CBSE Class 12th Chemistry Notes for Chapter 11 Alcohols, Phenols and Ethers

      UNIT 11 ALCOHOLS, PHENOLS AND ETHERS

 

 

• Alcohols and phenols are classified:
  • On the basis of the number of hydroxyl groups
  • According to the hybridization of the carbon atom, sp³ or sp² to which the –OH group is attached.

 

• Ethers are classified on the basis of groups attached to the oxygen atom.

 

• Alcohols may be prepared:
  • By hydration of alkenes
    • In presence of an acid
    • By hydroboration-oxidation reaction
  • From carbonyl compounds by:
    • Catalytic reduction
    • The action of Grignard reagents

 

• Phenols may be prepared by:
  • Substitution of:
    • Halogen atom in haloarenes
    • Sulphonic acid group in aryl sulphonic acods, by –OH group
  • By hydrolysis of diazonium salts
  • Industrially from cumene

 

• Alcohols have a higher boiling point than other classes of compounds like hydrocarbons, ethers and haloalkanes of comparable molecular masses. The ability of alcohols, phenols and ethers to form intermolecular hydrogen bonding with water makes them soluble in it.

 

• Alcohols and phenols are acidic in nature. Electron withdrawing group in phenol increase its acidic strength and electron releasing groups decreasing it.

 

 

 

• Alcohols undergo nucleophilic substitution with hydrogen halides to yield alkyl halides. Dehydration of alcohols gives alkenes. On oxidation, primary alcohols yield aldehydes with mild oxidizing agents and carboxylic acids with strong oxidizing agents while secondary alcohols yield ketones. Tertiary alcohols are resistant to oxidation.

 

• The presence of –OH group in phenols activates the aromatic ring towards electrophilic substitution and directs the incoming group to ortho and para positions due to resonance effect.

 

• Reimer-Tiemann reaction of phenol yields salicylaldehyde. In the presence of sodium hydroxide, phenol generates phenoxide ion which is even more reactive than phenol. Thus, in alkaline medium, phenol undergoes Kolbe’s reaction.

 

• Ethers may be prepared by:
  • Dehydration of alcohols
  • Williamson synthesis

 

• The boiling point of ethers resemble those of alkenes while their solubility id comparable to those of alcohols having the same molecular mass. The C-O bond in ethers can be cleaved by hydrogen halides. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the incoming group to ortho and para positions.