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Science Class 12 - Phenols And-Ethers Notes

Comprehensive study notes for Class 12 - Phenols And-Ethers olympiad preparation

Phenols and Ethers

Welcome to the chapter on Phenols and Ethers for Class 12. In this chapter, you will learn about the structure, properties, preparation, and uses of phenols and ethers. By the end of this chapter, you will be able to identify, prepare, and distinguish between phenols and ethers, and understand their importance in organic chemistry.

Introduction

Phenols and ethers are important classes of organic compounds. Phenols contain a hydroxyl group (-OH) attached directly to an aromatic ring, while ethers have an oxygen atom connected to two alkyl or aryl groups.

Phenols

  • Structure: Phenol is C6H5OH, with an -OH group attached to a benzene ring.
  • Nomenclature: Named as derivatives of benzene (e.g., o-cresol, m-cresol, p-cresol).

Preparation of Phenols

  • From benzene sulfonic acid (fusion with NaOH).
  • From diazonium salts (hydrolysis of benzene diazonium chloride).
  • From chlorobenzene (Dow’s process: fusion with NaOH at high temperature and pressure).

Physical Properties of Phenols

  • Colorless, crystalline solids (may turn pink on exposure to air).
  • Slightly soluble in water, more soluble in organic solvents.
  • Distinctive odor.

Chemical Properties of Phenols

  • Acidic nature: Phenols are more acidic than alcohols but less than carboxylic acids.
  • Electrophilic substitution: Undergoes nitration, halogenation, sulfonation easily due to activating -OH group.
  • Reactions with bases: Forms phenoxide ion with NaOH.
  • Reimer-Tiemann reaction: Forms salicylaldehyde with chloroform and NaOH.
  • Kolbe’s reaction: Forms salicylic acid with CO2 and NaOH.

Uses of Phenols

  • Antiseptics and disinfectants (e.g., Dettol).
  • Manufacture of plastics, drugs, dyes, and explosives.

Ethers

  • Structure: Ethers have the general formula R–O–R', where R and R' are alkyl or aryl groups.
  • Nomenclature: Named as alkoxyalkanes (e.g., ethoxyethane) or as simple ethers (e.g., diethyl ether).

Preparation of Ethers

  • Williamson synthesis (reaction of alkoxide ion with alkyl halide).
  • Dehydration of alcohols (acid-catalyzed, e.g., ethanol to diethyl ether).

Physical Properties of Ethers

  • Colorless, volatile liquids with pleasant smell.
  • Slightly soluble in water, highly soluble in organic solvents.
  • Lower boiling points than alcohols of similar molecular mass.

Chemical Properties of Ethers

  • Relatively unreactive due to absence of active hydrogen.
  • Undergo cleavage with strong acids (HI, HBr) to form alcohols and alkyl halides.
  • Form peroxides on exposure to air and light.

Uses of Ethers

  • Used as solvents in laboratories and industries.
  • Diethyl ether was used as an anesthetic.

Practice Questions

  1. Write the structure of phenol and name two of its derivatives.
  2. How is phenol prepared from benzene sulfonic acid?
  3. Explain the Williamson synthesis of ethers with an example.
  4. Why are phenols more acidic than alcohols?
  5. What happens when diethyl ether reacts with HI?

Challenge Yourself

  • Draw the mechanism of the Reimer-Tiemann reaction.
  • Compare the boiling points of alcohols, phenols, and ethers of similar molecular mass and explain why.

Did You Know?

  • Phenol was the first antiseptic used in surgery by Joseph Lister.
  • Ethers can form explosive peroxides if stored for a long time in the presence of air.

Glossary

  • Phenol: An aromatic compound with an -OH group attached to a benzene ring.
  • Ether: An organic compound with an oxygen atom bonded to two alkyl or aryl groups.
  • Williamson Synthesis: A method to prepare ethers by reacting an alkoxide with an alkyl halide.
  • Reimer-Tiemann Reaction: A reaction to form aldehydes from phenols using chloroform and base.

Answers to Practice Questions

  1. Phenol: C6H5OH; derivatives: o-cresol, p-nitrophenol.
  2. By fusing sodium benzene sulfonate with NaOH, followed by acidification.
  3. Williamson synthesis: Sodium ethoxide reacts with ethyl bromide to give diethyl ether and NaBr.
  4. Due to resonance stabilization of the phenoxide ion and the electron-withdrawing effect of the aromatic ring.
  5. Diethyl ether reacts with HI to form ethanol and ethyl iodide (and with excess HI, both are converted to ethyl iodide and water).

Understanding phenols and ethers is key to mastering organic chemistry and their applications in daily life!

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