CBSE Class 12 — Notes, Chapters & Practice Quizzes

Chapter 2: Alcohols, Phenols and Ethers

Summary

Alcohols and phenols are formed when a hydrogen atom of an aliphatic or aromatic hydrocarbon respectively is replaced by a hydroxyl (-OH) group, while ethers have an oxygen atom linking two alkyl or aryl groups. Alcohols are classified as primary, secondary or tertiary depending on the carbon bearing the -OH group, and are named by the IUPAC system. They are prepared by hydration of alkenes, reduction of aldehydes, ketones and carboxylic acids, and from Grignard reagents. Phenols are prepared from haloarenes, benzenesulphonic acids, diazonium salts and cumene. Alcohols and phenols show hydrogen bonding, which raises their boiling points and water solubility. Chemically alcohols react at the O-H bond (showing weak acidity and ester formation) and the C-O bond (substitution and dehydration to alkenes), and undergo oxidation. Phenols are more acidic than alcohols because the phenoxide ion is stabilised by resonance, and the -OH group activates the ring toward electrophilic substitution at ortho and para positions. Ethers are prepared by dehydration of alcohols and by the Williamson synthesis from alkyl halides and sodium alkoxides; they are relatively unreactive but the C-O bond can be cleaved by hydrogen halides. These compounds are widely used as solvents, antiseptics, fragrances and fuels.

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