenols (R-Ar).

4. Reaction with Concentrated Nitric Acid

• Phenols react with concentrated nitric acid to undergo nitration, resulting in the formation of nitrophenols.
• The reaction proceeds via the electrophilic substitution mechanism, with the nitro group (-NO₂) being introduced at the ortho and para positions of the benzene ring.
• The regioselectivity of nitration is influenced by the substituents on the benzene ring.

5. Reaction with Concentrated Sulphuric Acid

• Phenols react with concentrated sulphuric acid to undergo sulfonation, leading to the formation of phenolsulfonic acids.
• The reaction proceeds via the electrophilic substitution mechanism, with the sulfonic acid group (-SO₃H) being introduced at the ortho and para positions of the benzene ring.
• The regioselectivity of sulfonation is similar to nitration and is influenced by the substituents on the benzene ring.

6. Oxidation of Phenols

• Phenols can be oxidized by various oxidizing agents, such as potassium dichromate (K₂Cr₂O₇) or hydrogen peroxide (H₂O₂), to form quinones and other oxidized products.
• Quinones are cyclic compounds that contain two carbonyl groups (C=O) in a conjugated system.
• The oxidation of phenols is an important reaction in biological systems, such as the oxidation of dopamine to produce melanin.

7. Kolbe-Schmitt Reaction

• The Kolbe-Schmitt reaction involves the reaction of phenols with carbon dioxide (CO₂) and sodium hydroxide (NaOH) to form salicylic acids.
• The reaction proceeds via the formation of a phenoxide ion, which attacks the electrophilic carbon dioxide to form a carboxylate salt.
• Subsequent protonation and decarboxylation lead to the formation of salicylic acid.

8. Reimer-Tiemann Reaction

• The Reimer-Tiemann reaction is a method for synthesizing salicylaldehyde from phenols.
• It involves the reaction of phenols with chloroform (CHCl₃) and aqueous sodium hydroxide (NaOH) to form salicylaldehyde.
• The reaction proceeds via the formation of a dichloromethyl ether, which undergoes hydrolysis to form salicylaldehyde.

9. Williamson Ether Synthesis

• The Williamson ether synthesis is a method for synthesizing ethers from phenols and alkyl halides.
• It involves the reaction of a phenoxide ion, generated by the treatment of a phenol with a strong base (e.g., NaOH), with an alkyl halide.
• The reaction proceeds via a nucleophilic substitution mechanism, resulting in the formation of an ether.

10. Coupling Reactions

• Phenols can undergo various coupling reactions to form biaryl compounds (compounds with two benzene rings linked by a bond).
• Some important coupling reactions include the Gomberg-Bachmann reaction and the Ullmann reaction.
• In the Gomberg-Bachmann reaction, phenols react with diazonium salts in the presence of a copper(I) catalyst to form biaryls.
• In the Ullmann reaction, phenols react with copper powder in the presence of a copper(I) salt to form biaryls.

11. Carboxylation of Phenols

• Phenols can undergo carboxylation reactions in the presence of carbon dioxide (CO₂) and a Lewis acid catalyst to form salicylic acids.
• The reaction proceeds via the electrophilic addition of carbon dioxide to the phenoxide ion, followed by protonation and decarboxylation.

12. Dowtherm A Process

• The Dowtherm A process is an industrial method for the production of phenol from chlorobenzene.
• It involves the reaction of chlorobenzene with sodium hydroxide (NaOH) at high temperature and pressure to form a mixture of phenol and diphenyl oxide.
• The phenol is then separated from the mixture by distillation.

References

• NCERT Chemistry Textbook for Class 11 (Chapter 12: Alcohols, Phenols and Ethers)
• NCERT Chemistry Textbook for Class 12 (Chapter 13: Amines)